BTU Calculator
Find the air conditioner size your room needs in BTU per hour, tons and kilowatts, using the ENERGY STAR sizing chart, or estimate the heat a room needs.
Efficiency for power estimate
For a 300 sq ft room with normal sun and 2 people.
Your room on the ENERGY STAR chart, 100 to 2,500 sq ft. Each block is one chart row.
Unit size to look for
Room air conditioners and ductless mini splits are both rated in BTU per hour, so look for a model rated at this capacity. Avoid a bigger unit: ENERGY STAR notes that an oversized unit cools the room before it removes the humidity, leaving it damp and clammy.
Your calculation
The same steps the calculator runs, with your numbers.
- Floor area: 15 × 20 = 300 sq ft.
- ENERGY STAR chart row 300 up to 350 sq ft: 8,000 BTU/h.
- Sun exposure normal: no change.
- 2 people: no change (600 BTU/h is added per person above two).
- Not a kitchen: no change.
- Adjusted load: 8,000 + 0 + 0 + 0 = 8,000 BTU/h, rounded down to the nearest 1,000: 8,000 BTU/h.
- Tons: 8,000 ÷ 12,000 = 0.67 tons. Kilowatts: 8,000 × 0.2930711 ÷ 1,000 = 2.34 kW.
ENERGY STAR room air conditioner sizing chart
Your row is highlighted. Capacities assume an 8 ft ceiling, before adjustments.
| Area (sq ft) | Area (m²) | Capacity (BTU/h) |
|---|---|---|
| 100 up to 150 | 9.3 to 13.9 | 5,000 |
| 150 up to 250 | 13.9 to 23.2 | 6,000 |
| 250 up to 300 | 23.2 to 27.9 | 7,000 |
| 300 up to 350 | 27.9 to 32.5 | 8,000 |
| 350 up to 400 | 32.5 to 37.2 | 9,000 |
| 400 up to 450 | 37.2 to 41.8 | 10,000 |
| 450 up to 550 | 41.8 to 51.1 | 12,000 |
| 550 up to 700 | 51.1 to 65.0 | 14,000 |
| 700 up to 1,000 | 65.0 to 92.9 | 18,000 |
| 1,000 up to 1,200 | 92.9 to 111.5 | 21,000 |
| 1,200 up to 1,400 | 111.5 to 130.1 | 23,000 |
| 1,400 up to 1,500 | 130.1 to 139.4 | 24,000 |
| 1,500 up to 2,000 | 139.4 to 185.8 | 30,000 |
| 2,000 up to 2,500 | 185.8 to 232.3 | 34,000 |
A rule of thumb estimate for one room unit. Whole home systems need an ACCA Manual J load calculation by a qualified contractor.
How the BTU Calculator Works
This BTU calculator answers the question most people ask before buying a window unit, a portable unit or a ductless mini split: how many BTU do I need for this room? It follows the method ENERGY STAR publishes for room air conditioners. You measure the floor area, look up the matching capacity on the sizing chart, adjust it for sun, people and cooking, and round the total down to the nearest thousand BTU per hour.
A BTU per hour (BTU/h) is a rate of heat removal. Air conditioners are labeled by the heat they can pull out of a room in one hour, so a "10,000 BTU" window unit removes 10,000 BTU of heat per hour at its rated conditions. The calculator also converts the result to tons of refrigeration and kilowatts, which is how central systems, heat pumps and equipment from outside the United States are often rated.
The sun adjustment is a percentage of the chart value, while the occupant and kitchen adjustments are fixed amounts. ENERGY STAR notes that its chart capacities are calculated for an 8 foot ceiling, so the calculator flags taller rooms in the result.
How to Calculate BTU for a Room Manually
- Measure the floor. Measure length and width in feet and convert inches to decimals: 6 inches is 0.5 ft and 9 inches is 0.75 ft. Multiply length by width. For an L shaped room, split it into rectangles and add the areas.
- Find the chart row. Look up the area on the ENERGY STAR chart in the results. Each row starts at its lower number, so 150 sq ft belongs to the 150 up to 250 row.
- Adjust for sun. Subtract 10 percent of the chart value for a heavily shaded room or add 10 percent for a very sunny one.
- Adjust for people. Add 600 BTU/h for each person above two who regularly uses the room.
- Adjust for a kitchen. Add 4,000 BTU/h if the unit will cool a kitchen.
- Round down. Round the total down to the nearest 1,000 BTU/h. ENERGY STAR calls this the number to look for when choosing a unit.
Kitchen, 12 by 14 feet
Sunny home office, 12 by 15 feet, 3 people
Shaded living room, 20 by 25 feet, 4 people
Rounding down means small adjustments can disappear. A very sunny 120 sq ft bedroom works out to 5,000 × 1.10 = 5,500 BTU/h, which rounds back to 5,000. A metric room of 4 m × 5 m is 20 m², or 20 ÷ 0.09290304 = 215.28 sq ft, which falls in the 6,000 BTU/h row.
BTU to Tons, Watts and Kilowatts
A BTU (British thermal unit) is a unit of heat energy. NIST Special Publication 811 lists the International Table BTU as 1,055.056 joules and 1 BTU per hour as 0.2930711 watt. One ton of refrigeration is defined as 12,000 BTU per hour, which NIST gives as 3,516.853 watts. Converting between the units lets you compare a room unit labeled in BTU/h with a central system quoted in tons or a heat pump rated in kilowatts.
| BTU/h | Tons | Kilowatts | ENERGY STAR room area |
|---|---|---|---|
| 5,000 | 0.42 | 1.47 | 100 up to 150 sq ft |
| 6,000 | 0.5 | 1.76 | 150 up to 250 sq ft |
| 8,000 | 0.67 | 2.34 | 300 up to 350 sq ft |
| 10,000 | 0.83 | 2.93 | 400 up to 450 sq ft |
| 12,000 | 1 | 3.52 | 450 up to 550 sq ft |
| 14,000 | 1.17 | 4.10 | 550 up to 700 sq ft |
| 18,000 | 1.5 | 5.28 | 700 up to 1,000 sq ft |
| 24,000 | 2 | 7.03 | 1,400 up to 1,500 sq ft |
| 30,000 | 2.5 | 8.79 | 1,500 up to 2,000 sq ft |
| 36,000 | 3 | 10.55 | Above the chart; use Manual J |
The kilowatt figure is cooling output, not electricity use. A 12,000 BTU/h unit delivers 3.52 kW of cooling but draws far less power from the outlet, because an air conditioner moves heat instead of making it. The power draw tile in the results divides capacity by the efficiency rating, as the next section explains.
BTU per Square Foot and the 12,000 BTU Question
Dividing each chart capacity by its area range shows how many BTU per square foot ENERGY STAR assumes. Small rooms need the most per square foot: the 5,000 BTU/h row covers 33.3 to 50 BTU/h per sq ft. Mid sized rooms in the 8,000 BTU/h row need 22.9 to 26.7, and the largest row, 34,000 BTU/h for 2,000 up to 2,500 sq ft, works out to 13.6 to 17. A single "20 BTU per square foot" rule therefore oversizes large spaces and undersizes small ones.
A 12,000 BTU unit, one ton, matches 450 up to 550 sq ft on the chart, so a 500 sq ft room needs 12,000 BTU/h under normal conditions. Make that room very sunny and the total becomes 13,200, rounded down to 13,000 BTU/h; use it as a kitchen and it becomes 16,000. The Department of Energy warns that the popular rule of one ton per 400 to 500 square feet, which is 24 to 30 BTU/h per sq ft, ignores local climate and house efficiency, and says one ton can condition 800 to 1,000 square feet of a very efficient house.
Why a Bigger Air Conditioner Is Not Better
An air conditioner removes two things from a room: heat and moisture. Moisture condenses on the cold coil only while the unit runs, and it needs time to drip away. ENERGY STAR explains that an oversized unit cools the room before it has a chance to remove the humidity, leaving the air cool but damp and clammy.
Oversized units also short cycle, running in brief bursts. An ENERGY STAR fact sheet on right sized air conditioners states that efficiency is low at startup and reaches its peak after about 10 minutes, and that bursts of cold air can satisfy the thermostat before the rest of the house is cool. A Department of Energy fact sheet adds that frequent starts accelerate wear, and cites a Florida study where units oversized by 50 percent or more used about 9 percent more cooling electricity.
Undersizing has the opposite problem: the unit runs constantly on hot afternoons and may not reach the thermostat setting. If a room has unusual loads such as a wall of west facing glass, a home gym or a server rack, a professional load calculation will give a better answer than any chart.
Manual J: When a Room Chart Is Not Enough
The ENERGY STAR chart is meant for a single room air conditioner. For a central air conditioner, a heat pump, a furnace or a mini split system with several indoor heads, the accepted method is ACCA Manual J. The Air Conditioning Contractors of America describes the 8th edition as the national ANSI recognized standard for producing equipment sizing loads for homes, and states that a proper load calculation is required by national building codes and most state and local jurisdictions.
Manual J works room by room. It accounts for wall, ceiling and floor insulation, window area and orientation, shading, air leakage, duct location and the local design temperatures. The equipment is then chosen to match those loads, a step ACCA covers in Manual S. The Department of Energy advises homeowners to beware of casual sizing estimates based on floor area or contractor experience, and its consumer guide says contractors should use Manual J.
For mini split sizing, use this calculator for each room that will get its own indoor head, then let the installer confirm the total with a load calculation before choosing the outdoor unit.
Heating BTU Estimate
Heating mode multiplies floor area by a heating load factor in BTU/h per square foot. The factors come from a quick load calculation table that Northwest utilities created for contractors sizing ductless heat pumps for a main living area. The factor grows as wall insulation gets worse and as the winter design temperature, the cold outdoor temperature a system is designed for, gets lower.
| Wall insulation | Below −10 °F | −10 to 5 °F | 5 to 20 °F | Above 20 °F |
|---|---|---|---|---|
| No wall insulation | 47 | 41 | 35 | 27 |
| Insulated 2x4 walls | 25 | 22 | 19 | 14 |
| Insulated 2x6 walls | 18 | 15 | 13 | 10 |
| New construction, post 2012 | 16 | 14 | 12 | 9 |
The range in these examples, from 2,700 to 14,100 BTU/h for the same floor area, shows why insulation and climate matter more than size alone. Treat the heating result as a starting point for a conversation with a contractor, not as an equipment size.
EER, CEER and SEER2 Explained
Capacity tells you how much cooling you get. Efficiency tells you how much electricity it costs. All three ratings below are expressed in BTU per watt hour, and a higher number means less electricity for the same cooling.
- EER (energy efficiency ratio) is cooling capacity in BTU/h divided by power input in watts at a fixed outdoor temperature of 95 °F. The Department of Energy describes it as peak load efficiency.
- CEER (combined energy efficiency ratio) is the rating used for room air conditioners under the DOE test procedure, expressed in BTU per watt hour. ENERGY STAR certifies room units by minimum CEER.
- SEER2 (seasonal energy efficiency ratio 2) applies to central air conditioners and heat pumps. ENERGY STAR defines it as the total heat removed during the cooling season divided by the electricity used over the same season. Since 2023, ratings use the SEER2 and EER2 test conditions, which DOE notes are measured differently from the older SEER and EER.
To estimate power draw, divide capacity by the rating: 12,000 BTU/h at CEER 12 is about 1,000 W, and at EER 10 it is about 1,200 W. The Department of Energy consumer guide suggests looking for a room unit with an EER of 10 or higher.
How to Use the Calculator
- Mode and units. Pick cooling or heating, then feet or meters.
- Room size. Enter length and width, or type the total floor area to override them.
- Ceiling. Used for the volume tile and the tall ceiling note.
- Sun, people and room type. Apply the ENERGY STAR cooling adjustments.
- Insulation and design temperature. Pick the heating factor.
- Efficiency. Change the CEER to match a model and update the power draw.
Common Mistakes
- Using inches as decimals. 12 ft 6 in is 12.5 ft, not 12.6 ft.
- Counting guests. Count only people who regularly use the room.
- Rounding up. The method rounds the total down to the nearest 1,000.
- Adding open areas twice. Measure an open plan space once as one area.
- Reading kW as power use. Cooling kilowatts are output, not the electric bill.
- Sizing a house from one room. Whole homes need Manual J.
Frequently Asked Questions
How many BTU do I need for my room?
Measure the floor area in square feet and find it on the ENERGY STAR sizing chart, then adjust. Reduce the chart value by 10 percent for a heavily shaded room, add 10 percent for a very sunny room, add 600 BTU per hour for each regular occupant above two and add 4,000 BTU per hour for a kitchen. Round the total down to the nearest thousand. A 15 by 20 foot living room is 300 square feet, which falls in the 300 up to 350 row, so it needs about 8,000 BTU per hour with normal sun and two people.
How many square feet does a 12,000 BTU air conditioner cool?
On the ENERGY STAR room air conditioner chart, 12,000 BTU per hour matches 450 up to 550 square feet, about 41.8 to 51.1 square meters, under normal conditions. That is one ton of cooling, or 3.52 kilowatts. Sun, people and a kitchen change the match: a very sunny 500 square foot room works out to 13,200 BTU per hour, rounded down to 13,000, and the same room used as a kitchen needs 16,000. The Department of Energy notes that the common rule of one ton per 400 to 500 square feet ignores local climate and how efficient the house is.
How do you convert BTU to tons and watts?
One ton of refrigeration is defined as 12,000 BTU per hour, so divide BTU per hour by 12,000 to get tons: 18,000 BTU per hour is 1.5 tons and 24,000 is 2 tons. NIST lists 1 BTU per hour as 0.2930711 watt, so multiply by 0.2930711 to get watts: 12,000 BTU per hour is 3,516.85 watts, or 3.52 kilowatts. Going the other way, 1 kilowatt equals 3,412.14 BTU per hour. These figures describe cooling or heating output, not the electricity the unit draws from the outlet.
What happens if an air conditioner is too big for the room?
An oversized air conditioner cools the air quickly and shuts off before it removes much moisture, so the room feels cool but damp and clammy, as ENERGY STAR explains. These short runs, called short cycling, also waste energy: an ENERGY STAR fact sheet notes that air conditioners reach peak efficiency after about 10 minutes of running. A Department of Energy fact sheet adds that frequent starts increase wear and cites a Florida study in which units oversized by 50 percent or more used about 9 percent more cooling electricity. Choose the size the chart gives, not a bigger one.
What is Manual J and do I need one?
Manual J is the residential load calculation procedure published by the Air Conditioning Contractors of America (ACCA). Its 8th edition is the ANSI recognized standard for the heating and cooling loads of houses, condos, townhouses and manufactured homes, and ACCA states that national building codes and most states and local jurisdictions require a proper load calculation. It accounts for insulation, windows, orientation, air leakage, ducts and local design temperatures. Ask a qualified contractor for one before buying a central air conditioner, heat pump, furnace or multi room mini split system. A room chart suits only a single room unit.
How many BTU do I need to heat a room?
For a quick estimate, multiply the floor area by a heating load factor that depends on wall insulation and your winter design temperature. A table that Northwest utilities created for ductless heat pump contractors lists factors from 9 BTU per hour per square foot for post 2012 construction in mild winters up to 47 for uninsulated walls below minus 10 °F. A 300 square foot room with insulated 2x4 walls and a 5 to 20 °F design temperature needs about 300 × 19 = 5,700 BTU per hour, or 1.67 kilowatts. Confirm whole home heating with a Manual J calculation.
Sources & References
- ENERGY STAR (U.S. EPA). Room Air Conditioners, properly sized room air conditioners: cooling capacity chart, 8 foot ceiling basis and sun, occupant and kitchen adjustments. energystar.gov
- ENERGY STAR. How To Choose the Right Sized Window AC, Feb 2, 2024 (round the adjusted total down to the nearest thousand). energystar.gov
- ENERGY STAR. Right Sized Air Conditioners fact sheet, 2005 (short cycling, peak efficiency after about 10 minutes). energystar.gov
- U.S. Department of Energy. Right Size Heating and Cooling Equipment, Building America technology fact sheet (oversizing effects, Manual J and Manual S). energy.gov
- U.S. Department of Energy. Energy Saver: Consumer Guide to Home Heating and Cooling, Aug 2021 (one ton per 400 to 500 sq ft rule, Manual J, room unit EER). energy.gov
- U.S. Department of Energy. 2023 Central Air Conditioner Standards FAQ, Oct 2022 (EER, SEER, EER2 and SEER2). energy.gov
- ENERGY STAR. Tax Credit Definitions (SEER2 and EER2 definitions). energystar.gov
- Air Conditioning Contractors of America. Manual J Residential Load Calculation, 8th Edition (ANSI/ACCA 2 Manual J 2016). acca.org
- NIST. Guide to the SI (Special Publication 811), Appendix B.8: Factors for Units Listed Alphabetically (BTU, BTU per hour and ton of refrigeration). nist.gov
- BetterBuilt NW. Calculating Heating and Cooling Requirements, Tech Smart HVAC Sales (heating load factors by insulation and design temperature). betterbuiltnw.com
Results are rule of thumb estimates and do not replace a load calculation. Consult a licensed HVAC contractor for equipment sizing. Every number on this page was recomputed with the same formulas the calculator uses. Methodology reviewed: 2026.