What size radiator do I need?
Radiators are sold by heat output in BTU. Too small and the room never warms up; too big and you waste money and energy. Enter the room size and a few details and get the output you need — in BTU and watts. Free, no sign-up.
The room's heat loss — these make a big difference
A guide, not a heating design. It estimates the heat a room needs from its size and a few characteristics — real heat loss depends on construction, draughts, floor type and what's above and below. Radiator outputs are quoted at "Delta T 50"; if your system runs cooler (as heat pumps do), you'll need a larger radiator for the same room. For a whole-house system, a new boiler or a heat pump, get a heat-loss survey from a qualified installer. Nothing you type leaves your browser.
BTU, watts, and getting the size right.
BTU is just a measure of heat. British Thermal Units per hour is how radiators are sold in the UK; watts are the same thing in metric (1 watt ≈ 3.4 BTU). This works out roughly how much heat the room loses, then matches a radiator to replace it. We start from the room's volume and adjust for the things that drive heat loss — the type of room, the windows, how many walls face outside, the insulation and which way it faces.
Aim within the range, not below it. Pick a radiator at or a little above the figure — it's better to have a touch more output you can turn down on the thermostatic valve than too little on a cold day. We show a sensible range to shop between.
Heat pumps change the maths. Radiator outputs are quoted at "Delta T 50" — a flow temperature about 50°C hotter than the room. Heat pumps run much cooler, so the same radiator gives out far less heat. If you're on (or moving to) a heat pump, you'll typically need radiators rated 2–2.5× this figure, which is exactly why a professional heat-loss survey matters.
Big rooms, two radiators. If the figure is large, two radiators spread around the room heat it more evenly than one huge one — and give you more wall to play with. Add the room figures together for a whole floor.
Your home's details, always to hand.
Stead keeps your rooms, their sizes, your boiler and heating details and the jobs that keep them running in one place — so when something needs replacing you already have the numbers, not a tape measure and a guess.