Find the right air-conditioner size for a room from its dimensions, climate, sun, insulation and occupancy — results in BTU/hr, tons and kW with the recommended standard unit, updated live.
About the BTU / AC Sizing Calculator
This tool estimates how much cooling capacity a room needs and the air-conditioner size to match. It starts from the floor area and a climate-based BTU-per-square-foot figure, then adjusts for ceiling height, sun exposure, insulation, the number of people and any cooking load — the factors that make two rooms of the same size need different units.
It is built for homeowners and installers choosing a window or split unit, and for engineers wanting a quick check. For a new ducted system or a permit, use a full load calculation — our Cooling & Heat Load Calculator does this in detail.
How the size is worked out
Base = Area (ft²) × BTU/ft² × (Ceiling ÷ 8)
× Sun factor × Insulation factor
+ 600 BTU per person over 2
+ 4,000 BTU if a kitchen
= Required BTU/hr → round up to a standard unit
BTU/ft² rises with the climate (about 20 cool, 25 moderate, 30 hot, 35 very hot). One ton of cooling is 12,000 BTU/hr and one kilowatt is 3,412 BTU/hr, so the result is shown three ways and matched to the nearest standard unit.
Quick room-size guide (before adjustments)
| Room area | Approx. capacity |
|---|
| 150 – 250 ft² (14–23 m²) | 6,000 BTU |
| 250 – 350 ft² (23–33 m²) | 7,000 – 8,000 BTU |
| 350 – 450 ft² (33–42 m²) | 9,000 – 10,000 BTU (≈1 ton) |
| 450 – 700 ft² (42–65 m²) | 12,000 – 14,000 BTU (1–1.2 ton) |
| 700 – 1,000 ft² (65–93 m²) | 18,000 BTU (1.5 ton) |
| 1,000 – 1,400 ft² (93–130 m²) | 21,000 – 24,000 BTU (≈2 ton) |
Why right-sizing matters
- Too small — runs non-stop and never cools the room on peak days.
- Too big — short-cycles, cools fast but doesn't run long enough to remove humidity, leaving the room cold and clammy and stressing the compressor.
- Right-sized — steady temperature, good dehumidification and lower running cost.
Assumptions & limitations
- This is a rule-of-thumb estimate with sensible margins — fine for a window or split unit, not a substitute for ACCA Manual J or an ASHRAE load calculation.
- Very hot, glass-heavy or poorly insulated rooms can need more than the rule predicts; verify with a detailed load calculation where it matters.
- The recommended unit is the nearest standard size up; an inverter unit that modulates is more forgiving of a small mismatch.
Frequently asked questions
How many BTU do I need per square foot?
A common rule of thumb is about 20–25 BTU per square foot for a moderate climate and 8-foot ceiling, rising toward 30–35 BTU/ft² in hot or sunny conditions. This tool applies a climate-based base figure and then adjusts for ceiling height, sun exposure, insulation, occupancy and kitchen heat, so you get a size tailored to the room rather than a flat number.
What size air conditioner do I need for a room?
Enter the room dimensions and conditions and the calculator returns the required BTU/hr, the equivalent tons and kW, and the nearest standard unit. As a quick guide: a 150–250 ft² room needs roughly 6,000 BTU, 350–400 ft² about 9,000 BTU, 550–700 ft² about 14,000 BTU, and 1,000–1,200 ft² about 21,000 BTU — before adjustments.
How do I convert BTU to tons or kW?
One ton of cooling equals 12,000 BTU/hr, so divide BTU by 12,000 for tons. To get kilowatts of cooling, divide BTU/hr by 3,412. For example, 18,000 BTU is 1.5 tons or about 5.3 kW.
What happens if my AC is too big or too small?
An undersized unit runs constantly and never reaches setpoint on the hottest days. An oversized unit cools the air quickly but short-cycles, so it doesn't run long enough to remove humidity — leaving the room cold and clammy and wearing the compressor. Right-sizing gives steady temperature, good dehumidification and lower energy use.
Why does a kitchen need extra BTU?
Cooking appliances add a large sensible and latent heat gain, so ENERGY STAR recommends adding about 4,000 BTU when the unit serves a kitchen. The calculator adds this automatically when you tick the kitchen option.
Does sun exposure and insulation really change the size?
Yes. A very sunny room can need around 10% more capacity while a heavily shaded one needs about 10% less, and poor insulation or single-glazed windows push the load up further. These factors are built into the calculator as multipliers.
Is this accurate enough to buy a unit?
It is a solid rule-of-thumb estimate, ideal for choosing a window or split unit and for a quick check. For a new system, ducted equipment, a permit, or an unusual room, use a full load calculation (ACCA Manual J or an ASHRAE cooling-load method) — our Cooling & Heat Load Calculator does this in detail.
Last updated: 2026-01-03 · Estimate for unit selection — for new systems use a full load calculation and local code.