Radiator Heat Output and Delta T Calculations
Properly sizing a radiator requires more than just knowing the square footage of your room. You must calculate the required heat output, typically measured in BTUs (British Thermal Units) or Watts. A critical factor in this calculation is the ‘Delta T’ (ΔT), which is the difference between the average water temperature inside the radiator and the desired room temperature. Modern steel panel radiators heat up quickly and require different sizing rules compared to traditional, heavy cast-iron radiators that retain heat for much longer. Use the tool below to calculate your exact heating requirement based on your room’s insulation and window types.
♨️ What Size Radiator Does Each Room Need?
Work out the heat output each room needs in Watts and BTU/hr, then get the exact panel radiator size (Type 11, 21 or 22) to fit it. Includes the ΔT correction — essential if you're on a heat pump or running low flow temperatures.
🌍 Choose your units
📐1. Room Size
Standard UK ceilings are 2.4 m. Victorian and period rooms are often 3 m+, which needs noticeably more output.
🛋️2. Room Type
Different rooms are heated to different target temperatures — bathrooms warmer (22 °C), bedrooms cooler (18 °C).
🧱3. Insulation & Glazing
🔧4. Radiator Style
Panel radiators (Type 11/21/22) are the standard choice. Vertical suits narrow walls, column / cast iron suits period homes (sized in sections), and towel rails suit bathrooms — but note rails give far less heat, so a big bathroom may need a rail plus a radiator.
🌡️5. System Flow Temperature (ΔT)
Radiators are rated at ΔT50 (75 °C flow). Modern condensing boilers and especially heat pumps run cooler — which means you need a bigger radiator for the same heat. This is the step most calculators skip.
Radiator Size Chart by Room (Watts & BTU)
Typical heat output needed per room, for an average-insulated semi-detached home with double glazing and 2.4 m ceilings. Use the calculator above for your exact room.
| Room | Size | Heat needed | Type 22 radiator (600 mm high) |
|---|---|---|---|
| Small bedroom | 3 × 3.3 m (10 m²) | ~670 W / 2,280 BTU | 600 × 400 mm |
| Double bedroom | 4 × 3.5 m (14 m²) | ~945 W / 3,220 BTU | 600 × 600 mm |
| Small living room | 4 × 4 m (16 m²) | ~1,200 W / 4,095 BTU | 600 × 800 mm |
| Medium living room | 5 × 5 m (25 m²) | ~1,875 W / 6,400 BTU | 600 × 1000 mm |
| Large living / open plan | 6 × 6 m (36 m²) | ~2,700 W / 9,210 BTU | 600 × 1600 mm |
| Kitchen / diner | 4 × 5 m (20 m²) | ~1,350 W / 4,605 BTU | 600 × 800 mm |
| Bathroom | 2 × 3 m (6 m²) | ~495 W / 1,690 BTU | 600 × 400 mm or towel rail |
Figures assume a standard ΔT50 boiler system. On a heat pump (ΔT30) you need roughly double the radiator size for the same room — set ΔT in the calculator to see your figure.
How Radiator Size Is Calculated
Radiator sizing is a room-by-room heat-loss calculation, then a match to a radiator's rated output.
1. Room Heat Loss
Floor area × a heat-loss factor for the building's insulation (roughly 55 W/m² for a modern home, 75 W/m² for average cavity-wall, and 105 W/m² for solid-brick pre-1980), then adjusted for glazing, exposure (detached loses more than mid-terrace), room type (bathrooms warmer, bedrooms cooler) and ceiling height.
2. Radiator Types: 11, 21 and 22
UK panel radiators come in three common builds. Type 11 is a single panel with one convector fin set — slim, lowest output. Type 21 is a double panel with a single convector. Type 22 is a double panel with double convectors — the most common, and roughly 1.9× the output of a Type 11 the same size. So a Type 22 can be much shorter for the same heat.
3. The ΔT Correction (the step most guides miss)
Radiators are rated at ΔT50 — 75 °C flow, 65 °C return, 20 °C room. If your system runs cooler, output drops sharply, following CF = (ΔT ÷ 50)1.3:
- ΔT50 (traditional boiler) — rated output, factor 1.00
- ΔT40 (condensing boiler, lower flow temp) — factor 0.75, so you need ~33% more radiator
- ΔT30 (heat pump) — factor 0.51, so you need roughly double the radiator
This is why heat-pump retrofits so often require bigger radiators. Set the ΔT in the calculator to size correctly for your system.
Frequently Asked Questions
What size radiator do I need for my room?
How many BTU do I need per square metre?
What's the difference between Type 11, 21 and 22 radiators?
Do I need bigger radiators for a heat pump?
What is ΔT50 on a radiator?
Is it bad to oversize a radiator?
Can I use one big radiator or two smaller ones?
Related tools from Refcon HVAC
- Boiler Size Calculator (whole house)
- Heat Pump Sizing & Savings Calculator
- BTU Load Calculator
- Furnace Size Calculator
Last updated: 2026. Reviewed by the Refcon HVAC engineering team. Planning estimate — confirm with a qualified heating engineer. Tell us about an error.
Estimates vary by contractor. Compare free quotes from licensed local pros before you commit. Request a quote →
High-efficiency systems may qualify for utility rebates and federal or state incentives. Find current programs at ENERGY STAR & DSIRE.
Figures are reviewed by the Refcon HVAC engineering team using U.S. DOE, ENERGY STAR & ACCA Manual J data. Estimates only — confirm with a licensed pro.