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BTU Calculator

Cooling BTU for a room, adjusted for height, sun and occupancy.

Your measurementsEvery assumption editable01

The base figure assumes 8 ft. Taller rooms hold more air to cool.

Cooling load per square foot for your climate.

Each person beyond the first two adds about 600 BTU/hr.

Cooking appliances add roughly 4,000 BTU/hr.

Cooling loadCalculated live02

Result

10000BTU/hr

About 0.83 tons — nearest unit 9000 BTU

Base load400 sq ft × 25 BTU/sq ft
10000 BTU/hr
Adjusted for ceiling height8 ft against an 8 ft baseline
10000 BTU/hr
Total cooling load
10000 BTU/hr
In tons12,000 BTU/hr per ton
0.83 ton
Nearest nominal unit
9000 BTU/hr

Do not round up to be safe. An oversized air conditioner short-cycles — it satisfies the thermostat before it has run long enough to pull moisture out of the air, leaving a house that is cold and clammy while wearing the compressor out faster. Undersizing is uncomfortable; oversizing is uncomfortable and expensive.

This is a square-footage estimate, not a Manual J. A proper load calculation accounts for window area and glazing type, insulation values, air-tightness, duct losses and your local design temperature. For a whole-house system replacement, get one done.

Room-by-room figures do not simply add up to a whole-house size. Rooms peak at different times of day, so the house total is meaningfully less than the sum of its rooms.

Enter the room area and the adjustments that actually move the answer — ceiling height, sun exposure, how many people are usually in it, and whether it is a kitchen. The result is in BTU per hour, in tons, and rounded to the nearest unit you can actually buy.

Why use this tool?What it does differently03

Why use this tool?

Adjustments that matter

Ceiling height, sun exposure, occupancy and cooking load, rather than area alone. A sunny kitchen and a shaded bedroom of identical size need very different equipment.

BTU, tons and unit size

The raw figure, the tonnage, and the nearest nominal size — 9k, 12k, 18k and up — so you can shop from it directly.

Warns against oversizing

Every result explains why rounding up is the wrong instinct here, unlike every other material calculator on the site.

Shows the working

Base load, each adjustment and the total are itemised, so you can see which input is driving the answer.

How this worksThe method04

How this btu calculator works

The base figure is floor area multiplied by a climate load in BTU per square foot — roughly 20 in mild regions, 25 typical, and 30 to 35 where summers are long and humid. That product is then scaled by ceiling height against an eight-foot baseline, because what is being cooled is a volume of air rather than a floor.

Sun exposure applies a percentage either way: shaded rooms need around ten per cent less, west-facing glass and sunrooms considerably more. Occupancy adds about 600 BTU per hour for each person beyond the first two, and a kitchen adds roughly 4,000 for cooking appliances.

The total converts to tons at 12,000 BTU per hour per ton, and is matched to the nearest nominal equipment size. Nominal sizes are labels rather than measurements — a unit sold as 3 tons may be rated at 34,000 or 36,600 BTU/hr depending on the model.

How to use itStep by step05

How to use it

  1. Step 1: Measure the room

    Floor area in square feet, and ceiling height if it is not the standard eight feet.

  2. Step 2: Pick your climate load

    Mild, moderate, hot or very humid. This is the input with the largest effect on the result.

  3. Step 3: Set sun and occupancy

    Shaded, average, sunny or sunroom, and how many people are regularly in the room.

  4. Step 4: Check against nominal sizes

    Match the total to the nearest unit size rather than rounding up to the next one.

Example usageWorked figures06

Example usage

A shaded bedroom
A 300 sq ft bedroom with 8 ft ceilings in a moderate climate, heavily shaded, two occupants: 7,500 base, less 10% for shade, gives 6,750 BTU/hr — a 9,000 BTU unit is the nearest size.
A sunny open kitchen
The same 300 sq ft with very sunny exposure, four occupants and cooking appliances: 7,500 base, plus 10% sun, plus 1,200 for occupants, plus 4,000 for the kitchen — 13,450 BTU/hr, nearly double the bedroom.
A room with a vaulted ceiling
400 sq ft at 12 ft rather than 8 scales the base load by 1.5, from 10,000 to 15,000 BTU/hr before any other adjustment. Height is routinely forgotten and routinely undersizes vaulted rooms.
Frequently asked questionsCommon questions07

Frequently asked questions

How many BTU do I need per square foot?

Roughly 20 in a mild climate, 25 in a moderate one, and 30 to 35 where summers are long and humid. That is the starting point, before adjustments for ceiling height, sun, occupancy and cooking.

Should I round up to be safe?

No. Oversized cooling short-cycles — it reaches the set temperature before running long enough to remove moisture, leaving a room that is cold and clammy and wearing the compressor faster. Slightly under is more comfortable than slightly over.

How many BTU is a ton?

12,000 BTU per hour, by definition. The term comes from the cooling produced by melting a short ton of ice over 24 hours and has nothing to do with equipment weight.

Does this work for heating?

No — heating load behaves differently and depends heavily on your design temperature. Use the furnace size calculator, which works from a heating load per square foot instead.

Can I add up room figures for a whole house?

Not directly. Rooms peak at different times of day, so the sum of room-by-room loads overstates the whole-house figure. Use the AC size calculator for a whole-house estimate.

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