This BTU calculator sizes heating and cooling for a room: base load from square footage, with the standard adjustments for sunny rooms, extra occupants and kitchen heat — the estimate behind every window-AC and mini-split decision.
The 20-Per-Square-Foot Rule (and Its Limits)
The industry sizing heuristic: cooling load ≈ 20 BTU/h per square foot for typical rooms with 8-ft ceilings, then adjustments — +600 per extra person (bodies are 400–600 BTU heaters), +4,000 for kitchens, ±10% for sun. Whole-house precision work uses Manual J load calculations (insulation, windows, climate zone) — this estimator is the honest quick version for single rooms, window units, space heaters and sanity checks. What the unit costs to run is our Electricity Cost Calculator‘s question. Part of our Construction Calculators collection.
Quick Size Chart (Cooling, Rule-of-Thumb)
| Room size | BTU/h | Room size | BTU/h |
|---|---|---|---|
| 100–150 sq ft | 5,000–6,000 | 450–550 sq ft | 12,000 |
| 150–250 sq ft | 6,000–8,000 | 550–700 sq ft | 14,000 |
| 250–350 sq ft | 8,000–10,000 | 700–1,000 sq ft | 18,000 |
| 350–450 sq ft | 10,000–12,000 | 1,000–1,200 sq ft | 21,000–24,000 |
Worked Example (Verified)
- 350 sq ft living room, 2 people, average sun: 350 × 20 = 7,000 BTU/h — an 8,000 BTU window unit is the natural pick
- Add two more regular occupants: +1,200 → 8,200; make it sunny: × 1.1 ≈ 9,000 BTU/h
- The oversizing trap: a unit far above the load cools fast and shuts off before dehumidifying — cold but clammy. Correct size runs longer and drier, the actual comfort goal.
Related Tools
- Electricity Cost Calculator — what it costs to run
- Construction Calculators — the full collection
Frequently Asked Questions
How many BTU do I need for a 350 sq ft room?
About 7,000 BTU/h for cooling by the standard rule (20 per sq ft), ~8,000–9,000 with strong sun or several occupants — an 8k or 9k window unit.
How many BTU per square foot is standard?
20 BTU/sq ft for cooling in typical construction and moderate climates. Heating estimates in cold climates start higher (30–50/sq ft) and true whole-house sizing uses Manual J rather than any single multiplier.
Is a bigger air conditioner better?
No — oversized units short-cycle: they hit the set temperature quickly, shut off, and leave humidity unremoved (cold-and-damp). They’re also louder and pricier. The right size runs steadily, which is what dehumidification and comfort actually require.
What is a “ton” of air conditioning in BTU?
One ton = 12,000 BTU/h — the nominal output of the example living setup is about 0.75 tons; whole houses run 2–5 tons.
Do ceiling height and insulation change the number?
Yes — the 20/sq ft rule assumes ~8 ft ceilings and average insulation. Vaulted ceilings, single-pane walls of glass, or desert climates all push the load up; a Manual J calculation captures those properly for permanent systems.
Is this BTU calculator free?
Yes — free, instant, no sign-up.