Sizes underfloor heating pipe spacing and loop lengths. Add one zone per room; picks the widest (most economical) spacing that meets the target output, checks the BS EN 1264 floor surface-temperature limit, and splits large zones into parallel loops.
Tender-stage sizing check. Always verify against the emitter manufacturer's own output data.
Reference construction only: pipe diameter, screed depth and manifold hydraulic balancing are not parameterised. For design guidance only. Always verify with a qualified engineer.
About this underfloor heating loop sizing calculator
This free underfloor heating loop sizing calculator sizes wet underfloor heating room by room. Add one zone per room and it picks the widest, most economical pipe spacing that meets the target output, checks the BS EN 1264 floor surface-temperature limit for the zone type, and splits large zones into parallel loops within a pressure-drop budget. It is aimed at heating designers and installers laying out manifolds and loop lengths at tender stage across occupied areas, window strips and wet rooms. Everything runs in your browser. Nothing is uploaded.
How the loop sizing is calculated
Output and floor surface temperature come from BS EN 1264 output tables (16 mm PE-RT pipe, 75 mm screed reference construction) at spacings of 100/150/200/300 mm, bilinearly interpolated over the design mean water temperature (35–45 °C) and room design temperature (18–24 °C). The calculator walks the spacings widest-first and keeps the first that meets your target W/m², then checks the resulting surface temperature against the zone limit (occupied 29 °C, periphery 35 °C, bathroom 33 °C). Loop length is coverage = area ÷ (spacing ÷ 1000), split into ceil(coverage ÷ usable length) parallel loops, with a tail allowance of 2 × the manifold distance added per loop to give the total pipe length.
Frequently asked questions
Why does the floor covering matter so much?
The covering’s thermal resistance sits between the pipe and the room, so a carpet or thick timber floor passes far less heat than tile or stone at the same water temperature and spacing. The calculator holds a separate BS EN 1264 output table for tile/stone, wood/laminate, carpet and deep-pile/thick wood, so the achievable W/m² reflects the covering you choose.
What is the floor surface-temperature limit?
BS EN 1264 caps the floor surface temperature for comfort and safety: about 29 °C in occupied areas, up to 35 °C in peripheral window strips, and 33 °C in bathrooms and wet areas. The tool reports the surface temperature at the chosen spacing and flags a zone that would breach its limit, often a cue to widen the spacing or lower the water temperature.
Why does it split a zone into several loops?
A single loop has a practical length cap (typically 100 m of 16 mm pipe) set by its pressure drop, so a large zone must run as several parallel loops off the manifold. The calculator divides the pipe coverage by the usable loop length and rounds up, adding a flow-and-return tail for the manifold run. It uses a reference construction only. Verify against the manufacturer’s own output data and a qualified engineer.