Dew Point Temperature Calculator

Calculate

Dry-bulb temperature of the air

Relative humidity of the air (0–100%)

Overview

Dew point temperature is the temperature at which moist air becomes fully saturated — the point at which water vapor begins to condense into liquid water. It is the single most actionable moisture metric in HVAC engineering because it defines a precise threshold: any surface in contact with air that falls below the dew point temperature will experience condensation. Unlike relative humidity, which changes whenever temperature changes, dew point changes only when the actual moisture content of the air changes. This makes it the preferred parameter for condensation risk assessment, building envelope analysis, and moisture control design.

In HVAC practice, dew point appears in virtually every moisture-related design decision. Cooling coil selection requires knowing the apparatus dew point — the effective surface temperature at which the coil must operate to remove latent heat. Supply air dew point determines whether condensation will form on cold supply ducts. Room dew point governs whether cold water pipes, basement walls, or exterior glazing will sweat. Outdoor design dew point drives the latent cooling load calculation for any air handling unit processing outdoor air.

This calculator derives dew point from any of four common input combinations — dry-bulb temperature with relative humidity, wet-bulb temperature, humidity ratio, or vapor pressure entered directly — using the Magnus approximation for saturation pressure and standard ASHRAE psychrometric formulations. It outputs dew point as the primary result alongside dew point depression, relative humidity, humidity ratio, vapor pressure, and enthalpy, and classifies the moisture condition against ASHRAE 55 comfort thresholds and condensation risk benchmarks.

Both Imperial (°F, gr/lb, psi) and Metric (°C, g/kg, kPa) unit systems are fully supported. All formulas use the corrected Magnus constant (0.08855 for Imperial, 0.61078 for Metric) consistent with the psychrometric equations used across this calculator suite.

How to Use This Calculator

  1. Select input combination — choose from Dry-Bulb + Relative Humidity, Dry-Bulb + Wet-Bulb Temperature, Dry-Bulb + Humidity Ratio, or Vapor Pressure Only.

  2. Enter dry-bulb temperature — in °C or °F (not required for Vapor Pressure Only).

  3. Enter relative humidity — in %.

  4. Enter wet-bulb temperature — in °C or °F.

  5. Enter humidity ratio — in g/kg or gr/lb.

  6. Enter vapor pressure — in kPa or psi.

  7. Click "Calculate" — get dew point temperature, dew point depression (ΔT), relative humidity, humidity ratio, vapor pressure, and enthalpy.

Compare the dew point against the coldest surface temperature (pipes, ducts, glazing); insulate any surface that falls below it to prevent condensation.

Inputs & Outputs

Inputs

  • •Input Combination — Options: Dry-Bulb + Relative Humidity, Dry-Bulb + Wet-Bulb Temperature, Dry-Bulb + Humidity Ratio, Vapor Pressure Only
  • •Dry-Bulb Temperature (°C / °F)
  • •Relative Humidity (%)
  • •Wet-Bulb Temperature (°C / °F)
  • •Humidity Ratio (g/kg / gr/lb)
  • •Vapor Pressure (kPa / psi)

Outputs

  • •Dew Point Temperature (°C / °F)
  • •Dew Point Depression (ΔT) (°C / °F)
  • •Relative Humidity (%)
  • •Humidity Ratio (W) (g/kg / gr/lb)
  • •Vapor Pressure (kPa / psi)
  • •Saturation Pressure at T_db (kPa / psi)
  • •Specific Enthalpy (kJ/kg / BTU/lb)

Formula

Calculator Formulas

All calculations assume standard atmospheric pressure:

  • Imperial: P_atm = 14.696 psi
  • Metric: P_atm = 101.325 kPa

Saturation Vapor Pressure (Magnus Approximation)

Metric:   P_sat = 0.61078 × exp(17.625 × T / (243.04 + T))   [kPa]
Imperial: P_sat = 0.08855 × exp(17.625 × Tc / (243.04 + Tc)) [psi]
          where Tc = (T_°F − 32) / 1.8

Note: 0.08855 = 0.61078 × 0.1450377 (base Magnus constant × kPa-to-psi conversion)

Combination A: Dry-Bulb + Relative Humidity

Step 1: P_sat_db = Magnus formula at T_db Step 2: P_v = (RH / 100) × P_sat_db Step 3: Dew point (inverse Magnus)

Metric:
  T_dp(°C) = 243.04 × ln(P_v / 0.61078) / (17.625 − ln(P_v / 0.61078))
Imperial:
  P_v_kPa = P_v × 6.8948
  T_dp(°C) = 243.04 × ln(P_v_kPa / 0.61078) / (17.625 − ln(P_v_kPa / 0.61078))
  T_dp(°F) = 32 + 1.8 × T_dp(°C)

Step 4: Humidity ratio

Imperial: W = 4350 × P_v / (P_atm − P_v)    [gr/lb]
Metric:   W = 621.945 × P_v / (P_atm − P_v) [g/kg]

Step 5: ΔT = T_db − T_dp

Combination B: Dry-Bulb + Wet-Bulb

Step 1: P_sat_wb = Magnus formula at T_wb

Imperial: W_sat_wb = 4350 × P_sat_wb / (P_atm − P_sat_wb)    [gr/lb]
Metric:   W_sat_wb = 621.945 × P_sat_wb / (P_atm − P_sat_wb) [g/kg]

Step 2: Humidity ratio (Sprung equation)

Imperial: W = W_sat_wb − 0.000437 × (T_db − T_wb) × 7000   [gr/lb]
Metric:   W = W_sat_wb − 0.000799 × (T_db − T_wb) × (1000 + W_sat_wb)   [g/kg]

Step 3: Vapor pressure

Imperial: P_v = P_atm × W / (4350 + W)
Metric:   P_v = P_atm × W / (621.945 + W)

Step 4: RH = (P_v / P_sat_db) × 100 Step 5: Dew point — same as Combination A Step 3 Step 6: ΔT = T_db − T_dp

Combination C: Dry-Bulb + Humidity Ratio

Step 1:

Imperial: P_v = P_atm × W / (4350 + W)
Metric:   P_v = P_atm × W / (621.945 + W)

Step 2: RH = (P_v / P_sat_db) × 100 Step 3: Dew point — same as Combination A Step 3 Step 4: ΔT = T_db − T_dp

Combination D: Vapor Pressure Only

Step 1:

Imperial: P_v_kPa = P_v_psi × 6.8948
  T_dp(°C) = 243.04 × ln(P_v_kPa / 0.61078) / (17.625 − ln(P_v_kPa / 0.61078))
  T_dp(°F) = 32 + 1.8 × T_dp(°C)
Metric: apply inverse Magnus directly to P_v in kPa

Note: suppress RH, ΔT, and enthalpy — T_db not available

Enthalpy (Combinations A, B, C)

Imperial: h = 0.240 × T_db + (W / 7000) × (1061 + 0.444 × T_db)   [BTU/lb]
Metric:   h = 1.006 × T_db + (W / 1000) × (2501 + 1.86 × T_db)    [kJ/kg]

Verification

  • At RH = 100%: T_dp = T_db = T_wb, ΔT = 0
  • At RH = 0%: T_dp → −∞, W = 0
  • T_dp always ≤ T_wb always ≤ T_db
  • T_dp unchanged by sensible heating or cooling at constant W

Variable Reference

Variable Meaning Units
T_db Dry-bulb temperature °F / °C
T_wb Wet-bulb temperature °F / °C
T_dp Dew point temperature °F / °C
RH Relative humidity %
W Humidity ratio gr/lb / g/kg
P_v Partial vapor pressure psi / kPa
P_sat Saturation vapor pressure psi / kPa
ΔT Dew point depression (T_db − T_dp) °F / °C
h Specific enthalpy BTU/lb / kJ/kg
P_atm Standard atmospheric pressure 14.696 psi / 101.325 kPa

What Is Dew Point Temperature?

Dew point temperature is the temperature to which air must be cooled, at constant pressure and constant moisture content, for saturation to occur and condensation to begin. At the dew point, the partial pressure of water vapor in the air equals the saturation vapor pressure at that temperature — meaning the air can hold no more moisture and any further cooling causes water to condense out of the air onto available surfaces.

The name derives from the familiar phenomenon of morning dew — overnight radiative cooling brings outdoor surfaces below the atmospheric dew point, causing water vapor to condense on grass, leaves, and other surfaces. In HVAC engineering, exactly the same phenomenon occurs on any surface that falls below the local air's dew point: cold water supply pipes sweat in humid summer air, supply air ducts condensate when installed in unconditioned humid spaces, and cooling coils remove latent heat precisely by operating their surface temperature below the entering air's dew point.

What distinguishes dew point from relative humidity as a practical metric is its absolute character. A given dew point corresponds to a fixed vapor pressure and a fixed humidity ratio, regardless of dry-bulb temperature. When air is heated from 50°F to 75°F, the dew point remains constant while relative humidity drops from approximately 85% to 40%. This invariance through sensible temperature change makes dew point the preferred parameter for condensation risk assessment — engineers compare a fixed dew point against a fixed surface temperature, without needing to track how temperature changes affect the calculation.

Key Facts

  • Dew point is independent of dry-bulb temperature. When air is heated or cooled without adding or removing moisture, the dew point does not change. Only processes that add moisture (humidification, evaporation, infiltration) or remove moisture (condensation on a cooling coil below the dew point) change the dew point.
  • ASHRAE 55 identifies a dew point of approximately 62°F (17°C) as the upper comfort limit for occupied spaces. Above this dew point, occupants perceive the air as uncomfortably humid regardless of dry-bulb temperature. Summer outdoor dew points in hot-humid climates frequently exceed 70°F (21°C), which is why mechanical dehumidification is essential in these regions.
  • The apparatus dew point (ADP) of a cooling coil is the effective surface temperature at which the coil operates. For the coil to remove latent heat from the air, the ADP must be below the entering air's dew point. This is the fundamental constraint in cooling coil selection for humid climates.
  • Dew point depression (ΔT = T_db − T_dp) is an intuitive field metric. A depression below 5°F (3°C) indicates near-saturation conditions and imminent condensation risk. A depression of 15–35°F (8–19°C) is typical of comfort HVAC conditions. A depression above 55°F (31°C) indicates very dry air typical of cold outdoor conditions or over-heated winter supply air.
  • The Magnus approximation used in this calculator for dew point inversion is accurate to better than 0.1°C across the HVAC operating range of −40°F to 200°F (−40°C to 93°C). The Imperial constant 0.08855 = 0.61078 × 0.1450377 combines the base Magnus constant with the kPa-to-psi unit conversion.
  • Frost point is the temperature at which ice forms rather than liquid water — it occurs below 32°F (0°C) and is slightly higher than the dew point at the same moisture content. At temperatures below freezing, surfaces experience frost formation rather than liquid condensation.

Applications

  • Condensation risk assessment on building envelopes, cold water pipes, supply air ducts, and glazing systems — compare dew point against surface temperature to determine if condensation will occur
  • Cooling coil apparatus dew point determination — the coil surface must operate below the entering dew point to condense moisture and remove latent heat
  • Supply air duct condensation prevention in humid climates — determine the ambient dew point that duct insulation must protect against
  • Building envelope interstitial condensation analysis — compare dew point at each plane within a wall or roof assembly against the actual temperature at that plane
  • HVAC commissioning and field measurement — calculate dew point from dry-bulb and wet-bulb readings to verify supply air dehumidification performance
  • Weather station and outdoor design condition analysis — convert ASHRAE climate data (design dry-bulb and coincident wet-bulb) into full moisture conditions for equipment sizing

Example Calculation

Imperial Example

Given: Combination A — Dry-Bulb + Relative Humidity

  • T_db = 82°F, RH = 65%
  • P_atm = 14.696 psi

Step 1 — Saturation pressure at 82°F:

Tc = (82 − 32) / 1.8 = 27.78°C
P_sat = 0.08855 × exp(17.625 × 27.78 / (243.04 + 27.78))
P_sat = 0.08855 × exp(1.8063) = 0.08855 × 6.087 = 0.5390 psi

Step 2 — Vapor pressure:

P_v = 0.65 × 0.5390 = 0.3504 psi

Step 3 — Dew point:

P_v_kPa = 0.3504 × 6.8948 = 2.416 kPa
T_dp(°C) = 243.04 × ln(2.416 / 0.61078) / (17.625 − ln(2.416 / 0.61078))
T_dp(°C) = 243.04 × 1.375 / (17.625 − 1.375) = 334.2 / 16.250 = 20.6°C
T_dp(°F) = 32 + 1.8 × 20.6 = 69.1°F

Step 4 — Dew point depression:

ΔT = 82 − 69.1 = 12.9°F

Step 5 — Humidity ratio:

W = 4350 × 0.3504 / (14.696 − 0.3504) = 1524.2 / 14.346 = 106.3 gr/lb

Step 6 — Enthalpy:

h = 0.240 × 82 + (106.3 / 7000) × (1061 + 0.444 × 82)
h = 19.68 + 0.015186 × 1097.41 = 19.68 + 16.66 = 36.34 BTU/lb

Result summary:

  • Dew Point (T_dp): 69.1°F
  • Dew Point Depression: 12.9°F
  • Relative Humidity: 65.0%
  • Humidity Ratio (W): 106.3 gr/lb
  • Vapor Pressure: 0.3504 psi
  • Saturation Pressure: 0.5390 psi
  • Enthalpy: 36.34 BTU/lb
  • Status: LOW DEPRESSION — HIGH HUMIDITY (ΔT 5–15°F)
  • Condensation Risk: WARM DEW POINT (T_dp ≥ 55°F)

Metric Example

Given: Combination B — Dry-Bulb + Wet-Bulb

  • T_db = 28°C, T_wb = 21°C
  • P_atm = 101.325 kPa

Step 1 — Saturation humidity ratio at T_wb = 21°C:

P_sat_wb = 0.61078 × exp(17.625 × 21 / (243.04 + 21))
P_sat_wb = 0.61078 × exp(1.4075) = 0.61078 × 4.085 = 2.495 kPa
W_sat_wb = 621.945 × 2.495 / (101.325 − 2.495) = 1551.8 / 98.830 = 15.70 g/kg

Step 2 — Humidity ratio (Sprung):

W = 15.70 − 0.000799 × (28 − 21) × (1000 + 15.70)
W = 15.70 − 0.000799 × 7 × 1015.70 = 15.70 − 5.678 = 10.02 g/kg

Step 3 — Vapor pressure:

P_v = 101.325 × 10.02 / (621.945 + 10.02) = 1015.5 / 631.965 = 1.607 kPa

Step 4 — Saturation pressure at T_db = 28°C:

P_sat_db = 0.61078 × exp(17.625 × 28 / (243.04 + 28))
P_sat_db = 0.61078 × exp(1.8296) = 0.61078 × 6.232 = 3.806 kPa

Step 5 — Relative humidity:

RH = (1.607 / 3.806) × 100 = 42.2%

Step 6 — Dew point:

T_dp(°C) = 243.04 × ln(1.607 / 0.61078) / (17.625 − ln(1.607 / 0.61078))
T_dp(°C) = 243.04 × 0.9676 / (17.625 − 0.9676) = 235.2 / 16.657 = 14.1°C

Step 7 — Dew point depression:

ΔT = 28 − 14.1 = 13.9°C

Step 8 — Enthalpy:

h = 1.006 × 28 + (10.02 / 1000) × (2501 + 1.86 × 28)
h = 28.17 + 0.01002 × 2553.08 = 28.17 + 25.58 = 53.75 kJ/kg

Result summary:

  • Dew Point (T_dp): 14.1°C
  • Dew Point Depression: 13.9°C
  • Relative Humidity: 42.2%
  • Humidity Ratio (W): 10.02 g/kg
  • Vapor Pressure: 1.607 kPa
  • Saturation Pressure: 3.806 kPa
  • Enthalpy: 53.75 kJ/kg
  • Status: MODERATE DEPRESSION — COMFORTABLE (ΔT 8–19°C)
  • Condensation Risk: MODERATE DEW POINT (T_dp 2–13°C)

Standards & References

  • ASHRAE Handbook — Fundamentals (2021), Ch. 1 Psychrometrics; Ch. 27 Moisture in Building Construction — Primary references for dew point formulations, saturation pressure equations, psychrometric relationships, and interstitial condensation analysis implemented on this page.
  • ASHRAE Standard 55-2020 — Thermal Environmental Conditions for Human Occupancy — Identifies a dew point of approximately 62°F (17°C) as the upper humidity comfort limit for occupied spaces.
  • ASHRAE Standard 62.1-2022 — Ventilation and Acceptable Indoor Air Quality — References sustained relative humidity above 70% as a condition that promotes mold growth and represents an IAQ concern.
  • Magnus Formula (Alduchov-Eskridge, 1996) — Used for both saturation pressure and inverse dew point calculation. Constants 17.625 and 243.04 are optimized for the temperature range −40°C to +60°C.
  • Sprung Psychrometric Equation — Used for Combination B (wet-bulb input). Constants A = 0.000437 °F⁻¹ (Imperial) and A = 0.000799 °C⁻¹ (Metric) apply to mechanically ventilated wet-bulb sensors.
  • ASHRAE Handbook — Fundamentals (2021), Chapter 27: Moisture in Building Construction — Governs interstitial condensation analysis using dew point as the condensation threshold at each plane in a building assembly.

Limitations

  • All calculations assume standard atmospheric pressure of 14.696 psi (101.325 kPa). At elevations above approximately 1,000 ft (300 m), lower atmospheric pressure increases humidity ratio for the same dew point. Dew point itself is independent of total pressure and remains valid at any altitude, but the associated humidity ratio and enthalpy values require altitude-corrected atmospheric pressure for accuracy.
  • The Magnus approximation loses accuracy below approximately −40°F (−40°C) and above approximately 200°F (93°C). Frost point — the temperature at which ice rather than liquid water forms — deviates from the thermodynamic dew point by 1–3°F (0.5–1.5°C) at temperatures below 32°F (0°C).
  • The Sprung psychrometric constant (Combination B) applies to mechanically ventilated wet-bulb thermometers. Field readings from naturally ventilated or screen-shielded sensors use a slightly different constant and may produce dew point results that differ by 1–3°F (0.5–1.5°C).
  • Dew point calculated at a single state point does not account for spatial variation in moisture distribution within a space. In large or stratified spaces, the dew point may vary significantly with location.

Common Mistakes to Avoid

  • Confusing dew point with wet-bulb temperature. Both are temperatures associated with moisture content but they are fundamentally different quantities. Wet-bulb is the equilibrium temperature of an evaporating water surface in the air — it lies between dry-bulb and dew point, and depends on all three psychrometric state variables. Dew point depends only on vapor pressure. They are equal only at 100% relative humidity.
  • Assuming dew point changes when air is heated. Dew point is conserved through sensible heating and cooling — it changes only when moisture is added or removed. Air heated from 50°F to 80°F maintains exactly the same dew point; only relative humidity drops.
  • Using relative humidity alone for condensation risk assessment. Relative humidity cannot determine condensation risk without knowing the dry-bulb temperature and the surface temperature. A dew point of 65°F at 80°F DB and 60% RH will condense on any surface below 65°F — regardless of what the relative humidity reading appears to indicate. Always use dew point for surface condensation assessment.
  • Applying the wrong Imperial P_sat constant. The correct Imperial saturation pressure formula uses 0.08855 = 0.61078 × 0.1450377. Using only 0.1450377 (the unit conversion factor alone) produces P_sat values inflated by a factor of 6.89 and dew point results that are grossly incorrect.
  • Comparing dew point against surface temperature without accounting for surface temperature depression due to airflow. In practice, supply air impinging on a surface at high velocity can lower the local surface temperature below the bulk surface temperature, increasing condensation risk.
  • Treating the ASHRAE 55 dew point comfort limit of 62°F (17°C) as a hard code requirement. ASHRAE Standard 55 defines a comfort envelope that includes an upper dew point recommendation, but it is a thermal comfort standard, not a ventilation code.

Frequently Asked Questions

What is dew point temperature and why does it matter in HVAC?
Dew point temperature is the temperature at which moist air becomes saturated and water vapor begins to condense into liquid. In HVAC, it matters because it defines a precise condensation threshold: any surface below the dew point will have condensation form on it. Unlike relative humidity, dew point does not change when air is heated or cooled — it changes only when moisture is added or removed. This makes it the correct parameter for condensation risk assessment on pipes, ducts, windows, and building envelope components.
What is the difference between dew point and wet-bulb temperature?
Both relate to moisture content but measure different things. Wet-bulb temperature is the equilibrium temperature of a water-wetted surface evaporating into the surrounding air — it always lies between dry-bulb and dew point. Dew point depends only on vapor pressure. They are equal only at 100% relative humidity. In HVAC practice, wet-bulb is used for cooling coil entering conditions; dew point is used for condensation risk and moisture content assessment.
What is dew point depression and how is it used?
Dew point depression is the difference between dry-bulb temperature and dew point temperature: ΔT = T_db − T_dp. A depression below 5°F (3°C) means near-saturation and imminent condensation risk. A depression of 15–35°F (8–19°C) is typical of comfortable occupied spaces. A depression above 55°F (31°C) indicates very dry air common in winter heating conditions.
How does dew point relate to cooling coil design?
A cooling coil removes latent heat by operating its surface temperature — the apparatus dew point (ADP) — below the entering air's dew point. Moisture condenses on the coil fins when the fin surface falls below the dew point of the passing air. If the ADP is above the entering air's dew point, the coil performs sensible cooling only — it cannot remove moisture.
Why does dew point stay the same when air is heated?
Heating air without adding moisture is a sensible-only process — it raises dry-bulb temperature but does not change the mass of water vapor present per unit mass of dry air. Since dew point depends only on vapor pressure, which depends only on humidity ratio and atmospheric pressure, and neither changes during sensible heating, the dew point is unchanged.
What dew point should I design to for a comfortable occupied space?
ASHRAE Standard 55 identifies a dew point of approximately 62°F (17°C) as the upper comfort limit. For a typical occupied office, design dew points of 50–60°F (10–15°C) are common, corresponding to approximately 40–55% RH at 75°F (24°C). The supply air from a properly sized cooling coil typically has a dew point of 50–54°F (10–12°C).
Can I use this calculator for outdoor air dew point analysis?
Yes. This calculator accepts any two independent psychrometric inputs and is equally applicable to outdoor air conditions. Use Combination A (dry-bulb + RH) if you have percentage humidity data, or Combination B (dry-bulb + wet-bulb) if you have sling psychrometer or weather station wet-bulb data.
Is this calculator valid below freezing temperatures?
Yes, with a note about frost. At temperatures below 32°F (0°C), water condenses as ice (frost) rather than liquid water. The Magnus approximation used here is accurate down to −40°F (−40°C). Below 32°F, the calculated dew point is valid as the frost formation threshold, but it differs slightly from the thermodynamic frost point (typically 1–3°F higher).

Frequently Used Together

Engineers often use these calculators in combination for complete project workflows:

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Humidification Load Calculator

Dehumidifier Sizing For Pools

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