Free HVAC Sizing Tool

AC Size Calculator — Cooling Load in BTU, Tons & kW for a Room

A quick, transparent estimate of how much cooling a space needs — the number behind "what size AC / fan-coil / rooftop unit". Enter the room, climate and internal loads and get BTU/h, kW, tons and the supply airflow the air handler or fan-coil must move. For a code-compliant design use Manual J / ASHRAE; this gets you to the right equipment class in a minute.

No signup · Works on mobile · Formulas shown below · Updated 2026-08-23

Cooling Load Estimator

Base load per m² by climate × adjustments + people + equipment + lighting. Supply airflow at the ΔT you choose.

Quick preset

Floor area

Area unit

Ceiling height

Climate

Sun exposure / glazing

Insulation / building

Occupants

≈ 100 W sensible + 50 W latent each (seated); ×2 for active.

Equipment & appliance heat

PC 100–200, TV 100, fridge 100–200, server rack = its power, kitchen spill 20–40 % of appliance rating.

Lighting

W/m²

LED 5–10, fluorescent 10–15, halogen 25+

Supply air ΔT (for airflow)

Fan-coil / AHU 8–12 K; high-induction diffusers 12–16 K.

Results

Engineering estimate for preliminary sizing. Verify against the fan curve, local codes and your system measurements before purchase. Ask a Longwell engineer to check your duty point free of charge.

How the estimate works

Envelope load = base W/m² (climate) × sun factor × insulation factor × height factor × floor area
Internal gains = people × 150 W + equipment W + lighting W/m² × area
Total = envelope + internal // latent ≈ 15 % of envelope + 50 W per person
Supply airflow Q = sensible load ÷ (ρ × c_p × ΔT) // ≈ 3 × W ÷ ΔT in m³/h

Climate	Base load	Typical total with occupants
Cool / mild	55 W/m²	60–90 W/m² (20–30 BTU/h·ft²)
Temperate	75 W/m²	80–120 W/m² (25–40)
Warm	100 W/m²	110–150 W/m² (35–50)
Hot / tropical	130 W/m²	140–200 W/m² (45–65)

Adjustments: sun 0.75–1.25, insulation 0.85–1.2, +8 % per extra 0.1 m of ceiling height above 2.7 m. Occupants 150 W total (100 sensible + 50 latent) seated, double for gyms and dance floors. Equipment: take nameplate for IT and resistive loads, 20–40 % of rating for kitchen appliances under hoods.

From load to equipment and fans

Load	Typical equipment	Supply airflow	Fan type inside
< 3.5 kW (12k BTU)	Wall split, window AC	400–600 m³/h	Cross-flow (indoor), axial (outdoor)
3.5–7 kW (1–2 ton)	Split, cassette, fan-coil	600–1,400 m³/h	Forward-curved / cross-flow, axial condenser
7–18 kW (2–5 ton)	Ducted split, fan-coils, small AHU	1,400–3,500 m³/h	Forward-curved or EC backward-curved plug fan
18–100 kW (5–30 ton)	Rooftop unit, AHU + chiller/heat pump	3,500–20,000 m³/h	EC plug fans, fan walls, axial condenser fans
> 100 kW	Central AHUs, chillers, CRAH	> 20,000 m³/h	Fan arrays of EC plug fans

Frequently Asked Questions

How many BTU do I need per square foot?

Roughly 20 BTU/h per ft² in a temperate climate, 25–30 in warm and 35–45 in hot climates, before adding people and equipment. In metric: 65–140 W/m². A 300 ft² bedroom therefore needs about 6,000–9,000 BTU/h.

How do I calculate AC size in kW for a room?

Multiply the floor area by 75–130 W/m² depending on climate, adjust for sun and insulation, then add 150 W per person and the wattage of equipment. A 20 m² living room with 3 people and a TV in a warm climate: 20 × 100 × 1.1 + 450 + 150 ≈ 2.8 kW → a 3.5 kW (12,000 BTU/h) unit.

What is the airflow for a given cooling load?

About 400 CFM per ton (190 m³/h per kW) at a standard 11 K (20 °F) supply temperature difference; or calculate it exactly as sensible load ÷ (1.2 × 1006 × ΔT). A 7 kW room load needs roughly 1,300–1,500 m³/h from the fan-coil or duct system.

Is it better to oversize an air conditioner?

No. Oversized units cycle on and off, fail to dehumidify and wear faster. Size to the calculated load plus at most 15–25 %, and use inverter/EC equipment that modulates down rather than a bigger fixed-speed unit.

Does this replace a Manual J calculation?

No — it is a screening estimate for equipment class and fan sizing. Code submissions, large buildings and unusual loads (kitchens, pools, data rooms) need a full ASHRAE / Manual J / Manual N load calculation from a licensed designer.

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<a href="https://www.longwellfans.com/resources/btu-cooling-load-calculator/">AC Size & Cooling Load Calculator (BTU / Tons / kW)</a> by LONGWELL Fans

APA: LONGWELL Fans. (2026). AC Size & Cooling Load Calculator (BTU / Tons / kW). https://www.longwellfans.com/resources/btu-cooling-load-calculator/

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