Heat Pump Radiator Sizing Calculator Guide

Radiator sizing for heat pumps follows a simple formula: room heat loss divided by the correction factor for your flow temperature equals the required rated output. Understanding this calculation helps you verify your installer's recommendations and make informed decisions about radiator upgrades.

By Home Heat Pump GuidePublished: 18 March 2026

Radiator sizing calculations being performed for a heat pump installation — Understanding the sizing calculation helps you verify your installer's radiator recommendations

This guide walks through the radiator sizing process step by step. For background, see our sizing multiplier guide and pillar: do I need new radiators?

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The Sizing Formula

Required rated output (Delta T 50) = Room heat loss (watts) ÷ Correction factor

The room heat loss comes from the heat loss calculation. The correction factor depends on the target flow temperature.

Worked Example

Correctly sized radiator in a UK room based on heat pump sizing calculations — A living room with 1,200W heat loss at 45°C flow needs a radiator rated at 2,553W (Delta T 50)

Room	Heat Loss	Flow Temp	Correction	Required Rating
Living room	1,200W	45°C	0.47	2,553W
Main bedroom	600W	45°C	0.47	1,277W
Kitchen	500W	45°C	0.47	1,064W
Bathroom	400W	45°C	0.47	851W

Example calculations for a well-insulated 3-bed semi at 45°C flow temperature.

Correction Factor Table

0.47At 45°C flow

0.38At 40°C flow

0.29At 35°C flow

0.57At 50°C flow

Select a radiator from our best radiators guide that meets or exceeds the calculated rating. Solar panels complement efficient radiator sizing by reducing the electricity cost of running the heat pump.

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Frequently Asked Questions

How do I calculate the radiator size I need?

Divide room heat loss by the correction factor for your flow temperature. This gives the required rated output at Delta T 50.

Should I do my own radiator sizing?

Understanding the principles helps, but final sizing should be done by your MCS-certified installer based on measured data.

Radiator sizing calculations are fundamental to efficient heat pump operation. Connects to radiator assessment, heat loss calculations, and running costs. Solar panels further reduce costs.