Heating, Cooling & Water · Calculator
BTU Calculator
This BTU calculator sizes a window AC, mini split or space heater for one room. It uses the Energy Star cooling chart with its sun, occupant and kitchen adjustments, plus heating BTU by climate zone, and gives you the standard unit size to buy.
Your room
Over 8 ft scales the load up.
Area: 300 sq ft
Energy Star: minus 10% for heavy shade, plus 10% for a very sunny room.
Adds 600 BTU for each person over 2.
How the BTU calculator works
For cooling, the calculator starts with the Energy Star room air conditioner chart, which matches floor area to capacity for an 8 ft ceiling. Then it applies the Energy Star adjustments: minus 10 percent for a heavily shaded room, plus 10 percent for a very sunny one, 600 BTU for each regular occupant past two, and 4,000 BTU for a kitchen.
The chart assumes 8 ft ceilings, so taller rooms get scaled by volume (a 10 ft ceiling adds 25 percent). Then we round up to the next size you can actually buy.
For heating, it multiplies area by a BTU-per-square-foot figure for your climate zone, adjusted for insulation. That is a rule of thumb for a single room. For a whole-house furnace or heat pump, ask your installer for a Manual J load calculation.
Energy Star AC sizing chart
Energy Star capacities for room air conditioners, with the standard window unit and mini split size that covers each one. Use the base figure, then adjust for sun, people and kitchens as described above.
| Area to cool | Capacity needed | Window AC to buy | Mini split to buy |
|---|---|---|---|
| 100 to 150 sq ft | 5,000 BTU | 5,000 BTU | 6,000 BTU |
| 150 to 250 sq ft | 6,000 BTU | 6,000 BTU | 6,000 BTU |
| 250 to 300 sq ft | 7,000 BTU | 8,000 BTU | 9,000 BTU |
| 300 to 350 sq ft | 8,000 BTU | 8,000 BTU | 9,000 BTU |
| 350 to 400 sq ft | 9,000 BTU | 10,000 BTU | 9,000 BTU |
| 400 to 450 sq ft | 10,000 BTU | 10,000 BTU | 12,000 BTU |
| 450 to 550 sq ft | 12,000 BTU | 12,000 BTU | 12,000 BTU |
| 550 to 700 sq ft | 14,000 BTU | 14,000 BTU | 18,000 BTU |
| 700 to 1,000 sq ft | 18,000 BTU | 18,000 BTU | 18,000 BTU |
| 1,000 to 1,200 sq ft | 21,000 BTU | 24,000 BTU | 24,000 BTU |
| 1,200 to 1,400 sq ft | 23,000 BTU | 24,000 BTU | 24,000 BTU |
| 1,400 to 1,500 sq ft | 24,000 BTU | 24,000 BTU | 24,000 BTU |
| 1,500 to 2,000 sq ft | 30,000 BTU | Mini split | 30,000 BTU |
| 2,000 to 2,500 sq ft | 34,000 BTU | Mini split | 36,000 BTU |
Why a bigger air conditioner is worse
It is tempting to buy one size up "just in case." Do not. An oversized unit hits the thermostat setting in a few minutes and shuts off before the coil has pulled moisture out of the air. That is short cycling, and it leaves the room cool but damp, which feels worse than slightly warm and dry.
Short cycling also wears out the compressor, wastes power on every startup, and makes the room swing between too cold and too warm. A right-sized unit runs longer, quieter cycles and dehumidifies as it goes.
A smart thermostat helps a central system run efficient cycles, and our smart thermostat buying guide explains which features matter for heat pumps and multi-stage systems.
Running an AC on a generator?
Window AC vs mini split vs portable
- Window AC: the cheapest BTU you can buy. Sizes run from 5,000 to about 24,000 BTU. Fine for bedrooms and single rooms, but noisy, and it blocks the window.
- Ductless mini split: costs more up front but runs at variable speed, so it rarely short-cycles and stays quiet. The same unit heats as a heat pump. Common sizes are 9,000, 12,000, 18,000 and 24,000 BTU.
- Portable AC: the last resort. Single-hose models pull conditioned air out of the room, so a 10,000 BTU portable cools like a much smaller window unit. If you must, buy dual-hose and size up one step.
Pair any of them with a ceiling fan. Moving air lets you set the thermostat about 4 F higher with the same comfort. Size one with our ceiling fan size calculator.
Heating BTU by climate zone
Colder climates need more heat per square foot because the gap between indoor and outdoor temperature is bigger. These figures are for a room with average insulation and exterior walls, heated as its main heat source.
Using a space heater to top up a room that already has central heat? You can get by with about 10 watts per square foot, so one 1,500 W heater covers a 150 sq ft room. Never plug two into the same 15 amp circuit.
For a heat pump mini split in zone 5 or colder, look at its rated capacity at 5 F, not the headline number. Standard models can lose a third or more of their output in deep cold. Whatever you buy, run the BTU calculator for each room first, then pick a thermostat that works with it.
| Zone | BTU per sq ft | Examples |
|---|---|---|
| Zone 1 (hot) | 30 to 35 | South Florida, Hawaii |
| Zone 2 (warm) | 35 to 40 | Gulf Coast, most of Texas, Phoenix |
| Zone 3 (mild) | 40 to 45 | Atlanta, Dallas, Southern California, Las Vegas |
| Zone 4 (mixed) | 45 to 50 | Virginia, Kentucky, Kansas, Pacific Northwest |
| Zone 5 (cool) | 50 to 55 | Chicago, Ohio, Pennsylvania, Denver |
| Zone 6 (cold) | 55 to 60 | Minnesota (south), Wisconsin, Vermont, Montana |
| Zone 7 (very cold) | 60 to 65 | Northern Minnesota, North Dakota, mountain towns |
Frequently asked questions
How many BTUs do I need to cool a room?
About 20 BTU per square foot for small and medium rooms, per the Energy Star chart. A 150 sq ft bedroom needs 5,000 BTU, a 300 sq ft room 7,000 BTU, and a 500 sq ft living area 12,000 BTU. Add 10 percent for a very sunny room, 600 BTU per extra person over two, and 4,000 BTU for a kitchen.
What size AC do I need for a 12x12 room?
A 12 x 12 room is 144 sq ft, so a 5,000 BTU window air conditioner is enough under normal conditions. If the room gets strong afternoon sun, 5,500 BTU still rounds to a 6,000 BTU unit, which is the next standard size.
Is it bad to oversize an air conditioner?
Yes. An oversized AC cools the air so fast that it shuts off before it pulls moisture out, a problem called short cycling. The room ends up cold and clammy, the compressor wears faster, and you pay more for the unit. Buy the size the chart gives you, not the next one up.
How many BTUs is a 1,500 watt space heater?
A 1,500 watt heater produces 5,118 BTU per hour (1,500 x 3.412). Every plug-in electric heater is effectively 100 percent efficient, so a 1,500 watt model heats the same no matter what the box claims. In a moderate climate that covers roughly 100 to 150 sq ft as primary heat.
How many tons is 12,000 BTU?
One ton. Air conditioning tons measure cooling per hour, and 1 ton equals 12,000 BTU per hour. A 24,000 BTU mini split is a 2 ton system and a 36,000 BTU system is 3 tons.
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