Work out the right BTU rating for any room with a free calculator. Learn the formula, the adjustments that matter, and why an oversized AC is a costly mistake.
BTU (British Thermal Unit) measures how much heat an air conditioner removes per hour. The baseline rule is roughly 20 BTU per square foot (about 215 BTU per square metre), adjusted for ceiling height, sunlight, occupancy, and what the room is used for. Getting it wrong in either direction causes real problems and most people only know about one of them. Try the free BTU Calculator → Why Oversizing Is the Bigger Mistake Undersized AC is obvious: it runs constantly and never quite cools the room. Oversized AC fails in a way that's harder to diagnose. It cools the air to the target temperature very fast, hits the thermostat setpoint, and shuts off before it has run long enough to pull moisture out of the air. The result is a room that's cold and clammy rather than comfortable. That short-cycling pattern also means: Higher energy bills , because startup draws far more power than steady running Faster wear on the compressor from constant stopping and starting Uneven temperatures , since the unit never runs long enough to circulate air properly Humidity problems , which in bad cases means condensation and mould Bigger is not safer. Correct is safer. The Base Formula Base BTU = Room area (sq ft) × 20 Or in metric: room area (m²) × 215 . Room size Base BTU 15 m² (~150 sq ft) ~3,200 25 m² (~270 sq ft) ~5,400 35 m² (~375 sq ft) ~7,500 50 m² (~540 sq ft) ~10,800 75 m² (~800 sq ft) ~16,000 That's the starting point, not the answer. The adjustments below often move the final figure by 30% or more. The Adjustments That Actually Matter Ceiling height. The base formula assumes standard ceilings of about 2.4 m (8 ft). You're cooling volume, not floor area for higher ceilings, scale up proportionally. A room with 3.5 m ceilings needs roughly 45% more capacity than the floor area alone suggests. Sun exposure. A room with large south- or west-facing windows takes on substantial heat through the afternoon. Add about 10% for a heavily sunlit room. A consistently shaded room can subtract about 10% . Occupancy. The formula assumes two people. Add roughly 600 BTU for each additional regular occupant. This matters most for offices and living rooms, not bedrooms. Kitchens. Cooking generates serious heat. Add about 4,000 BTU for a kitchen. Top-floor rooms. Heat rises, and roofs absorb sun all day. Add capacity for rooms directly under an uninsulated roof. Insulation and windows. Poor insulation, single-glazed windows, and draughty older buildings all increase the load. Good insulation and modern double glazing reduce it. Climate. The 20 BTU/sq ft baseline suits temperate conditions. Persistently hot, humid climates need more; mild ones need less. Worked Example A 30 m² top-floor living room, 2.8 m ceilings, large west-facing windows, four regular occupants: Base: 30 × 215 = 6,450 BTU Ceiling height (2.8 m vs 2.4 m standard): × 1.17 → 7,550 Heavy afternoon sun: +10% → 8,300 Two extra occupants beyond the assumed two: +1,200 → 9,500 Top floor allowance: +10% → ~10,450 BTU The base formula alone would have suggested 6,450 around 40% short of what the room actually needs. BTU, Tons, and Kilowatts Air conditioner capacity gets quoted in different units depending on where you are and what type of unit it is: Unit Equivalent 1 ton of cooling 12,000 BTU/hr 1 kW of cooling ~3,412 BTU/hr 12,000 BTU ~3.5 kW Important distinction: cooling capacity in kW is not the same as electrical consumption in kW. A unit rated at 3.5 kW of cooling might draw around 1 kW of electricity — the ratio between them is the efficiency rating (SEER, EER, or SCOP depending on the standard). When comparing running costs, compare efficiency ratings, not capacity. For converting between units, the Unit Converter handles energy and power conversions directly. When to Get a Professional Load Calculation The calculator above is a solid estimate for a single room with a portable or split unit. For whole-house systems, ducted installations, or unusual buildings, a professional performs a full load calculation (Manual J in the US, or the local equivalent), which accounts for wall construction, duct losses, air infiltration rates, and orientation in far more detail. Rule of thumb: single room, plug-in or split unit → the calculator is fine. Whole house, ducted, or a significant investment → pay for the proper calculation. Frequently Asked Questions How many BTU do I need per square metre? Roughly 215 BTU per square metre (20 BTU per square foot) as a baseline, before adjusting for ceiling height, sun exposure, occupancy, insulation, and room type. Is it better to buy a bigger air conditioner than I need? No. An oversized unit cools quickly then shuts off before removing humidity, leaving the room cold and damp. It also short-cycles, which raises energy costs and shortens the compressor's life. Correct sizing outperforms oversizing. How many BTU is a 1-ton air conditioner? One ton of cooling equals 12,000 BTU per hour. Do I need more BTU for a kitchen? Yes add roughly 4,000 BTU