Office Open Plan Cooling Load Calculator

Calculate

Net usable floor area of the open-plan office zone

Floor-to-ceiling height of the office space

Total number of people expected in the open-plan area at peak occupancy

Summer design dry-bulb temperature for your location

Target indoor dry-bulb temperature

Approximate ratio of window area to total exterior wall area

Installed lighting power per unit floor area

Heat output from computers, monitors, printers, and other office equipment per unit floor area

ASHRAE 62.1 typical: 17 CFM (8.5 L/s) per person for offices. Adjust for local codes.

Overview

The Office Open Plan Cooling Load Calculator estimates the total cooling capacity required to maintain comfortable indoor conditions in an open-plan office environment. Open offices present unique HVAC challenges: high occupant density, concentrated equipment loads from workstations, large glazing areas, and significant ventilation requirements under ASHRAE 62.1.

This calculator breaks the total cooling load into five components — envelope (walls, roof, and windows), occupants (sensible + latent), lighting, equipment, and ventilation (outside air) — then sums them to produce a total cooling load in watts, kilowatts, BTU/hr, and tons of refrigeration.

Use this tool for early-stage HVAC sizing, feasibility studies, and quick cross-checks against detailed load calculations. Enter your office parameters and the calculator returns the estimated cooling load with a classification indicating whether the result falls in a typical, high, or very high range for open-plan offices.

How to Use This Calculator

  1. Enter floor area — in m² or ft².

  2. Enter ceiling height — in m or ft.

  3. Enter number of occupants.

  4. Enter outdoor design temperature — in °C or °F.

  5. Enter indoor design temperature — in °C or °F.

  6. Select window-to-wall ratio — choose from 20% — Minimal glazing, 40% — Moderate glazing, 60% — Extensive glazing, 80% — Curtain wall / full glass.

  7. Select lighting power density — choose from 8 W/m² — LED, energy-efficient, 12 W/m² — Standard office, 16 W/m² — Older fluorescent systems, 20 W/m² — High-intensity or legacy lighting.

  8. Select equipment power density — choose from 10 W/m² — Light use, 15 W/m² — Standard office, 25 W/m² — Dense workstations, 40 W/m² — High-density (trading floors, labs).

  9. Enter outside air per person — in L/s or CFM.

  10. Click "Calculate" — get envelope, occupant, lighting, equipment, and ventilation loads, plus total cooling load and load intensity.

Compare the load intensity (W/m²) against the typical 80–150 W/m² office range; apply a 10–20% margin and an equipment diversity factor before sizing.

Inputs & Outputs

Inputs

  • •Floor Area (m² / ft²)
  • •Ceiling Height (m / ft)
  • •Number of Occupants
  • •Outdoor Design Temperature (°C / °F)
  • •Indoor Design Temperature (°C / °F)
  • •Window-to-Wall Ratio — Options: 20% — Minimal glazing, 40% — Moderate glazing, 60% — Extensive glazing, 80% — Curtain wall / full glass
  • •Lighting Power Density — Options: 8 W/m² — LED, energy-efficient, 12 W/m² — Standard office (ASHRAE 90.1), 16 W/m² — Older fluorescent systems, 20 W/m² — High-intensity or legacy lighting
  • •Equipment Power Density — Options: 10 W/m² — Light use (few devices), 15 W/m² — Standard office (monitors, laptops), 25 W/m² — Dense workstations (dual monitors, desktops), 40 W/m² — High-density (trading floors, labs)
  • •Outside Air per Person (L/s / CFM)

Outputs

  • •Envelope Load (Walls + Windows) (W / BTU/hr)
  • •Occupant Load (W / BTU/hr)
  • •Lighting Load (W / BTU/hr)
  • •Equipment Load (W / BTU/hr)
  • •Ventilation Load (W / BTU/hr)
  • •Total Cooling Load (W / BTU/hr)
  • •Cooling Load Intensity (W/m² / BTU/hr·ft²)

Formula

Calculator Formula

This calculator estimates the total cooling load for an open-plan office by summing five independent heat-gain components.

Step 1: Temperature Difference

ΔT = |T_outdoor − T_indoor|

Step 2: Envelope Load (Walls + Windows)

The calculator estimates the exterior wall area from the floor area using a square-plan assumption:

Perimeter = 4 × √(Floor Area)
Wall Area = Perimeter × Ceiling Height
Window Area = Wall Area × WWR
Opaque Area = Wall Area − Window Area

Heat gain through the envelope:

Q_envelope = (Opaque Area × U_wall × ΔT)
           + (Window Area × U_window × ΔT)
           + (Window Area × SHGC × Solar Irradiance)

Default assumptions:

  • U_wall (opaque) = 0.5 W/m²·K
  • U_window = 3.0 W/m²·K
  • SHGC = 0.4
  • Solar irradiance = 300 W/m² (average peak for design)

Step 3: Occupant Load

Q_occupants = Number of Occupants × 130 W/person

130 W per person includes both sensible (~75 W) and latent (~55 W) heat for seated office work (ASHRAE Fundamentals, Table 1, Chapter 18).

Step 4: Lighting Load

Q_lighting = Lighting Power Density × Floor Area

Step 5: Equipment Load

Q_equipment = Equipment Power Density × Floor Area

Step 6: Ventilation Load

Q_ventilation = Occupants × OA_rate × ρ × cp × ΔT

Where:

  • OA_rate = outside air per person (L/s, converted to m³/s internally)
  • ρ = 1.2 kg/m³ (standard air density)
  • cp = 1.006 kJ/kg·K (specific heat of air)

Step 7: Total Cooling Load

Q_total = Q_envelope + Q_occupants + Q_lighting + Q_equipment + Q_ventilation

Conversions:

  • kW = Q_total / 1000
  • TR = Q_total / 3517
  • Load Intensity = Q_total / Floor Area (W/m²)

What is Office Open Plan Cooling Load

Office open plan cooling load is the total amount of heat that must be removed from an open-plan office space to maintain comfortable indoor conditions. Unlike residential spaces, open offices combine high occupant density, concentrated equipment loads, large glazing areas, and significant ventilation requirements — all of which contribute to a substantially higher cooling demand per unit area.

Heat Gain Components

The total cooling load in an open-plan office is the sum of five main components:

  1. Envelope Load — heat transfer through walls and windows due to the indoor-outdoor temperature difference, plus solar heat gain through glazing
  2. Occupant Load — metabolic heat from people, including both sensible (temperature-raising) and latent (moisture-adding) components
  3. Lighting Load — heat output from all lighting fixtures in the space
  4. Equipment Load — heat from computers, monitors, printers, copiers, and other office devices
  5. Ventilation Load — the energy required to cool outside air brought in for ventilation

Why Open Offices Are Different

Open-plan offices typically require 80–150 W/m² of cooling capacity, compared to 50–80 W/m² for residential spaces. The key differences are:

  • Higher occupant density — open offices may have 8–15 m² per person, compared to 30+ m² in residential
  • More equipment — each workstation generates 100–300 W of heat
  • More ventilation — ASHRAE 62.1 requires significant outside air for occupied offices
  • Large glazing — modern office buildings often have 40–80% window-to-wall ratios

Engineering Applications

This calculator is used for:

  • Early-stage HVAC sizing — determining approximate chiller, AHU, and ductwork capacity before detailed design
  • Feasibility studies — evaluating whether existing HVAC infrastructure can support a new office fit-out
  • Design option comparison — quantifying the cooling load impact of different glazing, lighting, or occupancy scenarios
  • Energy benchmarking — comparing calculated load intensity against industry benchmarks (ASHRAE, CIBSE)
  • Quick cross-checks — validating detailed load calculations against simplified estimates

Practical Tips

  • Start with internal gains — in open offices, occupants + equipment + lighting often dominate the total load
  • Don't ignore ventilation — at high occupant density, ventilation load can be 20–40% of total
  • Consider diversity — not all equipment runs at full load simultaneously; apply 0.7–0.9 diversity factor for equipment
  • Check load intensity — if your result exceeds 150 W/m², verify inputs; if below 60 W/m², check that all loads are included
  • Add safety margin — 10–15% is typical for screening estimates; avoid excessive oversizing
  • Use detailed software for final sizing — this calculator is for screening, not final equipment selection

Key Facts

  • Open-plan offices typically require 80–150 W/m² of cooling capacity, depending on occupant density and equipment loads.
  • Occupant heat gain in offices is approximately 130 W per person (sensible + latent) for seated work.
  • Lighting power density in modern LED offices can be as low as 8 W/m², compared to 16–20 W/m² for older fluorescent systems.
  • Ventilation load can account for 20–40% of total cooling load in densely occupied open offices.
  • High window-to-wall ratios significantly increase solar heat gain, often making envelope load the dominant component.
  • ASHRAE 62.1 requires approximately 8.5 L/s (17 CFM) of outside air per person for office spaces.
  • Equipment power density in modern offices ranges from 10 W/m² (light use) to 40+ W/m² (trading floors).

Applications

  • Early-stage HVAC system sizing for new office fit-outs
  • Feasibility studies for open-plan office renovations
  • Quick cross-checks against detailed load calculations
  • Comparing cooling load impact of different glazing options
  • Evaluating the effect of occupant density on HVAC capacity
  • Screening equipment and lighting upgrades for energy savings
  • Preliminary chiller and AHU sizing for office buildings

Example Calculation

Metric Example

Given:

  • Floor Area = 500 m²
  • Ceiling Height = 2.7 m
  • Occupants = 50
  • Outdoor Temperature = 35°C
  • Indoor Temperature = 24°C
  • Window-to-Wall Ratio = 40%
  • Lighting Power Density = 12 W/m²
  • Equipment Power Density = 15 W/m²
  • Outside Air per Person = 10 L/s

Step 1: ΔT = |35 − 24| = 11°C

Step 2: Envelope Load

Perimeter = 4 × √500 = 89.44 m
Wall Area = 89.44 × 2.7 = 241.5 m²
Window Area = 241.5 × 0.4 = 96.6 m²
Opaque Area = 241.5 − 96.6 = 144.9 m²

Q_envelope = (144.9 × 0.5 × 11) + (96.6 × 3.0 × 11) + (96.6 × 0.4 × 300)
           = 796.9 + 3,187.8 + 11,592.0
           = 15,576.7 W

Step 3: Occupant Load

Q_occupants = 50 × 130 = 6,500 W

Step 4: Lighting Load

Q_lighting = 12 × 500 = 6,000 W

Step 5: Equipment Load

Q_equipment = 15 × 500 = 7,500 W

Step 6: Ventilation Load

Q_ventilation = 50 × 0.010 × 1.2 × 1006 × 11 = 6,636 W

(Note: 10 L/s = 0.010 m³/s; cp = 1.006 kJ/kg·K = 1006 J/kg·K)

Step 7: Total Cooling Load

Q_total = 15,577 + 6,500 + 6,000 + 7,500 + 6,637 = 42,214 W ≈ 42.2 kW ≈ 12.0 TR
Load Intensity = 42,214 / 500 = 84.4 W/m²

Imperial Example

Note: This example uses different inputs than the Metric example above (5,000 ft² vs 5,382 ft²; 20 CFM vs 17 CFM per person); results are not directly comparable.

Given:

  • Floor Area = 5,000 ft² (≈ 464.5 m²)
  • Ceiling Height = 9 ft (≈ 2.74 m)
  • Occupants = 50
  • Outdoor Temperature = 95°F
  • Indoor Temperature = 75°F
  • Window-to-Wall Ratio = 40%
  • Lighting Power Density = 12 W/m²
  • Equipment Power Density = 15 W/m²
  • Outside Air per Person = 20 CFM (≈ 9.44 L/s)

Step 1: ΔT = |95 − 75| = 20°F (= 11.1°C for internal calculation)

Step 2: Envelope Load

Perimeter = 4 × √464.5 ≈ 86.2 m
Wall Area = 86.2 × 2.74 = 236.4 m²
Window Area = 236.4 × 0.4 = 94.6 m²
Opaque Area = 236.4 − 94.6 = 141.8 m²

Q_envelope = (141.8 × 0.5 × 11.1)
           + (94.6 × 3.0 × 11.1)
           + (94.6 × 0.4 × 300)
           ≈ 787 + 3,149 + 11,352
           ≈ 15,288 W ≈ 52,162 BTU/hr

Step 3: Occupant Load

Q_occupants = 50 × 130 = 6,500 W = 22,178 BTU/hr

Step 4: Lighting Load

Q_lighting = 12 × 464.5 ≈ 5,574 W ≈ 19,018 BTU/hr

Step 5: Equipment Load

Q_equipment = 15 × 464.5 ≈ 6,968 W ≈ 23,775 BTU/hr

Step 6: Ventilation Load

OA = 20 CFM × 0.000472 = 0.00944 m³/s per person
Q_ventilation = 50 × 0.00944 × 1.2 × 1006 × 11.1 ≈ 6,317 W ≈ 21,553 BTU/hr

Step 7: Total Cooling Load

Q_total = 15,288 + 6,500 + 5,574 + 6,968 + 6,317 ≈ 40,647 W
        ≈ 138,688 BTU/hr ≈ 40.6 kW ≈ 11.6 TR
Load Intensity ≈ 40,647 / 464.5 ≈ 87.5 W/m² ≈ 27.7 BTU/hr·ft²

Standards & References

  • ASHRAE Handbook — Fundamentals — Ch. 18 (Nonresidential Cooling and Heating Load Calculations)
  • ASHRAE 62.1 — Ventilation for Acceptable Indoor Air Quality (office ventilation rates)
  • ASHRAE 90.1 — Energy Standard for Buildings (lighting and envelope requirements)
  • CIBSE Guide A — Environmental Design (UK/international office cooling loads)
  • ACCA Manual N — Commercial Load Calculation

Limitations

  • This calculator provides a simplified screening estimate, not a detailed hour-by-hour load analysis.
  • Envelope geometry is approximated as a square plan — actual building shapes will differ.
  • Fixed assumptions are used for wall U-value (0.5 W/m²·K), window U-value (3.0 W/m²·K), SHGC (0.4), and solar irradiance (300 W/m²).
  • Solar heat gain is estimated using a single average irradiance value — actual gains vary by orientation, shading, and time of day.
  • Latent ventilation load is not separated — only sensible ventilation load is calculated.
  • No safety factor or diversity factor is applied — actual equipment sizing typically adds 10–20% margin.
  • For final equipment selection, use detailed ASHRAE load calculation software with hourly weather data.

Common Mistakes to Avoid

  • Using residential cooling rules of thumb (e.g. 20 BTU/ft²) for commercial open offices, which have much higher internal gains.
  • Ignoring ventilation load — outside air can add 20–40% to the total cooling load in densely occupied spaces.
  • Underestimating equipment heat gain by not accounting for all workstation devices (monitors, docking stations, chargers).
  • Applying the same insulation assumptions to curtain-wall buildings as to traditional masonry construction.
  • Forgetting that occupant heat includes both sensible and latent components — using only 75 W/person misses the latent portion.
  • Not adjusting outside air rates for local code requirements that may exceed ASHRAE 62.1 minimums.

Frequently Asked Questions

What is a typical cooling load for an open-plan office?
Open-plan offices typically require 80–150 W/m² (25–48 BTU/hr·ft²) of cooling capacity. The wide range reflects differences in occupant density, equipment loads, glazing, and climate. High-density offices with extensive glazing and many workstations tend toward the upper end.
How many watts per person should I assume for office occupants?
ASHRAE Fundamentals recommends approximately 130 W per person for seated office work, which includes both sensible heat (~75 W) and latent heat (~55 W). This value covers metabolic heat generation for typical light office activity.
What is a reasonable equipment power density for offices?
Standard offices with laptops and single monitors typically range from 10–15 W/m². Dense workstation environments with desktop computers and dual monitors can reach 25 W/m². High-intensity spaces like trading floors or server-adjacent areas may exceed 40 W/m².
How does window-to-wall ratio affect cooling load?
Higher window-to-wall ratios increase both conductive heat gain (through the glass U-value) and solar heat gain (through SHGC). A curtain-wall building at 80% WWR can have 2–3 times the envelope cooling load of a building with 20% WWR, making glazing selection and shading critical design decisions.
What ventilation rate should I use for offices?
ASHRAE 62.1 specifies a combined rate of approximately 8.5 L/s (17 CFM) per person for office spaces, consisting of a per-person component (2.5 L/s) and a per-area component (0.06 L/s per m²). Local codes may require higher rates. Always check the applicable ventilation standard for your jurisdiction.
Why is ventilation load significant in open offices?
Open offices have high occupant density, which means more outside air must be conditioned. In hot climates with a large indoor-outdoor temperature difference, ventilation can account for 20–40% of the total cooling load. This is why dedicated outdoor air systems (DOAS) are popular for high-occupancy spaces.
How accurate is this calculator compared to detailed load software?
This calculator provides a screening estimate typically within ±20–30% of detailed calculations for standard office conditions. It uses simplified geometry, fixed U-values, and average solar assumptions. For final equipment sizing, always use detailed ASHRAE-compliant load calculation software with hourly weather data and actual building geometry.
Should I add a safety factor to the result?
This calculator does not include a safety factor. In practice, engineers typically add 10–20% margin to account for uncertainties, future load growth, and part-load performance. However, excessive oversizing leads to higher costs and poor part-load efficiency, so the margin should be justified.

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