BTU Calculator

By Calxy · Calculation methodology

Estimate how many BTU per hour a room loses in cold weather from its size, insulation, windows and air leakage, with every heat-loss term shown so you can check it.

Printed from Calxy · https://www.calxy.net/other/btu-calculator

Estimated heating load2,616 Btu/h
ft

0.001 – 100000

ft

0.001 – 100000

ft

0.001 – 1000

ft

Add up only the walls that face outdoors or an unheated space. A corner room with two outside walls of 15 ft and 12 ft is 27 ft.

ft²

0 – 10000000

ft²·°F·h/Btu

Whole-wall R-value. Framing lowers it below the batt label, so an R-13 cavity wall is roughly R-11 overall; replace with your own figure.

Btu/(h·ft²·°F)

From the window's NFRC label. Older single glazing can be above 1.0.

ft²·°F·h/Btu

Insulation R-value of the ceiling or roof above the room.

ACH

Outdoor air leaking in or ventilated per hour, as a fraction of room volume. A blower-door test gives the real figure.

°F

-100 – 200

°F

-100 – 200

Estimated heating load

2,616 Btu/h

  • Heating load0.77 kW
  • Temperature difference60.0 °F
  • Room volume1,440 ft³
  • Air leakage12.0 cfm
Heat loss by component
ComponentArea (ft²)FormulaBtu/h
Walls186A × ΔT ÷ R = 186 × 60 ÷ 111015
Windows and doors30U × A × ΔT = 0.3 × 30 × 60540
Ceiling180A × ΔT ÷ R = 180 × 60 ÷ 38284
Air leakage—1.08 × CFM × ΔT = 1.08 × 12 × 60778
Total2616

Steady-state heating estimate from conduction and air leakage only. It ignores floor and below-grade losses, thermal bridges, duct losses and sun or appliance heat, and it does not estimate cooling loads (solar and internal gains dominate those). Size real equipment with a full load calculation such as ACCA Manual J.

How this was calculated

ΔT = 70 − 10 = 60 °F.

Walls: net area 27 ft × 8 ft − 30 ft² = 186 ft²; Q = 186 × 60 ÷ 11 = 1,014.545455 Btu/h.

Windows: Q = 0.3 × 30 × 60 = 540 Btu/h.

Ceiling: Q = 180 × 60 ÷ 38 = 284.2105263 Btu/h.

Air leakage: CFM = 1,440 ft³ × 0.5 ÷ 60 = 12; Q = 1.08 × 12 × 60 = 777.6 Btu/h.

Total = 2,616.355981 Btu/h = 0.7667782473 kW.

The heat-loss method

Heat escapes through each surface by conduction at a rate Q = A × ΔT ÷ R, in Btu/h, where A is the area in ft², ΔT the indoor–outdoor temperature difference in °F, and R the surface's thermal resistance in ft²·°F·h/Btu. Windows are rated with a U-factor instead, Q = U × A × ΔT, where U = 1 ÷ R.

Outdoor air leaking in must also be warmed: Q = 1.08 × CFM × ΔT. The 1.08 comes from standard air, 0.075 lb/ft³ × 0.24 Btu/(lb·°F) × 60 minutes per hour. Leakage in CFM is room volume × air changes per hour ÷ 60.

Heat-loss terms
TermFormula (Btu/h)
Wallsnet wall area × ΔT ÷ R
Windows and doorsU × area × ΔT
Ceilingceiling area × ΔT ÷ R
Air leakage1.08 × volume × ACH ÷ 60 × ΔT

Worked example

A 15 × 12 ft corner room with 8 ft ceilings, 27 ft of exterior wall, 30 ft² of windows, R-11 walls, U-0.30 windows, an R-38 attic above and 0.5 air changes per hour, heated to 70 °F when it is 10 °F outside (ΔT = 60 °F).

Walls: (27 × 8 − 30) × 60 ÷ 11 = 1,015. Windows: 0.30 × 30 × 60 = 540. Ceiling: 180 × 60 ÷ 38 = 284. Air: 1,440 × 0.5 ÷ 60 = 12 CFM, × 1.08 × 60 = 778. Total about 2,616 Btu/h, or 0.77 kW.

What it leaves out

This is a steady-state heating estimate. It ignores floor and basement losses, thermal bridging beyond the whole-wall R-value you enter, duct losses, and free heat from sun, people and appliances. It does not estimate air-conditioning loads, which depend heavily on solar gain and humidity.

The default R-values, U-factor and air-change rate are example inputs, not measurements of your home. Replace them with your own figures, and size real heating equipment with a full load calculation such as ACCA Manual J.

How to use the BTU Calculator

Describe the room's size, insulation and the temperatures, then read the load.

  1. Enter room size

    Length, width, ceiling height and the length of walls facing outdoors.

  2. Enter insulation

    Wall and ceiling R-values and the window U-factor from labels or specs.

  3. Set leakage and temperatures

    Air changes per hour, indoor temperature and outdoor design temperature.

  4. Read the load

    See total Btu/h, kW and each heat-loss term in the table.

References

Frequently asked questions

Why not just use BTU per square foot?

Square-foot rules hide the assumptions about insulation, windows, leakage and climate. Working from R-values and temperature difference shows exactly where the heat goes and scales correctly for your conditions.

What outdoor temperature should I use?

Use your local winter design temperature, the cold condition that is only exceeded a small fraction of hours in a typical year, rather than the record low.

How do I convert BTU/h to watts?

Multiply by 0.29307. A 2,616 Btu/h load is about 767 W, so a 1,000 W heater would cover it with some margin.

Last updated . Results are estimates for informational purposes only.