Cooling Load Calculator (kW, Tons, BTU)

❄️ Cooling Load Calculator

Estimated AC size in kW, tons and BTU/hr for a room.

Floor area (m²)

Area heat gain (W/m²)

Occupants

Heat gain per person (W)

Equipment / lighting (W)

Cooling Load

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Refrigeration Tons

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BTU / hr

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Suggested Unit

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⚠️ Rule-of-thumb sensible estimate (area gain + people + equipment). For an accurate design use ASHRAE Manual J / CIBSE with solar orientation, glazing, fabric U-values and ventilation. Verify before professional use.

Standards & method

✓ Independently verified 12 July 2026

Governing standard

ASHRAE Fundamentals — Cooling and Heating Load Calculations

Clauses applied

ASHRAE Fundamentals Ch. 18 · sensible and latent load components · CLTD / RTS method for envelope gains · internal gains from people, lighting and equipment

Core formula

Q_sensible = U·A·ΔT + solar + internal · Q_total = Q_sensible + Q_latent

Why this matters

Sizing on floor area alone (the "one ton per 500 ft²" rule) ignores orientation, glazing, insulation and internal gains. And an oversized unit is worse than a slightly undersized one — it short-cycles and never runs long enough to dehumidify.

Independently verified

12 July 2026 — Re-derived from the governing standard and checked numerically against worked reference cases from the standard itself — not merely tested for “returns a number”.

Results are for guidance. Verify against the current edition of the governing standard and have a qualified professional review before use in practice.

Cooling Load — Method

A first-pass cooling load sums the dominant heat gains in a room: the building fabric, glazing and infiltration (captured by an area gain in W/m2), plus the heat from people (ASHRAE Fundamentals gives about 115-130 W total per person - sensible plus latent - for seated or light office work, so this tool defaults to 120 W and lets you change it) and from equipment and lighting. Q = area x area-gain + occupants x W-per-person + equipment. Divide watts by 3517 for refrigeration tons or multiply by 3.412 for BTU/hr. This rule-of-thumb is fine for picking a unit size, but a real design uses ASHRAE Manual J or CIBSE methods with orientation, glazing, U-values and ventilation.

Frequently Asked Questions

How do I calculate the cooling load for a room?⌄

Add the heat gains: floor area x area gain (about 120-150 W per m2) + occupants x 120 W (ASHRAE: ~115-130 W per person) + equipment watts. Divide the total watts by 3517 for tons, or multiply by 3.412 for BTU/hr.

How many BTU do I need per square metre?⌄

As a rough guide, 400-600 BTU/hr per m2 for typical rooms, more for kitchens, sunny rooms or high occupancy. This calculator builds it up from area, people and equipment instead of a flat figure.

How do I convert kW to tons of refrigeration?⌄

One refrigeration ton equals 3.517 kW (12,000 BTU/hr). Divide the cooling load in watts by 3517 to get tons.

Is a rule-of-thumb good enough?⌄

It is fine for selecting a unit size. For load that must be right (large glazing, high solar gain, tight efficiency targets) use a full Manual J or CIBSE calculation with fabric and solar data.

Should I oversize the air conditioner?⌄

Avoid large oversizing - an oversized unit short-cycles, controls humidity poorly and wastes energy. Pick the next standard size above the calculated load, not several sizes up.

HVAC Cooling Load Explained

The cooling load is the rate of heat the air conditioner must remove to hold the room at setpoint. It comes from the building fabric and glazing, outside air leaking in, the people inside, and electrical equipment and lighting.

The quick estimate

Bundle fabric, glazing and infiltration into an area gain (W/m2), add sensible heat from people and equipment, and you have the sensible cooling load. Convert to tons or BTU/hr to match catalogue units.

When to go further

For accuracy, a Manual J or CIBSE calculation separates solar gain by orientation, glazing type and shading, fabric U-values and fresh-air ventilation, and adds latent (moisture) load. Pair with the HVAC duct sizing and heat exchanger tools.

Related: HVAC Duct Sizing, Heat Exchanger.

Cooling Load Calculator

Cooling load — Quick answer