BTU Calculator
Estimate the cooling capacity (BTU/hr) your room needs from an air conditioner. Uses the ENERGY STAR sizing rule of 20 BTU per square foot, with adjustments for sun, occupants, and kitchens.
Recommended cooling capacity
6,000
- Base load (20 BTU per sq ft)
- 6,000
- Sun-exposure adjustment
- 0
- Extra-occupant adjustment
- 0
- Kitchen adjustment
- 0
- Room area
- 300
Recommended BTU/hr = (Square feet × 20) × sun factor + occupant and kitchen adjustments. Sun factor is 0.9 for a heavily shaded room, 1.1 for a very sunny room, 1.0 otherwise. Add 600 BTU/hr for every regular occupant beyond two, and 4,000 BTU/hr if the unit cools a kitchen. Round the result up to the next available unit size (room air conditioners are sold in 1,000 or 2,000 BTU increments). This is the ENERGY STAR consumer sizing rule for a room with 8 ft ceilings and average insulation; use ACCA Manual J for whole-house or non-standard rooms.
How to use this calculator
Measure the room's floor area in square feet (length × width). Pick the sun exposure: heavily shaded (north-facing or tree cover), average, or very sunny (south or west-facing, large windows). Enter the number of people who regularly use the room, and choose whether it is a kitchen. The result is the cooling capacity in BTU per hour. Round up to the nearest available unit size — room air conditioners are sold in 1,000 or 2,000 BTU steps starting at 5,000 BTU.
How the calculation works
The starting point is the ENERGY STAR rule of thumb: 20 BTU per hour per square foot of conditioned space, which assumes 8 ft ceilings and average insulation. The base load is then adjusted: multiply by 0.9 for a heavily shaded room or by 1.1 for a very sunny one, add 600 BTU for each regular occupant beyond two (bodies are space heaters), and add 4,000 BTU if the unit cools a kitchen (cooking and appliances dump a lot of heat). For non-standard rooms — high ceilings, poor insulation, or whole-house systems — use the ACCA Manual J load calculation instead. The 20 BTU/sq ft rule is a cooling-only consumer estimate, not a substitute for an HVAC contractor.
Worked example
A 300 sq ft very sunny kitchen with four regular occupants. Base load = 300 × 20 = 6,000 BTU. Sun adjustment = 6,000 × 1.1 = 6,600 BTU (so +600 from sun). Extra occupants = (4 − 2) × 600 = 1,200 BTU. Kitchen bonus = 4,000 BTU. Total = 6,600 + 1,200 + 4,000 = 11,800 BTU/hr. Round up to a 12,000 BTU unit.
Frequently asked questions
What is a BTU?
A British Thermal Unit (BTU) is the energy needed to raise one pound of water by one degree Fahrenheit — about 1,055 joules. In air conditioning, capacity is quoted in BTU per hour (often shortened to "BTU" on packaging), which is the rate at which the unit can remove heat from a room. A 12,000 BTU unit removes 12,000 BTU per hour, equivalent to one ton of refrigeration or about 3.5 kilowatts of cooling power.
How many BTUs do I need per square foot?
The ENERGY STAR rule of thumb is 20 BTU per hour per square foot of floor area, assuming an 8 ft ceiling and average insulation. A 200 sq ft room needs roughly 4,000 BTU; a 500 sq ft room needs roughly 10,000 BTU. Adjust upwards for sunny rooms, kitchens, or rooms with more than two regular occupants, and downwards for heavily shaded rooms. For rooms with very high ceilings, poor insulation, or unusual orientations, the rule of thumb undersizes — use a Manual J calculation.
Does ceiling height matter for BTU sizing?
Yes, but the 20 BTU per square foot rule assumes a standard 8 ft ceiling. If your ceiling is higher, scale the room area by ceiling height ÷ 8 before applying the rule. A 300 sq ft room with a 10 ft ceiling acts more like a 300 × (10 ÷ 8) = 375 sq ft room, needing about 7,500 BTU instead of 6,000. For cathedral ceilings or open lofts, add 25–50% to the result and consider a professional load calculation.
What happens if I buy an air conditioner that is too big?
An oversized AC short-cycles: it cools the room to the thermostat setpoint very quickly, shuts off, then restarts a few minutes later. Short cycles dehumidify poorly (moisture removal needs sustained run-time), waste energy through repeated startup spikes, and shorten compressor life. The room feels cold and clammy. Oversizing by 10–15% is fine and is what rounding up to the next available unit size does; oversizing by 50% or more is a noticeable problem.
Why does a kitchen need extra BTUs?
Cooking appliances — ovens, stoves, microwaves, refrigerators — dump significant heat into the room. A working oven can add 5,000+ BTU per hour to the cooling load, and a kitchen-in-use typically runs 4,000–6,000 BTU per hour hotter than a comparable bedroom. ENERGY STAR's rule is to add a flat 4,000 BTU on top of the square-footage calculation. If the kitchen has multiple ovens, professional ranges, or sees heavy daily cooking, add more.
Is the BTU rating the same as cooling watts?
They measure the same thing in different units. 1 BTU per hour ≈ 0.293 watts of cooling. A 12,000 BTU/hr air conditioner provides about 3,516 watts (3.5 kW) of cooling capacity. In Europe, AC units are usually rated in kW; in the US and most Asia-Pacific markets, BTU/hr is the standard. To convert: divide BTU by 3,412 to get kW. Power consumption (the wattage on the label) is different and depends on efficiency: a 12,000 BTU unit with SEER 14 draws about 1,000 watts of electricity.