How many BTU do I need per square metre?

As a rough guide, you need approximately 100-200 BTU per square metre of floor area, depending on the ceiling height and room characteristics. For a standard 2.4m ceiling height, this equates to about 40-80 Watts per cubic metre. Our calculator gives a more accurate figure by factoring in insulation, glazing, and exposure.

What is the difference between BTU and Watts for radiators?

BTU (British Thermal Units) and Watts both measure heat output. 1 Watt = 3.412 BTU. UK radiator manufacturers typically list both values. BTU is the traditional UK measurement, while Watts (W or kW) aligns with European standards. When comparing radiators, ensure you are using the same unit.

Should I use Delta T 50 or Delta T 60 radiator ratings?

Modern UK practice uses Delta T 50 (DT50) ratings, which assume a flow temperature of 75°C, return of 65°C, and room temperature of 20°C. Older catalogues may use Delta T 60 which gives higher output figures. If your radiator is rated at DT60, you will need a larger radiator to achieve the same output at DT50. Heat pumps use even lower flow temperatures (DT30), requiring significantly larger radiators — see our Heat Pump Sizing Calculator for more guidance.

Do I need a bigger radiator for a bathroom?

Yes. Bathrooms typically need 15-20% more heat output than other rooms because the desired temperature is higher (22°C vs 21°C for living rooms and 18°C for bedrooms) and there is additional heat loss from ventilation. Our calculator applies a bathroom uplift automatically.

Can I use one large radiator instead of two smaller ones?

Both approaches work. Two smaller radiators distribute heat more evenly across the room and are useful when wall space is limited. A single large radiator is simpler to install with less pipework. For rooms over 20m², two radiators on opposite walls often provide better comfort.

Radiator Size Calculator — BTU and Watt Calculator for Any Room

How We Calculate This

Our radiator sizing calculator uses the volumetric heat-loss method, which is the standard approach used by UK heating engineers. It calculates the heat output required to maintain a comfortable room temperature based on the room's size and characteristics.

The formula

Heat required (Watts) = Room volume (m³) x Adjusted W/m³

The base rate of 50 W/m³ (for an average room with double glazing and moderate insulation) is adjusted by multiplying the following factors:

Room type: Bathrooms +15%, conservatories +30%, kitchens -10%
Insulation: Poor +40%, average +10%, good baseline, excellent -15%
Glazing: Single +20%, double baseline, triple -10%
External walls: +10% per external wall

Converting to BTU

The result in Watts is converted to BTU by multiplying by 3.412. When shopping for radiators, match the BTU/Watt output to your calculated requirement — ensuring you compare values at the same Delta T rating (DT50 is the current UK standard).

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Radiator Sizing Calculator — BTU & Watt Calculator for Any Room