Underfloor heating calculator (radiant floor heating)

This underfloor heating calculator (also called a radiant floor heating calculator) sizes one room of a water-based system for early planning. It shows the heat output in Btu/h per sq ft or W/m² for your pipe spacing, floor covering and water temperature, whether that covers the room's heat demand, and whether the floor surface would get too warm.

It also answers the practical question of how much pipe you need for underfloor heating: field pipe, the leader runs to the manifold, the number of loops, and the flow rate per room. The materials list covers pipe, manifold ports and perimeter edge strip, with prices you can edit. US units are the default with a one-click switch to metric for UK and European projects. Export to PDF or Excel for free, no sign-up. Everything runs in your browser.

This is a simplified estimate. For the final design, the pipe manufacturer's design tables and your designer's heat-loss calculation govern.

1. Log-mean temperature difference. Water cools along the loop, so the calculator uses the log-mean excess temperature, not the simple average:

ΔθH = (θsupply − θreturn) / ln((θsupply − θroom) / (θreturn − θroom))

2. Heat transfer. A bare screed (about 1-3/4 in over 5/8 in pipe) has an equivalent heat transfer coefficient that falls as spacing grows: 8.0, 7.1, 6.3, 5.6 and 5.0 W/m²K at 4, 6, 8, 10 and 12 in (10 to 30 cm), approximated from EN 1264 curves. The floor covering's resistance R lowers it: K = 1 / (1/K0 + 0.9 × R) and q = K × ΔθH.

3. Surface temperature limit. The EN 1264 basic characteristic curve is q = 8.92 × (θfloor − θroom)^1.1 (in W/m²). Comfort limits are 29 °C (84.2 °F) in living areas and 33 °C (91.4 °F) in bathrooms. In a 68 °F room, 84.2 °F allows about 100 W/m², or 31.7 Btu/h per sq ft. If the raw output is higher, the calculator caps it.

4. Pipe and flow. Field pipe = area / spacing. Loops = field pipe / (max loop length − 2 × manifold distance), rounded up. Flow in gpm = Btu/h / (500 × ΔT in °F).

Inputs: 192 sq ft, 6 in spacing, ceramic tile, 104 °F supply and 95 °F return (40/35 °C), 68 °F room, 300 ft maximum loop, manifold 15 ft away.

1. Log-mean excess temperature: 17.4 K (31.3 °F).
2. Raw output: about 34.5 Btu/h per sq ft (109 W/m²). That would need a floor at about 85.5 °F, above the 84.2 °F living-area limit.
3. Capped output: about 31.7 Btu/h per sq ft, about 6,090 Btu/h (1,784 W) for the room.
4. Field pipe: 192 / 0.5 = 384 ft. Each loop can hold 300 − 2 × 15 = 270 ft of field pipe, so you need 2 loops. Total pipe with leaders: 444 ft, 222 ft per loop.
5. Flow: 6,090 / (500 × 9) = about 1.35 gpm.

Metric check (UK): a 5 × 4 m room with 150 mm spacing and tile at 40/35 °C gives 109 W/m² raw, limited to about 100 W/m² by the 29 °C surface, with 2 loops, 153 m of pipe and 344 l/h.

Raw output by spacing in a 68 °F (20 °C) room, in W/m² (multiply by 0.317 for Btu/h per sq ft):

Covering resistance used by the calculator (m²K/W, US R-value in brackets):

EN 1264 calculations stop at R = 0.15 m²K/W. A thicker rug or pad will cut output further than the calculator shows. For a heat pump, lower water temperatures mean better efficiency, so tight spacing and a low-resistance floor pay off. To cut the heat demand itself, try the insulation calculator.

• Loops that are too long have high pressure drop and cold ends. Use the manufacturer's maximum loop length for your pipe size.
• Forgetting leader pipe. The run to and from the manifold counts toward loop length; the calculator adds it per loop.
• Changing the floor later. Swapping tile for carpet raises R and lowers output. Design for the final covering.
• Turning up the water instead of fixing the design. A floor above the surface limit is uncomfortable; reduce heat loss or tighten spacing instead.
• No balancing. Set each loop's flow at the manifold so short loops do not steal the water.