BTU Calculator
Find the exact BTU for your AC, furnace, mini split, or radiator — instantly. Based on ACCA Manual J methodology.
- Air conditioner & cooling BTU
- Home heating & furnace sizing
- Mini split & heat pump BTU
- Radiator heat output calculator
- BTU unit conversions (kW, watts, tons)
Quick BTU Estimator
All BTU Calculators
Choose the right tool for your heating or cooling need. Each calculator uses specialized formulas for accurate sizing.
AC BTU Calculator
Calculate the exact BTU for window AC, portable AC, central air, and ductless systems. Enter room size, climate & insulation.
Heating BTU Calculator
Size your gas furnace, electric heater, or boiler. Uses climate zone, insulation level, and AFUE efficiency for accurate results.
Mini Split BTU Calculator
Single-zone and multi-zone mini split sizing. Works for Mitsubishi, Daikin, Mr. Cool, LG, Fujitsu & all major brands.
Radiator BTU Calculator
Find the BTU output for panel, column, electric & cast iron radiators. Supports UK metric (ΔT50) and imperial measurements.
BTU Conversion Calculator
Convert BTU to watts, kW, tons, joules, CFM, therms & horsepower. Bidirectional instant conversion for all energy units.
Special Spaces BTU
Specialized formulas for garages, server rooms, grow rooms, walk-in coolers, wine cellars & workshops.
Fireplace & Pool BTU
Size gas fireplaces, wood stoves, outdoor fire pits, pool heaters & hot tubs with appliance-specific BTU formulas.
BTU Energy Cost Calculator
Compare cost per BTU for gas, electric, propane & oil. Calculate annual heating bills and fuel switching savings.
How to Calculate BTU
Complete guide: BTU formula, per square foot charts, step-by-step examples for AC, heating, and mini splits.
What is a BTU?
- BTU — British Thermal Unit
- The amount of heat needed to raise 1 pound of water by 1°F. In HVAC, BTU/hr measures how much heating or cooling an appliance delivers per hour.
Key BTU Conversions
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❄️
1 Ton of AC = 12,000 BTU/hr Standard US measurement for air conditioner capacity
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⚡
1 kW = 3,412 BTU/hr Converts electric power to heat energy for comparisons
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🌡️
1 Watt = 3.412 BTU/hr Used to calculate server room and electronics heat loads
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🔥
1 Therm = 100,000 BTU Used for natural gas billing and furnace sizing
How Many BTU Do I Need?
The answer depends on room size, climate zone, and insulation. Use these starting rules:
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❄️
Cooling (AC): ~20 BTU per sq ft 500 sq ft room → ~10,000 BTU. Adjust for climate and insulation.
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🔥
Heating (Furnace): 30–60 BTU per sq ft Hot climates: 30 BTU/sq ft. Very cold climates: 60 BTU/sq ft.
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Mini Split: Same as AC + 10–20% for heating Size up slightly in heating-dominant climates for efficiency.
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🌡️
Radiator: 60–100 watts (205–341 BTU) per m² UK standard for panel radiators at ΔT50 system temperature.
BTU Requirements by Room Size (2026)
Quick reference for cooling and heating BTU. Adjust for your climate using the calculators above.
| Room Size | Cooling BTU (AC) | Heating BTU | AC Tonnage | Suggested Unit |
|---|---|---|---|---|
| 100–150 sq ft | 5,000 BTU | 5,000–7,500 BTU | 0.4 ton | Small window AC |
| 150–250 sq ft | 6,000 BTU | 7,500–12,500 BTU | 0.5 ton | Window AC / portable |
| 250–350 sq ft | 8,000 BTU | 12,500–17,500 BTU | 0.67 ton | Window AC |
| 350–500 sq ft | 10,000 BTU | 17,500–25,000 BTU | 0.83 ton | 1-ton mini split |
| 500–700 sq ft | 12,000 BTU | 25,000–35,000 BTU | 1 ton | 1-ton mini split |
| 700–1,000 sq ft | 18,000 BTU | 35,000–50,000 BTU | 1.5 tons | 1.5-ton mini split |
| 1,000–1,500 sq ft | 24,000 BTU | 50,000–75,000 BTU | 2 tons | 2-ton central AC |
| 1,500–2,000 sq ft | 30,000 BTU | 75,000–100,000 BTU | 2.5 tons | 2.5-ton central AC |
| 2,000–2,500 sq ft | 36,000 BTU | 100,000–125,000 BTU | 3 tons | 3-ton central AC |
| 2,500–3,500 sq ft | 48,000 BTU | 125,000–175,000 BTU | 4 tons | 4-ton central AC |
| 3,500–5,000 sq ft | 60,000 BTU | 175,000–250,000 BTU | 5 tons | 5-ton central AC |
* Cooling: 20 BTU/sq ft baseline (moderate climate, average insulation). Heating: 35–50 BTU/sq ft. See the full BTU chart and formula →
BTU Calculator — Frequently Asked Questions
Direct answers to the most common BTU questions.
How many BTU do I need per square foot?
Cooling (AC): approximately 20 BTU per square foot as a starting point.
Heating (furnace): depends on climate zone:
- Hot climates (FL, TX, AZ): 25–35 BTU/sq ft
- Mixed climates (Midwest, SE): 40–45 BTU/sq ft
- Cold climates (Northeast): 50–55 BTU/sq ft
- Very cold (Canada, Mountain): 55–60 BTU/sq ft
Use our Heating BTU Calculator for a climate-adjusted result.
How do I calculate BTU for a room?
Basic formula: Room sq ft × 20 = Base BTU (cooling)
Then adjust for:
- Ceiling above 8 ft: multiply by (height ÷ 8)
- Sunny/south-facing room: add 10%
- Shaded/north-facing room: subtract 10%
- More than 2 occupants: add 600 BTU each
- Kitchen: add 4,000 BTU
- Poor insulation: add 15–25%
Full guide: How to Calculate BTU →
What size AC do I need for 1,000 sq ft?
A 1,000 sq ft space typically needs 18,000–21,000 BTU (1.5 tons) in a moderate climate.
- Hot climate (TX, FL): up to 24,000 BTU (2 tons)
- Cool climate (PNW, NE): 15,000–18,000 BTU
- High ceilings (10 ft+): add 10–25%
- Poor insulation: add 15–25%
Get your exact result: AC BTU Calculator →
How many BTU is 1 ton of AC?
1 ton of air conditioning = 12,000 BTU/hr.
- 1.5 tons = 18,000 BTU/hr
- 2 tons = 24,000 BTU/hr
- 2.5 tons = 30,000 BTU/hr
- 3 tons = 36,000 BTU/hr
- 4 tons = 48,000 BTU/hr
- 5 tons = 60,000 BTU/hr
Convert instantly: BTU to Tons Calculator →
How to convert BTU to watts or kW?
BTU/hr → Watts: multiply by 0.2931 (12,000 BTU = 3,517 W)
BTU/hr → kW: multiply by 0.000293 (12,000 BTU = 3.52 kW)
Watts → BTU/hr: multiply by 3.412 (1,000 W = 3,412 BTU/hr)
kW → BTU/hr: multiply by 3,412 (1 kW = 3,412 BTU/hr)
What BTU mini split do I need for 500 sq ft?
A 500 sq ft room typically needs a 12,000 BTU (1-ton) mini split for cooling in a moderate climate.
For heating-dominant climates (Zone 4–6), size up to 15,000–18,000 BTU to maintain efficiency at low outdoor temperatures.
What size furnace do I need for a 2,000 sq ft home?
A 2,000 sq ft home typically needs a 60,000–100,000 BTU furnace:
- Warm climate (Zone 2–3): 60,000 BTU (60K)
- Mixed climate (Zone 4): 80,000 BTU (80K)
- Cold climate (Zone 5–6): 100,000 BTU (100K)
How do I calculate BTU for a garage or server room?
Garage: An uninsulated 2-car garage (400–500 sq ft) needs 24,000–36,000 BTU for heating in cold climates.
Server room: Add up total server wattage, then multiply by 3.412 to get BTU/hr heat output that must be removed.
Why Use BTUCalc
Formulas based on the ACCA Manual J standard — the method used by licensed HVAC contractors.
All calculations run in your browser. No account, no data stored, works offline on any device.
Fully responsive. Use it on your phone at the hardware store or on desktop for detailed planning.
Authoritative HVAC References
Sources used in our BTU calculation formulas.