Heat Loss Calculator

Estimate the heat loss and radiator or boiler size needed for a room from its dimensions, insulation level and target temperature.

Part of:Heating & HVAC hub
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How to Use This Calculator

How to Use This Calculator

  1. Units – Choose UK (room in metres, result in kW) or US (room in feet, result in BTU). Temperatures are °C for UK and °F for US.
  2. Room dimensions – Enter the length, width and ceiling height of the room.
  3. Insulation level – Poor (older, uninsulated), average (typical modern home) or good (well-insulated / new build).
  4. Inside & outside temperature – Your desired room temperature and the coldest design outdoor temperature.

This is a quick volume-based estimate of how much heat a room loses, and therefore the radiator or heater output you need. It gives a sensible starting figure; for a full design a room-by-room heat loss survey (SAP / Manual J) is recommended.

Common Questions

How accurate is a volume-based heat loss estimate?

It gives a good ballpark figure for sizing a radiator or heater. A detailed survey accounts for each wall, window, floor and air change, but the volume method is fine for quick planning.

What inside temperature should I use?

Living rooms are typically designed for 21°C (70°F), bedrooms around 18°C (64°F) and bathrooms 22–23°C. Use the temperature you actually want to maintain.

What outside temperature should I use?

Use the coldest design temperature for your area – around -1 to -3°C (28–30°F) for much of the UK, or your local winter design temperature in the US. This ensures the heating copes on the coldest days.

How do I convert kW to BTU?

1 kW equals about 3,412 BTU per hour. The calculator shows both so you can size radiators (often rated in watts/BTU) or heat pumps and furnaces (often rated in kW or BTU).

Does better insulation really cut heat loss this much?

Yes. A well-insulated room can lose less than half the heat of a poorly insulated one of the same size, which is why the insulation level has such a large effect on the result.

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