Underfloor heating calculator
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| System | Wet (water pipes) |
| Heated floor area | 15 m² |
| Room heat loss | 1,200 W |
| Floor surface temperature | 29 °C |
| Room temperature | 20 °C |
| Pipe spacing | 150 mm |
| Tails to the manifold | 0 m |
| Output coefficient | 8.92 W/m²K |
| Output exponent | 1.1 |
100.0 W/m² (1500 W); 100 m of pipe
- Limit not yet confirmed Floor surface temperature: 29 °C The limit for this check hasn't been confirmed yet: check the source. UK-wide: BS EN 1264 (maximum floor surface temperature). Check with the source.
- Within the guidance Floor output vs room heat loss: 1,500.109 W UK-wide: BS EN 1264 (design heat output to cover the room heat loss). Check with the source.
| Result | Value |
|---|---|
| Output per m² | 100 W/m² |
| Output for the room | 1,500 W (5,119 BTU/h) |
| Pipe needed | 100 m |
Show the working
- Output: 8.92 × (29 − 20)^1.1 = 100 W/m²
- Room: 100 W/m² × 15 m² = 1,500 W
- Pipe: 15 m² ÷ 0.15 m (150 mm spacing) + 0 m tails = 100 m
- Heat loss to meet: 1,200 W
How it works
- Wet output q (W/m²) = 8.92 × (floor surface temperature − room temperature)^1.1 (BS EN 1264 basic characteristic)
- Room output (W) = q × heated floor area
- Pipe (m) = heated floor area ÷ pipe spacing + tails to the manifold; loops = pipe ÷ the maker's longest loop, rounded up
- Electric: output (W) = area × mat W/m²; current (A) = W ÷ supply voltage
The floor finish (its tog or R-value) doesn't change the output at a given surface temperature, but a thicker or more insulating finish needs warmer water to reach it.
Electric mats need their own circuit with RCD protection; bathrooms have extra rules (BS 7671 Section 701).
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Output coefficient | 8.92 W/m²K^n | Our assumption Not yet checked against the source | Our choice: change it to suit your job Coefficient in q = 8.92 × (θfloor − θroom)^1.1, brief v2 H6 (BS EN 1264 characteristic). Brief says "Researcher verifies". |
| Output exponent | 1.1 | Our assumption Not yet checked against the source | Our choice: change it to suit your job Exponent in q = 8.92 × (θfloor − θroom)^1.1, brief v2 H6 (BS EN 1264 characteristic). Brief says "Researcher verifies". |
| Maximum floor surface temperature | To be confirmed from the source | Regulation or HSE figure BS EN 1264 (maximum floor surface temperature, occupied zone) Not yet checked against the source | Source to be added by the Researcher Brief v2 H6: max floor surface temperature is a limit from the BS EN 1264 explainer. Researcher to fill the value and cite the clause. |
| Supply voltage | 230 V | Our assumption Not yet checked against the source | Our choice: change it to suit your job UK nominal single-phase supply voltage (brief v2 E2). Researcher to cite the statutory source. |
Regulations and standards
- BS EN 1264 (floor heating output and surface temperature)
- BS 7671:2018+A4:2026 (electric mats)
Explained: BS EN 1264 (underfloor heating), BS 7671 (the IET Wiring Regulations).
Our checks use the guidance for England unless a badge says otherwise. Wales, Scotland and Northern Ireland have their own rules: check the source for where you live.