Radiator Size Calculator (BTU & kW)

Radiator Sizing Guide

How BTU and kW Relate?

BTU (British Thermal Unit) is the traditional unit for heating output; kW is the metric equivalent. 1 kW = 3,412 BTU/hr. Radiator catalogues often list both. A typical 60×60cm single-panel radiator (Type 11) outputs approximately 700–900 BTU/hr. A double-panel with convector fins (Type 22) of the same size outputs approximately 1,500–2,000 BTU/hr. The Type designation (11, 21, 22, 33) indicates panel count and convector fin rows — Type 22 is the most common for living rooms.

What's the key thing to understand about Rule of Thumb Calculation?

A simple starting point: multiply room volume (m³) by a factor depending on insulation — well-insulated: 25 BTU/m³; average: 35 BTU/m³; poor: 45 BTU/m³. Add 10% for each external wall, 20% for north-facing rooms in colder climates, 15% for bathrooms (higher target temperature). The result is total BTU needed — split between multiple radiators if required. Online BS EN 442 heat loss calculators (used by heating engineers) are more accurate for large installations.

What should I know about Heat Pumps Require Larger Radiators?

Air source heat pumps operate at lower flow temperatures (40–55°C) than gas boilers (65–80°C). At lower temperatures, the same radiator outputs less heat — approximately 30–50% less at 45°C vs 70°C flow temperature. This means a house switching from gas to a heat pump typically needs significantly larger radiators (or underfloor heating, which is ideal for heat pumps as it operates at 30–40°C). When specifying radiators for a heat pump system, size for 45°C flow temperature, not the standard 70°C flow temperature that older gas boiler systems were designed around.

What do I need to know about Common Radiator Sizes?

Standard heights: 300mm, 450mm, 600mm, 900mm. Standard lengths: from 400mm to 2000mm+. A 600×1200mm Type 22 is one of the most common sizes — outputs approximately 3,200–4,500 BTU depending on manufacturer. For a 15–20m² living room with average insulation, you typically need 3,500–5,000 BTU, which a single 600×1200 Type 22 can cover. Bathrooms often use towel radiators which output less heat for their size — always check the BTU rating, not just physical dimensions.

What's the key thing to understand about Insulation Level's Impact on Sizing?

The insulation level input has a bigger effect on the final BTU figure than most people expect, because heat loss through walls, windows and roof is the main driver of how much heating output a room actually needs — room size tells you the air volume, but insulation tells you how fast that heat escapes. A poorly insulated Victorian or Edwardian room with solid, uninsulated walls, single glazing, and no loft insulation above can need genuinely double the radiator output of an identically sized, well-insulated modern room, because far more heat is constantly leaking out and needs to be continuously replaced. This is why an accurate insulation setting matters more than being precise to the last few centimetres on room dimensions — if you're unsure whether a room counts as 'average' or 'poor', err toward the lower setting, since an undersized radiator leaves a room permanently a few degrees short of comfortable, while a modestly oversized one just runs at reduced output more efficiently via its TRV. If you've since added loft or cavity wall insulation, double glazing, or draught-proofing, re-running this calculator with the improved setting usually shows a worthwhile reduction in required output.

What do I need to know about Multiple External Walls?

Each external wall a room has represents an additional surface losing heat directly to outside air rather than to an adjacent heated room, which is why this calculator asks for the number of external walls separately from room type. A corner room with two external walls loses meaningfully more heat than an identically sized room with only one external wall and the rest shared with heated neighbouring rooms, because internal walls lose very little heat (the room on the other side is at a similar temperature), while external walls constantly lose heat to colder outside air regardless of the weather. Rooms above an unheated space, such as an integral garage, or below a cold loft without adequate insulation effectively have an additional cold surface even though it isn't a wall, and often need a similar upward adjustment as an extra external wall would cause. Corner rooms, end-of-terrace end walls, and rooms over unheated garages are consistently among the hardest to heat comfortably, so it's worth double-checking the external wall count is accurate for these specifically, since underestimating it is a common reason a 'correctly sized' radiator still leaves a room feeling cold.

What BTU Output Do Conservatories and Extensions Typically Need?

Conservatories and modern glazed extensions present a particular heat-loss challenge for radiator sizing because they typically have a much higher proportion of glass-to-wall area than a standard room, and glass loses heat far faster than insulated wall even when double or triple glazed. A standard BTU-per-square-metre rule of thumb, calibrated around typical room insulation levels, significantly underestimates heat loss in a room where most of the external surface area is glazing rather than wall, which is why many conservatories struggle to stay comfortably warm even with a radiator that would be more than adequate for a similarly-sized standard room. Older, poly-carbonate-roofed conservatories built under permitted development rules before more recent building regulation changes are often particularly poorly insulated, with high heat loss through both the glazed walls and the roof, and may need radiator output uplifted by 30% or more above a standard room calculation to stay comfortably heated through winter. Modern extensions built to current building regulations, with proper insulated walls, high-performance double or triple glazing, and a solid insulated roof, perform much closer to a standard room and need a more modest uplift, if any, over typical sizing. Because the exact heat loss depends heavily on glazing specification, roof type and how much of the structure is glazed versus solid wall, a room-specific heat loss calculation accounting for the actual glazed and solid areas gives a far more reliable radiator size for a conservatory or glazed extension than a general floor-area estimate.

Worked Example: Sizing a Radiator for a Specific Room

Take a living room measuring 4m x 3m (12m² floor area) with a standard 2.4m ceiling height, in a semi-detached house with average insulation (double glazing, some loft insulation, one external wall). A commonly used rule-of-thumb figure for this insulation level and room type is roughly 250-300 BTU per square metre of floor area — useful as a quick starting point, though it's a simplification and not a substitute for a full room-by-room heat-loss survey. Using the midpoint of about 280 BTU/m²: 12m² x 280 BTU/m² ≈ 3,360 BTU required. In practice, installers typically round this up to the nearest common radiator output rather than down, both to allow a margin and because available radiator models come in fixed output steps — so this room would usually be matched to a radiator rated at around 3,400-3,800 BTU rather than one sized exactly to 3,360.

These figures are widely used rule-of-thumb ranges rather than a formal engineering standard; a kitchen or bathroom generally needs more output per square metre than a living room both because of greater heat loss (more external glazing, extractor use, higher target temperature in a bathroom) and because rooms are often used for shorter, more intense heating bursts rather than being kept warm all day.

Why Two Radiators of the Same Size Can Have Different BTU Output

A radiator's physical size (height x width) and its rated BTU output are related but not interchangeable, because manufacturers also vary the internal construction. A single-panel radiator has one heat-emitting panel; a double-panel version doubles the water-carrying surface in the same footprint; adding convector fins behind the panel (a "double-panel double-convector", often labelled "P2" or "K2") increases output further still by using passing air to carry away more heat, without increasing the radiator's height or width. So a 600mm x 1200mm single-panel radiator and a 600mm x 1200mm double-panel double-convector radiator occupy the same wall space but can differ in rated output by a factor of two or more — which is why two radiators that look the same size on a spec sheet can carry very different BTU ratings, and checking the actual output rating (not just the dimensions) matters when replacing a radiator like-for-like. The other detail worth checking on a product spec sheet is the temperature difference (ΔT) the BTU rating was measured at. Most UK radiator outputs are quoted at ΔT50 (a mean water temperature of around 70°C against a 20°C room), which matches a traditional gas boiler system. A heat pump typically runs at a lower flow temperature, often tested and quoted at ΔT30, so a radiator's real output on a heat pump system is meaningfully lower than its ΔT50 "box" rating — commonly 35-45% lower — which is the main reason heat pump installations often need noticeably larger radiators than the same room would need on a gas boiler, even though the room's heat-loss requirement in BTU hasn't changed. This also explains why simply swapping a boiler for a heat pump without reviewing radiator sizing is a common source of disappointing heat pump performance, independent of the heat pump unit itself being correctly specified.