CALTRX

BTU Calculator

Convert BTU to watts, kilowatts, and other energy units. Includes room cooling BTU estimator and HVAC sizing guide for air conditioners and heaters.

BTU

Enter your values above to see the results.

Tips & Notes

  • ✓HVAC sizing rule of thumb: 20 BTU per square foot of living space for moderate climates. Add 10% for sunny rooms, add 600 BTU for each person regularly in the room beyond 2, add 4,000 BTU if the space is a kitchen.
  • ✓Air conditioner capacity is rated in BTU/hr or tons: 1 ton = 12,000 BTU/hr. A 2-ton AC = 24,000 BTU/hr. The "ton" refers to the cooling capacity of melting one ton of ice per day.
  • ✓Energy unit conversions: 1 kWh = 3,412 BTU. A 1,500 W space heater running 1 hour produces 1.5 kWh = 5,118 BTU of heat. A natural gas furnace burning 1 cubic foot of gas (1,020 BTU content) delivers 80-96% as useful heat.
  • ✓Cooling loads require both sensible heat (temperature change) and latent heat (humidity) capacity. In humid climates, latent load can be 30-50% of total cooling load — size equipment accordingly.
  • ✓SEER (Seasonal Energy Efficiency Ratio) for AC: higher is more efficient. SEER 13 is minimum, SEER 16-20 is good, SEER 20+ is high efficiency. Annual energy cost = (BTU/hr × hours/yr) / (SEER × 1,000) in kWh.

Common Mistakes

  • ✗Confusing BTU (energy) with BTU/hr (power) — a furnace rated at 80,000 BTU/hr delivers that heat rate continuously, not total energy. BTU alone is a quantity of energy; BTU/hr is a rate of heat transfer.
  • ✗Undersizing AC for kitchen or sunny rooms — kitchens generate 4,000+ BTU/hr from cooking appliances. South-facing rooms with large windows absorb solar radiation equivalent to hundreds of watts. Standard room-area calculations underestimate these loads.
  • ✗Ignoring insulation quality in BTU requirements — a poorly insulated room may need 2-3× the BTU capacity of a well-insulated room of the same area. R-value of walls and ceiling significantly affects heat loss/gain calculations.
  • ✗Using cooling BTU ratings for heating — AC units designed for cooling can run in reverse for heating (heat pumps) but at reduced efficiency in very cold weather. A 12,000 BTU/hr cooling unit may deliver only 9,000-10,000 BTU/hr heating at 35°F outdoor temperature.
  • ✗Forgetting latent heat load in humid climates — a 10,000 BTU/hr AC in humid Houston removes less sensible heat than the same unit in dry Phoenix because more capacity goes to moisture removal. Select equipment rated for your climate zone.

BTU Calculator Overview

BTU (British Thermal Unit) is the fundamental energy unit for HVAC, heating, and heat transfer calculations in the United States. Whether sizing an air conditioner, comparing heater outputs, or converting between energy units, BTU provides the common language of thermal engineering.

BTU energy conversions:

1 BTU = 1,055.06 J = 0.2931 Wh = 252.16 cal | 1 kWh = 3,412 BTU | 1 ton AC = 12,000 BTU/hr
EX: Convert 5,000 BTU/hr window AC to watts → 5,000 × 0.2931 = 1,465 W. Running 8 hr/day: Energy = 1.465 kW × 8 = 11.72 kWh/day. At $0.15/kWh: cost = $1.76/day, $52.70/month.
HVAC sizing — cooling BTU/hr by room area:
BTU/hr (cooling) = Room Area (sq ft) × 20 (moderate climate) | Adjusted for sun, occupants, kitchen load
EX: 400 sq ft living room, south-facing windows → Base = 400 × 20 = 8,000 BTU/hr → +10% sun = 8,800 → +2 extra people × 600 = 9,200 BTU/hr → select 10,000 BTU/hr unit
Energy unit conversion table:
UnitBTUWhkWhJoulesCalories
1 BTU10.29310.0002931,055.1252.2
1 Wh3.41210.0013,600860.4
1 kWh3,4121,00013,600,000860,420
1 therm100,00029,30729.31105,480,40025,200,000
1 ton-hr (AC)12,0003,5173.51712,660,7203,024,000
AC sizing guide by room area:
Room AreaBTU/hr NeededAC CapacityCommon Application
100–150 sq ft2,000–3,0005,000 BTU/hrSmall bedroom
150–250 sq ft3,000–5,0006,000 BTU/hrStandard bedroom
250–350 sq ft5,000–7,0008,000 BTU/hrLarge bedroom, studio
350–450 sq ft7,000–9,00010,000 BTU/hrLiving room
450–550 sq ft9,000–11,00012,000 BTU/hr (1 ton)Large open plan
700–1,000 sq ft14,000–20,00018,000–24,000 BTU/hrCentral AC zone
The efficiency of HVAC equipment has improved dramatically: a window AC from the 1990s might have an EER (Energy Efficiency Ratio) of 7, while modern units reach EER 12-15. This means for the same BTU/hr of cooling, a modern unit uses roughly half the electricity. Over a summer of 1,000 operating hours at $0.15/kWh, upgrading a 10,000 BTU/hr unit from EER 7 to EER 14 saves (10,000/7 − 10,000/14) × 1,000 × 0.15 / 1,000 = $107 in electricity annually.

Frequently Asked Questions

BTU (British Thermal Unit) is the amount of heat energy required to raise the temperature of one pound of water by one degree Fahrenheit. In HVAC, BTU/hr measures heating or cooling capacity. A 12,000 BTU/hr air conditioner can remove 12,000 BTU of heat per hour from a space. A 60,000 BTU/hr furnace delivers heat at that rate. To size equipment: estimate the heat gain (cooling) or heat loss (heating) of the space in BTU/hr, then select equipment with at least that capacity. Rule of thumb: 20 BTU/hr per square foot for moderate climates.

1 BTU/hr = 0.2931 watts. 1 watt = 3.412 BTU/hr. 1 kW = 3,412 BTU/hr. 1 ton of refrigeration = 12,000 BTU/hr = 3.517 kW. Examples: a 24,000 BTU/hr (2-ton) AC = 24,000 × 0.2931 = 7,034 W = 7.03 kW electrical equivalent. A 1,500 W space heater = 1,500 × 3.412 = 5,118 BTU/hr. A 100,000 BTU/hr gas furnace = 100,000 × 0.2931 = 29,310 W = 29.3 kW of heat output.

Basic sizing formula: BTU/hr = Room Area (sq ft) × 20. Adjust for: add 10% for very sunny rooms or high ceilings (above 8 ft); add 600 BTU/hr per person beyond 2 occupants; add 4,000 BTU/hr for kitchen use; reduce 10% if room is heavily shaded. Examples: 150 sq ft bedroom → 3,000 BTU/hr → use 5,000 BTU/hr window unit (next available size). 350 sq ft living room → 7,000 BTU/hr → use 8,000 BTU/hr unit. 500 sq ft open plan → 10,000 BTU/hr (plus kitchen load if applicable).

BTU/hr is the capacity — how much heat the unit can move per hour. SEER (Seasonal Energy Efficiency Ratio) is the efficiency — how much cooling capacity you get per unit of electrical energy consumed. SEER = Total BTU of cooling / Total Watt-hours of electricity over the season. Higher SEER means lower operating cost for the same cooling capacity. A 24,000 BTU/hr unit with SEER 14 uses 24,000/14 = 1,714 W of electricity running at full load. The same capacity at SEER 20 uses only 1,200 W — 30% less electricity for the same cooling.

Heat loss calculation: BTU/hr = (Surface Area × U-value × Temperature Difference) + Infiltration losses. Simplified rule: BTU/hr per sq ft = 0.133 × degrees of temperature difference between inside (68°F) and outside design temperature. Example: 300 sq ft room, outside design temp 10°F, inside 68°F → ΔT = 58°F → BTU/hr = 300 × 0.133 × 58 = 2,315 BTU/hr for an average-insulated room. Double this for poorly insulated rooms; halve for well-insulated. A 1,500 W electric space heater (5,118 BTU/hr) can heat a 150-300 sq ft space depending on insulation.

One ton of refrigeration = 12,000 BTU/hr of cooling capacity. The unit comes from the era of ice-based cooling: melting one ton (2,000 lbs) of ice over 24 hours absorbs 2,000 × 144 BTU = 288,000 BTU total ÷ 24 hours = 12,000 BTU/hr. Today it remains the standard commercial HVAC sizing unit. Common residential AC sizes: 1.5 ton (18,000 BTU/hr) for 600-900 sq ft; 2 ton (24,000) for 900-1,200 sq ft; 3 ton (36,000) for 1,500-2,100 sq ft; 4 ton (48,000) for 2,100-2,800 sq ft; 5 ton (60,000) for 2,800-3,500 sq ft.