BTU Calculator
Size air conditioners and heat pumps by room area, climate, sun exposure, insulation quality, occupants, and kitchen heat load. Outputs BTU/h + tons + kW.
BTU Calculator
How to use the BTU Calculator
Measure the room
Enter length and width of the room you want to cool. For open-plan kitchen-dining-living combinations, measure the combined area as if it's one space — air conditioning doesn't respect arbitrary walls. For partial-wall layouts (kitchen with island opening to dining), add 20% to the combined area to account for adjacent space pulling cooled air. Standard residential ceiling heights are 8 ft (2.5 m); if higher, select "High" from the ceiling dropdown.
Pick the right climate
Three climate tiers drive base BTU loading: Temperate (20 BTU/ft²) — most of the US, EU, northern China, northern Japan. Tropical (25 BTU/ft²) — most of ASEAN (Singapore, Malaysia, Indonesia, Philippines, Thailand, Vietnam), southern China, southern India, Florida, tropical Australia. Hot desert (30 BTU/ft²) — Middle East, Arizona, outback Australia, Sahara. Tropical climates need 25% more cooling capacity than temperate for the same room because humidity adds latent-heat load on top of sensible heat.
Adjust for sun + insulation + occupants
Sun exposure matters: south- and west-facing rooms (or top-floor rooms under a hot roof) get 10% more load; shaded north-facing or interior rooms get 10% less. Insulation: well-insulated modern construction with double-pane windows saves 10%; poor insulation (older single-pane, no wall insulation) adds 10%. Occupants beyond 2 add 600 BTU each (each person is a 100 W heat source). Kitchens with active cooking add 4000 BTU as a flat lump — refrigerators, ovens, stovetops all generate significant heat.
Round UP to the nearest standard size
The "Nearest standard AC size" recommendation shows the next standard unit size at or above your computed BTU. Always round UP, never down. Under-sizing means continuous compressor runtime, inadequate dehumidification, and a unit that struggles on hot days. Over-sizing by 10–20% is fine; over-sizing by 50%+ causes short-cycling that wastes energy and underchills/dehumidifies. In ASEAN markets, AC units are sold by "horsepower" (HP) — see the tropical advisory in the results panel.
HVAC sizing — why under-sized AC is the biggest residential mistake
BTU (British Thermal Unit) sizing for air conditioners is one of the few home-improvement decisions where the right answer isn't "buy the cheapest". An under-sized AC unit will run continuously, fail to cool on the hottest days, fail to dehumidify properly (because it doesn't run long enough to remove moisture), and ultimately use MORE energy than a properly-sized unit because the compressor never gets to ramp down. Over-sized units are also bad — they "short cycle" (turn on, cool quickly, turn off, repeat), which means the dehumidification doesn't happen and energy bills climb. The Goldilocks zone is to compute your actual cooling load and pick the next standard size above it.
Manual J vs the rule-of-thumb you'll see in stores
Professional HVAC contractors use ACCA Manual J — a detailed calculation that considers window U-values, air-leakage rates, duct losses, infiltration rates, internal heat gains, latent vs sensible load splits, and climate-specific design temperatures. Manual J is the gold standard for new construction and major retrofits. The "20 BTU/ft²" rule-of-thumb (used in this calculator with climate adjustments) is a simplification that's accurate within 10–15% for typical residential rooms. The simplification works because most rooms in similar climates have similar window-to-wall ratios, ceiling heights, and air-leakage rates. For tricky cases (vaulted ceilings, all-glass walls, severely under-insulated old homes), Manual J is worth paying for; for a standard bedroom or living room, the rule-of-thumb is fine.
Under-sized AC is the most expensive mistake in residential cooling. The unit runs constantly, fails to dehumidify, never satisfies on hot days, and shortens its own lifespan from continuous duty. Always round up.
The tropical climate multiplier — why ASEAN needs more BTU
Tropical climates like Singapore, Malaysia, Indonesia, Philippines, Thailand, Vietnam, and southern India face a double cooling load: sensible heat (warming air from outdoor 32°C+ down to indoor 24°C) AND latent heat (condensing moisture out of 80%+ humidity air). The latent load is invisible in a thermometer reading but consumes 30–50% of total AC energy in humid climates. This is why tropical-rated AC units (Daikin, Mitsubishi, LG, Samsung's APAC-spec models) often use larger evaporator coils than their US counterparts of the same nominal BTU rating — they're optimised for moisture removal. The 25 BTU/ft² base rate for tropical climates accounts for this; using temperate (20 BTU/ft²) sizing in Singapore guarantees an under-cooled, over-humid room.
The ASEAN AC market — sized in horsepower
One regional quirk: AC units in ASEAN are typically marketed in "horsepower" (HP) rather than BTU. The HP rating maps roughly to compressor power, not cooling capacity, but conventional sizing follows: 1 HP ≈ 9,000 BTU/h (good for 10–14 m² bedrooms), 1.5 HP ≈ 12,000 BTU/h (15–20 m² living rooms), 2 HP ≈ 18,000 BTU/h (25–35 m² open spaces), 2.5 HP ≈ 24,000 BTU/h (large living-dining combos), 3 HP ≈ 30,000 BTU/h (open-plan whole-floor). Inverter-type units have variable cooling output and can downshift on partial-load days; for tropical climates where you run AC 8+ hours daily, inverter premium pays back in 18–24 months via reduced electricity bills. Top brands across ASEAN: Daikin (Japanese, market leader in SG/MY/ID), Mitsubishi Electric (premium tier), Panasonic, LG / Samsung (Korean, strong in PH/VN/TH), Hitachi, Carrier (US, common in TH for commercial), and Midea / Gree / Haier (Chinese, budget tier). Match BTU to room size, not the HP label alone — a "1.5 HP" unit from a cheap brand can deliver 10K BTU vs 12K from a premium brand of the same nominal rating.
10 Things to Know About AC Sizing
1 ton of cooling = 12,000 BTU/h. The unit comes from the cooling capacity of melting 1 ton of ice over 24 hours — a 19th-century reference that stuck.
Tropical climates need 25% more BTU per square foot than temperate climates because of latent heat — the energy needed to condense moisture out of humid air.
An under-sized AC runs continuously, fails to dehumidify, and uses MORE energy than a properly-sized unit because the compressor never ramps down. Round up, never down.
An over-sized AC short-cycles — turns on, cools quickly, turns off, repeats. Dehumidification needs sustained runtime, so over-sized units leave rooms cold AND damp.
Each person generates ~100W of body heat — roughly 340 BTU/h. Four people in a room add 1,360 BTU/h, equivalent to a small additional AC requirement.
A typical kitchen adds 4,000 BTU/h from refrigerator + oven + stovetop heat. Kitchen-living combos need ~20% more cooling than equivalent-area bedrooms.
South- and west-facing rooms in the Northern Hemisphere get 10–15% more solar gain than north-facing rooms — they need correspondingly more cooling capacity.
Inverter AC units (variable-speed compressors) save 30–40% energy vs fixed-speed units in tropical climates because they downshift at partial load instead of cycling on/off.
ASEAN AC units are typically sold in "horsepower" (HP). Conversion: 1 HP ≈ 9,000 BTU/h, 1.5 HP ≈ 12,000, 2 HP ≈ 18,000, 2.5 HP ≈ 24,000, 3 HP ≈ 30,000.
ACCA Manual J (Air Conditioning Contractors of America) is the gold-standard HVAC sizing method. It's used for new construction and major retrofits — far more accurate than rule-of-thumb but takes 2–4 hours per house.
Frequently Asked Questions
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Over-sized AC is almost as bad as under-sized AC. An over-sized unit cools the room quickly, hits thermostat setpoint, shuts off — then turns back on a few minutes later when temperature drifts up. This "short cycling" prevents the AC from removing moisture (dehumidification requires sustained 15+ minute runtime), wastes energy from compressor start/stop losses, and shortens the unit's lifespan. The result: a cold, clammy room with high electricity bills. The right size is the one that runs steadily for 20–40 minutes per cycle on a hot day.
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Standard conversions: 1 ton = 12,000 BTU/h (US HVAC convention). 1 kW = 3,412 BTU/h (metric / European). 1 HP ≈ 9,000 BTU/h (ASEAN convention — note this varies by manufacturer; check the actual BTU spec on the unit, not the HP label alone). A 1.5-HP / 12,000-BTU / 1-ton / 3.5-kW unit is the most common bedroom-sized AC in tropical Asia. A 2-HP / 18,000-BTU / 1.5-ton / 5.3-kW unit is the typical living-room size.
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Tropical air carries much more moisture than temperate air. Cooling that air down requires two energy expenditures: sensible heat removal (lowering temperature) AND latent heat removal (condensing moisture out). The latent load can be 30–50% of total cooling energy in humid climates. A Singapore room at 80% relative humidity needs ~25% more total BTU/h than the same room in a dry temperate climate to feel equally comfortable. The 25 BTU/ft² tropical base rate accounts for this; using the 20 BTU/ft² temperate rate in Singapore guarantees an under-cooled, over-humid room.
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In tropical climates where AC runs 8+ hours daily, almost always yes. Inverter units have variable-speed compressors that adjust output to match the actual cooling load — running at 30% capacity on a mild day vs 100% on a hot day. Non-inverter units only run at full power, cycling on/off. Energy savings: 30–40% vs non-inverter for daily-use scenarios. Premium: 30–50% on the purchase price. Payback period in Singapore / Malaysia / Indonesia / Philippines: typically 18–24 months at current electricity rates. For weekend-only-use AC (a holiday home), the payback stretches; for primary residence, it's a no-brainer.
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ACCA Manual J is the professional gold-standard HVAC sizing method, used by licensed contractors for new construction and major retrofits. It considers: specific window U-values, air-leakage rates measured by blower-door test, duct losses, infiltration rates, internal heat gains from appliances, latent vs sensible load splits, and ASHRAE design temperatures for your specific climate zone. Takes 2–4 hours per house. The simplified rule-of-thumb used here is accurate within 10–15% for typical residential rooms — sufficient for picking a window AC unit, but worth upgrading to a real Manual J for whole-house HVAC, vaulted ceilings, all-glass walls, or severely under-insulated old buildings.
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Yes, with caveats. Heat pumps for cooling use the same BTU calculation. For heating mode, the same BTU/h is a reasonable starting estimate, but heating loads vary much more by climate severity (a -10°C winter day needs far more BTU/h than a 5°C winter day). For climates where heating is the dominant load (northern Europe, Canada, northern US, northern China, northern Japan), use this estimate as a floor and consult a contractor for proper Manual J. For mild-winter climates (Mediterranean, southern US, ASEAN highlands), this estimate works for both cooling and heating.
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Set "Ceiling height" to "High" — this applies a 25% multiplier to account for the extra air volume. For very high vaulted ceilings (10 ft / 3 m+) or domed ceilings, the 25% multiplier may under-estimate; bump waste by another 10% manually. Skylights add solar heat gain — treat the room as "Sunny" exposure even if walls are shaded. For all-glass walls or atriums, this simplified calculator is too crude — get a real Manual J calculation from an HVAC contractor.
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No. All calculations run entirely in your browser via JavaScript. There's no server roundtrip — open DevTools → Network and confirm zero outbound requests as you change inputs. Room measurements stay on your device. Safe for confidential commercial estimates, HVAC contractor work-ups, or any room data that shouldn't leave your machine. Close the tab and nothing remains.
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Measure the total combined floor area as if it's one room. Add 15–20% for adjacent space pulling cooled air through partial-wall openings. Check the "Kitchen" box for the +4000 BTU additional load. For very large open spaces (whole floors, lofts, large open warehouses), a single AC unit may be insufficient — consider a multi-zone system (one outdoor unit, multiple indoor units) sized per zone, or a ducted central AC. Manual J calculations become essential at this scale.
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These are efficiency ratings: EER (Energy Efficiency Ratio) = cooling output / power input at one rated condition. SEER (Seasonal EER) = cooling output / power input across a whole season's typical conditions (more representative). COP (Coefficient of Performance) = output / input in metric, used for heat pumps. Higher = better. In tropical climates where AC runs daily, prioritise high SEER (or its tropical equivalent CSPF) — every point of SEER above the regulatory minimum saves 3–5% on annual electricity. Singapore mandates 5-tick (NEA Energy Label) for ASHP residential systems; Malaysia has SIRIM 5-star ratings. Buy 5-tick / 5-star units for primary-residence AC.
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