Enthalpy Calculator

Overview

How to Use This Calculator

1. Select operating mode — choose from Single State Point, Two State Points.

2. Enter dry-bulb temperature — in °C or °F.

3. Select moisture input type — choose from RH, humidity ratio, wet-bulb, or dew point.

4. Enter relative humidity — in %.

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

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

7. Enter dew point temperature — in °C or °F.

8. Enter state point 2 — dry-bulb temperature — in °C or °F.

9. Select state point 2 — moisture input type — choose from Relative Humidity, Humidity Ratio, Wet-Bulb Temperature.

10. Enter state point 2 — relative humidity — in %.

11. Enter state point 2 — humidity ratio — in g/kg or gr/lb.

12. Enter state point 2 — wet-bulb temperature — in °C or °F.

13. Enter state point 2 — dew point temperature — in °C or °F.

14. Enter airflow (optional) — in m³/h or CFM.

15. Click "Calculate" — get specific enthalpy, sensible and latent components, SHR, and (in two-point mode) enthalpy difference and coil load.

Use Δh × airflow for coil load, and compare SHR against equipment-rated SHR at entering conditions.

Inputs & Outputs

Formula

Metric:   P_sat = 0.61078 × exp(17.625 × T / (243.04 + T))   [kPa]
Imperial: P_sat = 0.1450377 × exp(17.625 × Tc / (243.04 + Tc)) [psi]
          where Tc = (T_°F − 32) / 1.8

P_v = (RH / 100) × P_sat

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

h = 0.240 × T_db + (W / 7000) × (1061 + 0.444 × T_db)

h = 1.006 × T_db + (W / 1000) × (2501 + 1.86 × T_db)

SHR = h_s / h × 100   [%]

Δh = h₁ − h₂
Δh_s = 0.240 × (T_db1 − T_db2)   [Imperial]
Δh_s = 1.006 × (T_db1 − T_db2)   [Metric]
Δh_l = h_l1 − h_l2
SHR = Δh_s / Δh × 100   [%]

Q = 4.5 × CFM × Δh   [BTU/h]
Q_s = 1.1 × CFM × ΔT_db   [BTU/h]
Q_l = Q − Q_s   [BTU/h]

Q = (m³/h × Δh × 1.2) / 3600   [kW]
Q_s = (m³/h × 1.2 × 1.006 × ΔT_db) / 3600   [kW]
Q_l = Q − Q_s   [kW]

Key Facts

• The enthalpy formula h = 0.240 × T_db + (W / 7000) × (1061 + 0.444 × T_db) requires W in gr/lb to be divided by 7,000 to convert to lb/lb before the latent term is applied. Omitting this division produces values 7,000 times too large — the most common manual calculation error with Imperial enthalpy.
• Enthalpy is referenced to 0°F (Imperial) or 0°C (Metric) for dry air with zero moisture content. At 0°C and W = 0, h = 0 kJ/kg by definition. Negative enthalpy values are physically valid and occur for cold dry air below these reference temperatures.
• The coil load formula Q = 4.5 × CFM × Δh (Imperial) assumes standard air density of 0.075 lb/ft³. At 5,000 ft elevation, actual density is approximately 0.062 lb/ft³ — the correct factor becomes approximately 3.72 instead of 4.5.
• Sensible heat ratio (SHR) is the ratio of sensible load to total load. Cooling equipment is rated at specific SHR conditions — typically 0.75–0.80 for standard commercial equipment.
• The latent heat of vaporization at 0°C is 2,501 kJ/kg (1,061 BTU/lb). It decreases with increasing temperature — the ASHRAE enthalpy formula captures this variation through the temperature-dependent term.
• Enthalpy difference is the fundamental driver of energy recovery ventilation performance. An ERV with 70% total effectiveness recovers 70% of the enthalpy difference between outdoor and exhaust air — a much larger energy quantity than sensible recovery alone.

Applications

• Cooling coil load calculation — total coil load in BTU/h equals 4.5 × CFM × Δh (Imperial) or (m³/h × Δh × 1.2) / 3600 in kW (Metric).
• Cooling coil selection — SHR must match the load to the equipment at actual entering conditions.
• Energy recovery ventilation sizing — total ERV load = airflow × Δh × effectiveness.
• Economizer control logic — differential enthalpy economizer control compares outdoor air enthalpy to return air enthalpy.
• Psychrometric process line plotting — enthalpy at each process endpoint defines the process on a psychrometric chart.
• Equipment performance verification during commissioning — measured Δh × airflow compared against design load.

Standards & References

• ASHRAE Handbook — Fundamentals (2021), Chapter 1: Psychrometrics — Primary reference for the enthalpy formulations used on this page
• ASHRAE Handbook — Fundamentals (2021), Chapter 18 — Defines the coil load formula Q = 4.5 × CFM × Δh based on standard air density
• AHRI Standard 210/240-2023 — Performance rating conditions at which manufacturer SHR data is published
• AHRI Standard 550/590-2023 — Chilled water coil rating conditions for two-state-point enthalpy calculations
• ASHRAE Standard 90.1-2022 — Energy recovery ventilation thresholds based on outdoor air enthalpy
• Magnus Formula (Alduchov-Eskridge, 1996) — Saturation vapor pressure approximation used for humidity ratio derivation

Common Mistakes to Avoid

• Entering W in gr/lb without applying the 7,000 divisor in the Imperial enthalpy formula. The correct calculation is h = 0.240 × T_db + (W / 7000) × (1061 + 0.444 × T_db). Without dividing W by 7,000, the latent term is 7,000 times too large.
• Using total supply airflow instead of outdoor airflow for enthalpy-based ventilation energy calculations. The enthalpy of mixed air (return plus outdoor) is the correct entering condition for a coil load calculation.
• Confusing SHR at design conditions with SHR at rated conditions. Manufacturers publish equipment SHR at standard ARI/AHRI test conditions. If design entering conditions differ significantly, the equipment SHR at actual conditions must be obtained from the manufacturer's performance data.
• Treating Δh as always positive. In a heating process, h_leaving > h_entering, so Δh = h1 − h2 is negative. Always confirm which state point is entering and which is leaving before interpreting the sign of Δh.
• Using the 4.5 multiplier for non-standard density conditions. The factor 4.5 = 60 min/h × 0.075 lb/ft³ is valid at sea-level standard conditions only. At high altitude, the actual density must be calculated and substituted.
• Forgetting that enthalpy is referenced to dry air mass, not moist air mass. When converting between mass flow and volumetric flow, specific volume (ft³/lb dry air) must be used correctly.

Frequently Asked Questions