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Free BTU Calculator for Air Conditioners & Heaters

Enter your room size and a few details to get the BTU per hour you need, in tons, kW and watts, plus the exact unit size to shop for. No signup, results update instantly.

Room size
sq ft

Cooling capacity needed

6,000BTU/hr

Likely range 5,400 to 6,600 BTU/hr

Tons
0.5
kW
1.76
Watts
1,758

Buy this size

6,000BTU

small window or portable AC

Shop for a 6,000 BTU small window or portable AC. Don't go bigger than this: an oversized air conditioner cools the air before it can pull out humidity, then switches off (short-cycling), leaving the room clammy and wearing out the compressor.

How we got this number
  • 300 sq ft × 20 BTU6,000 BTU base
  • Average sun exposureno change
  • Average insulationno change
  • Moderate climateno change
  • Rounded to nearest 1006,000 BTU/hr

How the BTU calculation works

A BTU (British thermal unit) is the heat needed to raise one pound of water by one degree Fahrenheit. Air conditioners and heaters are rated in BTU per hour: how much heat they can move out of a room (or into it) every hour. Sizing means matching that rating to the heat your room actually gains on a hot afternoon or loses on a cold night.

This calculator uses the industry rule-of-thumb method that HVAC contractors, Energy Star's room air conditioner chart and appliance retailers all build on. For cooling, the starting point is 20 BTU per square foot of floor area with an 8-foot ceiling. Each factor below then nudges that number up or down. For heating, the starting point depends on your climate: roughly 30, 40 or 55 BTU per square foot for hot, moderate and cold regions, adjusted for insulation. The result is rounded to the nearest 100 BTU and shown with a ±10% range, because no square-footage method can see your window area, air leakage or thermostat habits.

Method last reviewed September 2026. See where the formulas come from.

The factors, and how much each one matters

FactorCoolingHeatingWhy
Floor area× 20 BTU/sq ft× 30 / 40 / 55 BTU/sq ft by climateMore floor means more air and more wall exposed to outside.
Ceiling above 8 ft× height ÷ 8× height ÷ 8A 10 ft ceiling holds 25% more air than an 8 ft one.
Very sunny room+10%−5%Solar gain through glass is a load in summer and a gift in winter.
Heavily shaded room−10%+5%Same effect, reversed.
Poor insulation+15%+20%Thin walls, single-pane glass and drafts leak heat both ways.
Good insulation−10%−15%Modern, sealed construction with double-pane windows.
Climate zoneHot +10%, cold −10%Sets the base factorDesign temperatures differ by 30°F+ across the US.
Each person beyond 2+600 BTUignoredA resting adult gives off roughly 400–600 BTU/hr.
Kitchen+4,000 BTUignoredRange, oven and refrigerator all reject heat into the room.

Full formulas are documented in plain English in the sizing guide and in the calculator's “How we got this number” panel.

BTU by room size

Average conditions assume two people, 8 ft ceilings and a moderate climate. The band shows the spread from a shaded, well insulated room to a sunny, poorly insulated one. Open any size for full tables and unit recommendations.

  1. 100 sq ft2,000 BTU/hr, buy 5,000

    Shaded 1,600, sunny 2,500. Heating (moderate) 4,000

  2. 150 sq ft3,000 BTU/hr, buy 5,000

    Shaded 2,400, sunny 3,800. Heating (moderate) 6,000

  3. 200 sq ft4,000 BTU/hr, buy 5,000

    Shaded 3,200, sunny 5,100. Heating (moderate) 8,000

  4. 250 sq ft5,000 BTU/hr, buy 5,000

    Shaded 4,100, sunny 6,300. Heating (moderate) 10,000

  5. 300 sq ft6,000 BTU/hr, buy 6,000

    Shaded 4,900, sunny 7,600. Heating (moderate) 12,000

  6. 400 sq ft8,000 BTU/hr, buy 8,000

    Shaded 6,500, sunny 10,100. Heating (moderate) 16,000

  7. 500 sq ft10,000 BTU/hr, buy 10,000

    Shaded 8,100, sunny 12,600. Heating (moderate) 20,000

  8. 600 sq ft12,000 BTU/hr, buy 12,000

    Shaded 9,700, sunny 15,200. Heating (moderate) 24,000

  9. 800 sq ft16,000 BTU/hr, buy 18,000

    Shaded 13,000, sunny 20,200. Heating (moderate) 32,000

  10. 1,000 sq ft20,000 BTU/hr, buy 24,000

    Shaded 16,200, sunny 25,300. Heating (moderate) 40,000

  11. 1,200 sq ft24,000 BTU/hr, buy 24,000

    Shaded 19,400, sunny 30,400. Heating (moderate) 48,000

  12. 1,500 sq ft30,000 BTU/hr, buy 30,000

    Shaded 24,300, sunny 38,000. Heating (moderate) 60,000

  13. 2,000 sq ft40,000 BTU/hr, buy 48,000

    Shaded 32,400, sunny 50,600. Heating (moderate) 80,000

Reading the result

The big number is the load: the BTU per hour your room needs on a design day. The “buy this size” box rounds that up to the nearest capacity actually sold. Window and portable units come in 5,000, 6,000, 8,000, 10,000, 12,000, 14,000, 18,000 and 24,000 BTU steps; mini-splits in 9,000, 12,000, 18,000, 24,000 and 36,000; central systems in half-ton (6,000 BTU) increments.

When the load sits just above a class boundary, say 6,300 BTU, the calculator will recommend 8,000 BTU but tell you a 6,000 BTU unit is a legitimate choice. The closer-to-load option is usually right for cooling, because an oversized air conditioner short-cycles and leaves the room humid. For heating, a 10–20% margin over the load is harmless and gives faster warm-up.

One more note on portable air conditioners: since 2017 the US Department of Energy rates them by SACC (seasonally adjusted cooling capacity), which accounts for the heat their exhaust hose leaks back into the room. A portable labelled 10,000 BTU (ASHRAE) is typically 6,000–7,000 BTU SACC. Compare the SACC figure against your result. The window vs. portable guide explains the details.

Frequently asked questions

How many BTU do I need per square foot?

For cooling, the standard rule is 20 BTU per square foot of floor area with an 8-foot ceiling, then adjust: +10% for a very sunny room, −10% for heavy shade, +600 BTU for each person beyond two, and +4,000 BTU if the room is a kitchen. For heating, use 30 BTU per square foot in hot climates, 40 in moderate climates, and 55 in cold climates, adjusted for insulation.

What size air conditioner do I need for a 300 sq ft room?

About 6,000 BTU under average conditions (300 × 20). With a sunny exposure or poor insulation that rises to 6,600–6,900 BTU, so an 8,000 BTU window or portable unit is the usual buy; a 6,000 BTU unit works if the room is shaded and well insulated.

Is it better to oversize or undersize an air conditioner?

Slightly undersized is better than oversized. An oversized AC cools the air quickly and shuts off before it has removed much humidity, leaving the room cold but clammy, and the constant on-off cycling wears the compressor and wastes electricity. Choose the unit class closest to your calculated BTU, not the next one up 'to be safe'.

How many BTU is a ton of air conditioning?

One ton of cooling equals 12,000 BTU per hour. So an 18,000 BTU unit is 1.5 tons, 24,000 BTU is 2 tons, and 36,000 BTU is 3 tons. The term comes from the heat needed to melt one ton of ice in 24 hours.

How many BTU is a 1,500 watt space heater?

1,500 watts is about 5,118 BTU per hour (1 watt = 3.412 BTU/hr). Electric resistance heaters convert essentially all their electricity to heat, so a 1,500 W heater covers roughly 100–150 sq ft in a moderate climate, or up to 170 sq ft in a hot one.

Does ceiling height change the BTU calculation?

Yes. The 20 BTU per square foot rule assumes 8-foot ceilings. For taller ceilings multiply by the height divided by 8: a 10-foot ceiling adds 25%, a 12-foot ceiling adds 50%, because you are cooling that much more air.

Is the BTU calculator accurate enough to buy an air conditioner?

For a single room unit (window AC, portable, mini-split, or space heater), yes: the rule of thumb lands within one unit size of a full load calculation in the large majority of homes. For whole-house central systems, use it as a sanity check and have a contractor run an ACCA Manual J calculation before buying.

What is the difference between BTU and BTU/hr?

A BTU is a quantity of heat (the energy to raise one pound of water by 1°F). BTU per hour is a rate, which is what air conditioner and heater capacities describe. In everyday usage 'a 10,000 BTU air conditioner' means 10,000 BTU per hour; this site follows that convention.

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