Radiator BTU Calculator

Calculate the BTU and wattage needed for your radiators based on room size, glazing and insulation.

Last updated: April 2026 · Source: GOV.UK — Building Regulations Approved Document L

BTU Required

7,344

Watts Required

2,153 W

Room Volume

48.0 m³

Disclaimer

This calculator is provided for informational purposes only and should not be considered as financial or tax advice. All calculations are performed locally in your browser — no personal data is collected or sent to our servers. Rates and thresholds are sourced from HMRC and GOV.UK and are updated for the current tax year. Always verify results with HMRC or consult a qualified professional before making financial decisions.

How It Works

Radiator sizing is calculated in BTU/h (British Thermal Units per hour). The base calculation multiplies room volume (length × width × height in metres × a base factor of about 153 BTU per m³). This is then adjusted for room characteristics: external walls, window type and size, insulation level, location in the house and desired temperature.

Common correction factors include: +15% for each external wall, +20% for single-glazed windows (or −10% for triple glazing), +15% for rooms above a garage or unheated space, and +10–15% for north-facing rooms. Bathrooms typically require a higher target temperature (22°C versus 21°C for living rooms).

Enter room dimensions and characteristics. The calculator shows the total BTU/h requirement and suggests appropriate radiator sizes. Most UK radiator manufacturers publish BTU outputs at Delta T50 (the standard comparison condition). If your system runs at a lower flow temperature (e.g. with a heat pump), you will need larger radiators.

Example: Living room 5 m × 4 m × 2.4 m, 2 external walls, double glazing

  1. Volume: 5 × 4 × 2.4 = 48 m³
  2. Base BTU: 48 × 153 = 7,344 BTU/h
  3. Two external walls (+15% each): × 1.30 = 9,547 BTU/h
  4. Double glazing (no adjustment): 9,547 BTU/h
  5. Recommended: radiator(s) totalling ~10,000 BTU/h

Source: GOV.UK — Building Regulations Approved Document L

Frequently Asked Questions

What does the Radiator BTU Calculator do?
Calculate the BTU and wattage needed for your radiators based on room size, glazing and insulation. All calculations are performed in your browser using official UK rates and thresholds.
Does this include waste allowance?
Yes. Most building material calculations include a standard 10% waste allowance. You can adjust this figure depending on the complexity of your project and cutting requirements.
Are prices accurate?
Material prices are indicative estimates based on average UK prices. Actual costs vary by region, supplier, brand and current market conditions. Always get quotes from local suppliers.