Free tool
HVAC BTU Calculator
Enter the space and get the cooling load — BTU needed and the AC size in tons, adjusted for ceiling height, sun, climate, and occupants. A fast field estimate for scoping and quoting. Free, no signup.
Cooling load
0 BTU
- Base load (20 BTU/ft²)0 BTU
- Ceiling & exposure adj0 BTU
- Occupants—
- Kitchen—
- Total cooling load0 BTU
- Approx. unit size—
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Or download it now (PDF)How to size an air conditioner
Sizing cooling equipment comes down to estimating the heat load of the space, then matching the nearest nominal unit. The baseline is roughly 20 BTU per square foot, but a flat per-foot number ignores the things that actually swing the load — ceiling volume, sun, climate, and the people and appliances putting out heat.
1. Start from the floor area
Measure the conditioned area in square feet and multiply by about 20 BTU. That is the base load for a standard eight-foot ceiling in a moderate climate — the anchor everything else adjusts from.
2. Adjust for volume, sun, and climate
A taller ceiling means more air to cool, so the load scales with ceiling height. A sun-baked room with west-facing glass runs hotter than a shaded one, and a southern climate carries more cooling hours than a northern one. The calculator applies a multiplier for each so the estimate reflects the real space, not an average one.
3. Add internal heat gains
People and appliances add heat. Budget roughly 600 BTU for each occupant beyond the first two who are regularly in the space, and about 4,000 BTU for a kitchen, where the range, oven, and refrigerator all dump heat into the room.
4. Match the unit — and don’t oversize
Convert the load to tons (12,000 BTU per ton) and pick the nearest nominal unit. Resist rounding way up: an oversized AC short-cycles, cools without dehumidifying, and wears out early. For the final specification, run a full Manual J — this estimate is for scoping and quoting.
Sizing the job is step one. Turning it into a priced, professional quote is faster inside CrewRivet. See the job pricing calculator, the flooring calculator, or the paint calculator.
HVAC BTU calculator FAQ
- How many BTUs do I need per square foot?
- A rough rule of thumb is about 20 BTU of cooling per square foot of living space, but the real number moves with ceiling height, sun exposure, climate, how many people use the room, and whether it is a kitchen. This calculator starts from the 20-BTU baseline and adjusts for each of those, which is why it lands closer than a flat per-foot number.
- What size air conditioner do I need?
- Size to the cooling load, not the biggest unit you can afford. Add up the BTU for the space, then match the nearest nominal unit — 12,000 BTU is one ton, 24,000 is two tons, and so on. An oversized AC short-cycles: it cools the air fast but shuts off before it pulls out humidity, so the room feels cold and clammy and the compressor wears out early.
- Is bigger always better for an AC unit?
- No — oversizing is one of the most common HVAC mistakes. A unit that is too large cools to the thermostat setpoint in short bursts and never runs long enough to dehumidify, leaving the space damp and uneven. It also costs more upfront and cycles harder. Sizing to the actual load gives longer, gentler run times, better humidity control, and a longer equipment life.
- Does this replace a Manual J load calculation?
- No. A full Manual J accounts for insulation values, window types and orientation, infiltration, and duct losses, and it is what you want before specifying and installing equipment. This calculator is a fast field estimate for scoping a job, quoting a ballpark, or sanity-checking a size — treat it as a starting point, not the final spec.
- Is this HVAC calculator free?
- Yes — the live estimate is free with no email required. Enter your email only if you want the free Job Pricing Kit (pricing worksheet, markup cheat-sheet, and invoice + quote templates) sent to your inbox.