How the size is worked out
Everything starts with the floor area: length × width in feet gives square feet. If you type the square feet yourself, the calculator uses that instead.
Cooling: window AC, portable AC and mini split
The calculator looks your square feet up in the ENERGY STAR sizing table for room air conditioners (printed below), then makes the adjustments ENERGY STAR lists, in its words:
- "If the room is heavily shaded, reduce capacity by 10 percent."
- "If the room is very sunny, increase capacity by 10 percent."
- "If more than two people regularly occupy the room, add 600 BTUs for each additional person."
- "If the unit is used in a kitchen, increase capacity by 4,000 BTUs."
The percentage goes on first, then the people and the kitchen. The result is your adjusted figure in BTU per hour. Units are sold in steps, so the answer is the next size up on the ENERGY STAR table: a figure of 6,600 points you to a 7,000 BTU unit. Both numbers are on the result so you can see the gap.
Three assumptions to know about:
- The table is for 8-foot ceilings. ENERGY STAR only says that with higher ceilings "you may want to select a room air conditioner with a higher CADR". For a ceiling above 8 feet we scale the table figure by height ÷ 8, so a 10-foot ceiling adds 25%. That scaling is our adjustment, not ENERGY STAR's. A ceiling below 8 feet gets no reduction.
- Portable ACs and mini splits use the same table. ENERGY STAR writes it for room air conditioners. We use it for all three types here because it is the published table we have; check the room size printed on the box of the unit you pick.
- Rooms under 100 sq ft sit below the first row of the table. The smallest size on it, 5,000 BTU, covers them.
Heating with a plug-in electric heater
The rule comes from Stelpro, a maker of electric heaters: "Multiply the area in feet by 10". That is watts = square feet × 10, and Stelpro says it is "valid for houses containing rooms whose ceilings are 8 feet high". For higher ceilings the rule is "1.25 watts per cubic foot": watts = square feet × height × 1.25 (Stelpro, calculating the right wattage for a room). At 8 feet both give the same answer.
Stelpro also says to "add a few percentage points" if the walls or ceiling are poorly insulated or the room has oversized windows. The checkbox adds 10%. The 10% is our reading of "a few percentage points", not Stelpro's figure.
To compare with a gas heater or an AC, the calculator converts watts to BTU per hour: watts × 3.412, because 1 kWh = 3,412 Btu (EIA, British thermal units).
The hard limit: a plug-in heater on a standard US outlet tops out at 1,500 W. A 1,500 W heater draws about 12.5 amps, most of what a common 15 amp circuit carries (our guide to space heaters that won't trip a breaker). So at 10 W per square foot, any room over 150 sq ft needs more than one heater or another heat source.
Heating with a propane or gas room heater
Ferrellgas, a propane supplier, gives BTU per square foot by climate: "Warmer areas (Zone 1 or 2): 30 to 40 BTUs per square foot. Moderate areas (Zone 3 or 4): 40 to 45 BTUs per square foot. Colder areas (Zone 5): Up to 60 BTUs per square foot." (Ferrellgas, how many BTUs you need). The calculator shows the range for your climate and suggests the top of it, so the heater keeps up on the coldest nights. As a cross-check it prints Mr. Heater's rule of thumb, "Square Footage x 40 = required BTUs" (Mr. Heater, general product info).
Neither source gives a ceiling rule. Mr. Heater says ceiling height "will also factor" in, so above 8 feet we apply the same height ÷ 8 scaling we use for cooling. That scaling is ours.
Worked examples
- 12 × 14 = 168 sq ft.
- 168 sq ft falls in the ENERGY STAR band "150 up to 250": 6,000 BTU per hour.
- Very sunny: 6,000 + 10% = 6,600 BTU per hour. Two people, not a kitchen, 8-foot ceiling: nothing else to add.
- 6,600 sits between the table sizes 6,000 and 7,000, so the next size up is a 7,000 BTU unit.
The same room heavily shaded: 6,000 − 10% = 5,400, so a 6,000 BTU unit.
- 20 × 20 = 400 sq ft.
- 8-foot ceiling: 400 × 10 = 4,000 W. (Stelpro uses this same room as its example.)
- In BTU per hour: 4,000 × 3.412 = 13,648 BTU/h.
- One plug-in heater gives at most 1,500 W. 4,000 ÷ 1,500 = 2.7, so it takes three 1,500 W heaters, or another heat source.
With a 9-foot ceiling Stelpro's rule gives 400 × 9 × 1.25 = 4,500 W.
The sizing tables
ENERGY STAR's table for room air conditioners, copied exactly. The capacities assume an 8-foot ceiling. When you use the calculator in cooling mode, your row is highlighted.
| Area to be cooled | Capacity needed |
|---|---|
| 100 up to 150 sq ft | 5,000 BTU/h |
| 150 up to 250 sq ft | 6,000 BTU/h |
| 250 up to 300 sq ft | 7,000 BTU/h |
| 300 up to 350 sq ft | 8,000 BTU/h |
| 350 up to 400 sq ft | 9,000 BTU/h |
| 400 up to 450 sq ft | 10,000 BTU/h |
| 450 up to 550 sq ft | 12,000 BTU/h |
| 550 up to 700 sq ft | 14,000 BTU/h |
| 700 up to 1,000 sq ft | 18,000 BTU/h |
| 1,000 up to 1,200 sq ft | 21,000 BTU/h |
| 1,200 up to 1,400 sq ft | 23,000 BTU/h |
| 1,400 up to 1,500 sq ft | 24,000 BTU/h |
| 1,500 up to 2,000 sq ft | 30,000 BTU/h |
| 2,000 up to 2,500 sq ft | 34,000 BTU/h |
Source: ENERGY STAR, room air conditioners, checked 20 October 2026.
Ferrellgas's climate bands for propane and gas heat. In propane mode your climate is highlighted.
| Climate | BTU per sq ft |
|---|---|
| Warmer areas (zone 1 or 2) | 30 to 40 |
| Moderate areas (zone 3 or 4) | 40 to 45 |
| Colder areas (zone 5) | Up to 60 |
Source: Ferrellgas, checked 20 October 2026. Ferrellgas does not say which zone map it uses, so pick by what your winters feel like.
When a room unit is the wrong tool
These rules size a unit for one closed room. They get shaky when the space is not one room:
- Whole floors and open plans. The tables size a unit for one room. A unit in the living room does not spread its air evenly round corners and up stairs, so adding up the square feet of a whole floor overstates what one unit can do.
- Rooms over 2,500 sq ft. The ENERGY STAR table stops at 2,500 sq ft and 34,000 BTU. Past that you are sizing a central or multi-zone system, which is a job for a pro.
- Electric heat for big rooms. Past 150 sq ft one plug-in heater cannot meet the 10 W per sq ft rule, and each extra 1,500 W heater wants a circuit of its own. A wired heat source, such as a heat pump or a wall heater, is the other route.
Rather have a pro size it?
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Guides that go with this tool
- 6 best 8,000 BTU window air conditionersBuying guide
- 7 quietest window air conditionersBuying guide
- Portable air conditionersBuying guide
- Compact mini split systems under 12,000 BTUBuying guide
- 12 energy-efficient space heatersBuying guide
- 11 space heaters that won't trip breaker boxesBuying guide
- 7 best propane wall heaters for indoor useBuying guide
FAQ
How many BTUs do I need to cool a 12 by 14 room?
A 12 by 14 foot room is 168 square feet, which falls in the ENERGY STAR band of 150 up to 250 square feet: 6,000 BTU per hour. If the room is very sunny, ENERGY STAR says to add 10 percent, which makes 6,600, so a 7,000 BTU unit is the next size up. If it is heavily shaded, take 10 percent off instead.
How many watts of electric heat does a room need?
Stelpro's rule of thumb is 10 watts per square foot for a room with an 8 foot ceiling, or 1.25 watts per cubic foot for higher ceilings. A 20 by 20 foot room is 400 square feet, so it needs about 4,000 watts. One plug-in heater draws at most 1,500 watts, so that room needs three heaters or another heat source.
How many BTUs does a propane or gas heater need for one room?
Ferrellgas puts it at 30 to 40 BTU per square foot in warmer areas, 40 to 45 in moderate areas and up to 60 in colder areas. Mr. Heater's rule of thumb is square feet times 40. A 168 square foot room in a moderate climate comes out between 6,720 and 7,560 BTU.
Sources
- Cooling table and adjustments: ENERGY STAR, room air conditioners, "How to choose the right size".
- Electric heat, 10 W per sq ft and 1.25 W per cubic foot: Stelpro, calculating the right wattage for a room.
- Watts to BTU, 1 kWh = 3,412 Btu: U.S. Energy Information Administration, British thermal units.
- Propane and gas heat by climate: Ferrellgas, how many BTUs you need to heat your home, shop, garage and more.
- Square feet × 40 cross-check: Mr. Heater, general product info.
- 1,500 W heaters and 15 amp circuits: Home Caprice, space heaters that won't trip breaker boxes.
- Our own adjustments, named where they are used: height ÷ 8 scaling above 8-foot ceilings (cooling and propane), 10% for poor insulation (our reading of Stelpro), rounding up to the next ENERGY STAR size.
Tables last checked 20 October 2026. The calculator does not store anything you type.