BTU calculator
Heat loss, surface by surface
For a real number rather than a rule of thumb: each surface's area × its U-value × the temperature difference, plus the air that leaks through. Everything here is typed by you — the U-values from the building, the design temperatures from your own local data.
Voir le calcul — avec vos chiffres
Arithmetic on the factor, areas, U-values and temperatures you type. It is not a Manual J or a full heat-loss survey — it ignores solar gain, internal gains, ducts and thermal bridges. Size equipment with a proper load calculation for anything that matters.
Exemple détaillé
Quick size, imperial: a 400 sq ft room at 25 BTU per square foot is 400 × 25 = 10,000 BTU/h — about 2,930 W, or 0.83 of a ton. A 10,000–12,000 BTU unit is the shelf to look at. Metric: 37 m² at 80 W per m² is 2,960 W — about 10,100 BTU/h.
Heat loss, imperial: 70 °F inside, 10 °F outside, so ΔT = 60 °F. Walls 320 ft² × 0.07 × 60 = 1,344; windows 40 × 0.30 × 60 = 720; ceiling 400 × 0.03 × 60 = 720; air 0.018 × 3,200 ft³ × 0.5 × 60 = 1,728. Total 4,512 BTU/h, about 1,320 W.
Heat loss, metric: 21 °C inside, −12 °C outside, ΔT = 33 K. Walls 30 m² × 0.4 × 33 = 396 W; windows 4 × 1.7 × 33 = 224 W; ceiling 37 × 0.17 × 33 = 208 W; air 0.33 × 90 m³ × 0.5 × 33 = 490 W. Total about 1,318 W — 4,500 BTU/h.
Sur le chantier
Bigger is not better for cooling. An oversized air conditioner cools the air fast and shuts off before it has pulled the moisture out — cold and clammy. Size to the number, not up two sizes "to be safe".
Air leakage is often the biggest line. In the example the draughts cost more than the walls. Sealing and weather-stripping move the number more than another inch of insulation in an already-insulated wall.
R to U. U is 1 ÷ total R. An R-13 wall assembly with its framing and finishes might come out near R-14 overall: 1 ÷ 14 = 0.071. In metric, RSI 2.5 gives U = 0.4 W/m²K.
Quick referencefor heat
- BTU/h to watts
- × 0.293 · watts to BTU/h × 3.412
- Ton of cooling
- 12,000 BTU/h = 3.517 kW
- U from R
- U = 1 ÷ R (either system)
- U imperial → metric
- 1 BTU/h·ft²·°F = 5.678 W/m²K
- Air load
- 0.018 × ft³ × ACH × ΔT°F (BTU/h) · 0.33 × m³ × ACH × ΔT (W)
- ΔT
- 1 °C of difference = 1.8 °F of difference
La formule
Q = Σ (area × U × ΔT) + 0.018 × volume × ACH × ΔT
Imperial gives BTU per hour; the metric form uses 0.33 for the air term and gives watts. The quick size is simply area × your factor.
Questions
- How many BTU do I need for a room?
- The quick answer is floor area times a BTU-per-square-foot factor — 20 to 30 is the usual rule of thumb for cooling a room with ordinary ceilings, which puts a 400 sq ft room around 10,000 BTU/h. It is a starting point for choosing a unit, not a load calculation: sun, windows, insulation and climate all move it.
- How do I convert BTU to watts?
- One BTU per hour is 0.293 watts, so multiply BTU/h by 0.293 — 10,000 BTU/h is about 2,930 W (2.9 kW). Going the other way, one watt is 3.412 BTU/h.
- What is a ton of cooling?
- 12,000 BTU per hour — the heat it takes to melt a ton of ice in a day. A 24,000 BTU/h air conditioner is a 2-ton unit; in metric it is about 7 kW.
- Where do the U-values come from?
- From the wall, window or ceiling itself: a window label, the insulation manufacturer, or 1 ÷ the total R-value of the assembly (R in ft²·°F·h/BTU gives U in BTU/h·ft²·°F; RSI in m²K/W gives U in W/m²K). This page does not supply them — type the ones for the building in front of you.
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