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Room Heating Calculator

Work out the heating a room needs — from its size, the temperatures and the insulation — in watts, kilowatts and BTU per hour.

Heating a room is its volume times the temperature rise times how fast it loses heat. This calculator is that multiplication, with the insulation factor yours to set.

Enter the room and the temperatures; the heater size follows.

The temperature you want inside.
The coldest outside temperature to design for.
About 1.2 for a well-insulated room, 1.8 typical, 2.5 for poor insulation. It does not cover draughts or large windows separately — adjust it for those.

Formula

heat required (W) = room volume (m³) × temperature rise (°C) × heat loss factor (W/m³/°C) temperature rise = inside temperature − outside temperature BTU/h = watts × 3.41214 The heat loss factor is user-entered: about 1.2 W/m³/°C well-insulated, 1.8 typical, 2.5 poorly insulated. It does not cover draughts, large windows or high ceilings separately.

All lengths are converted to a single unit before the calculation: 1 ft = 0.3048 m, 1 in = 25.4 mm, 1 ft² = 0.09290304 m², 1 yd² = 9 ft². Material quantities are rounded up — you cannot buy part of a tile, plank or bag.

How to use this calculator

  1. Enter the room length, width and height.
  2. Enter the inside and outside temperatures.
  3. Enter the heat loss factor for your insulation.
  4. Read the heat required in watts, kilowatts and BTU/h.

Worked example

Room4 m × 5 m × 2.7 m = 54 m³
Temperature rise21 °C − (−1 °C) = 22 °C
Heat loss factor1.8 W/m³/°C (typical)
Heat required54 × 22 × 1.8 = 2,138.4 W = 2.14 kW
BTU/h2,138.4 × 3.41214 = 7,296.52 BTU/h

Check it by hand: every step above uses the formula in the box. If your numbers match the example inputs but the calculator gives a different answer, please report it via the contact page.

Frequently asked questions

What heat loss factor should I use?

About 1.2 W/m³/°C for a well-insulated room, 1.8 for a typical room, 2.5 for poor insulation. If the room has draughts, large windows or a high ceiling, use a higher figure.

What outside temperature should I design for?

The coldest temperature your area usually reaches in winter — not the record low. A heating engineer or local climate data will give you the design temperature.

How many watts per square metre is that?

It depends on the ceiling height, but roughly 70–100 W/m² for a typical room. This calculator works by volume, which is more accurate for rooms with different heights.

Is this a substitute for heating design?

No. This is an estimate for planning. Heating system sizing requires qualified professionals and local code compliance — see the safety notice.

Related calculators

Estimates only. Results are for planning. Measure twice, confirm box coverage and tile sizes on the packaging you buy, and follow the manufacturer's instructions. See the disclaimer.