Radiator size calculator
A sizing estimate. Radiator outputs are quoted at a specific flow temperature — a heat pump running at 45 °C gets far less from the same radiator than a boiler at 75 °C.
Radiator sizing starts from room volume rather than floor area. A standard-construction living room wants about 40 watts per cubic metre, so a 4 × 3.5 m room with 2.4 m ceilings is 33.6 m³ and needs 1,344 W, or 4,586 BTU per hour. Every one of those figures assumes the radiator is quoted at Delta T 50.
How to size a radiator
The trap in radiator sizing is not the room, it is the temperature the output was measured at. European radiators are tested to EN 442, whose reference condition is 75 °C flow, 65 °C return and a 20 °C room. Average the flow and return and the water is at 70, which is fifty degrees above the room, and that is the Delta T 50 every catalogue figure is quoted at. Change the water temperature and the output changes with it, along a curve rather than in proportion: output scales as the temperature difference divided by fifty, raised to the radiator’s own exponent. EN 442-2 measures that exponent for each model alongside its output, and it is on the datasheet; 1.3 is the value conventionally used for a panel radiator when the real one is not to hand, and it is what every figure below comes from.
Work that curve through and the numbers are stark. A boiler at 75/65 gives you the rated output, by definition. A system run at Delta T 30 gives about 52 per cent of it. A heat pump flowing at 45 °C with a five degree drop across the radiator averages 42.5, so Delta T 22.5, and the same radiator delivers around 35 per cent of its catalogue figure. That single fact is why converting a house to a heat pump means upsizing radiators, and it also explains why an old catalogue can look so generous: British manufacturers once quoted at Delta T 60, which flatters the same radiator by 27 per cent against a modern Delta T 50 figure.
The volume rule underneath all this is deliberately coarse, and its coarseness sits mostly in the construction setting. Modern and well insulated is 30 W per cubic metre; poorly insulated is 70. Those two multiply the same room by more than two, and no other input here comes close to that leverage, so it is the one to get right rather than guess. The room-type factor is smaller and mostly about how the room is used: a bathroom adds twenty per cent because people stand still in it while wet, a bedroom drops ten because it is heated to a lower target, and a kitchen drops ten because ovens, hobs and a fridge are all quietly contributing.
The radiator count is a division and should be read as one. A 1,344 W room served by 1,000 W radiators comes out as two, which is 2,000 W of radiator for a 1,344 W load. Radiators are not sold in that few sizes, so the useful reading is the watts, not the count: one radiator of roughly 1,400 W, or two of 700, either of which fits the room better than the rounding does. Deliberate oversizing is fine here in a way it never is with cooling, because a larger radiator simply reaches the room temperature at a lower flow temperature, and a lower flow temperature is where both a condensing boiler and a heat pump do their best work.
What people use it for
- Replacing a single radiator in an existing room
- Sizing radiators for an extension or a loft conversion
- Checking whether existing radiators will cope with a heat pump
- Converting a catalogue output between Delta T ratings
- Comparing towel rail outputs for a bathroom
- Translating a watts figure into the BTU a UK merchant quotes
- Working out whether one large radiator or two small ones suits a room
Questions
Sizing works better on volume. About 40 W per cubic metre for standard construction, 30 for a modern insulated build, 55 for an older solid-wall house and up to 70 for a poorly insulated one.
The gap between the average water temperature in the radiator and the room temperature. Outputs are quoted at Delta T 50, and away from that the output falls as the ratio to 50 raised to the radiator’s exponent. That exponent is measured per model under EN 442-2 and published with it; the figures here use 1.3, the conventional value for a panel radiator, so check your own datasheet if the margin is tight.
A flow of 75 °C, a return of 65 and a room at 20. The water averages 70, which is fifty above the room, and every European catalogue figure comes from that EN 442 test condition.
About 52 per cent of the rated figure. A radiator listed at 1,500 W delivers around 770.
Not at their catalogue output. A 45 °C flow gives roughly Delta T 22.5, at which the same radiator returns about 35 per cent of its rating, so most rooms need a larger radiator or a second one.
Because it was probably measured at Delta T 60, the old 90/70/20 condition. That flatters the same radiator by 27 per cent against a Delta T 50 figure, and comparing across the two without converting is how radiators end up undersized.
Yes, about twenty per cent more, because people are wet and standing still. A towel rail on its own rarely does it for anything but a small bathroom.
An oven, a hob, a fridge and a dishwasher all put heat into the room, and a kitchen is usually the busiest one in the house. The ten per cent reduction is a nod to that rather than a measurement.
Yes, unlike with air conditioning. A larger radiator reaches the target at a lower flow temperature, and both a condensing boiler and a heat pump run more efficiently the cooler the water they can get away with.
Multiply by 3.412. The 1,344 W room above is 4,586 BTU per hour, and a 1,000 W radiator is 3,412.
Less than it once did. The old rule of putting one under a window was about countering the cold draught off single glazing; with modern glazing, position matters mostly for furniture. A radiator boxed in or blocked by a sofa loses real output, because it works partly by convection.
It divides the load by the output you entered and rounds up, so 1,344 W against 1,000 W units becomes two. Read the watts rather than the count and pick radiators that actually add up to it.
Whenever the system rather than one radiator is at stake. A whole-house design, and any heat pump, wants a room-by-room calculation to EN 12831 or the equivalent, which accounts for fabric, glazing and ventilation instead of multiplying a volume.