Underfloor Heating Pipe Length Calculator

How much pipe does underfloor heating need?

Pipe is laid in the floor at a set spacing. The length that goes into the room is the area divided by the spacing: L_room = A / a. That is 6.67 m per square metre at 150 mm spacing, 5 m at 200 mm and 10 m at 100 mm.

The flow and return tails between manifold and room are added. Every circuit covers that distance twice: L = L_room + N × 2 × L_tail.

Why does the number of circuits matter?

If a circuit is too long its pressure drop grows, the pump cannot cope and the end of the loop stays cold. Circuit length is therefore limited and large rooms are split into several circuits. The number of circuits is also the number of manifold ports needed.

Example

A 20 m² room at 150 mm spacing takes 133 m of pipe. With the manifold 5 m away and a 100 m circuit limit, 90 m per circuit is left for the room, so 2 circuits are needed. Total pipe is 133 + 2 × 10 = 153 m, or 77 m per circuit. In 16 × 2 mm pipe that run holds about 17 litres of water.

If the floor has to give 80 W per square metre the room needs 1600 W. At a 5 K temperature difference each circuit must carry about 2.3 litres per minute.

How do I choose the spacing?

Spacing is an input here, not a result. Closer spacing gives more heat at the same water temperature. The right spacing follows from heat demand, floor covering and water temperature using the EN 1264 method or the pipe manufacturer's tables. Spacing is often tightened along external walls and in front of windows.

Limits

The tool gives quantities and circuit count, not a thermal design. Pressure drop, pump selection, surface temperature limits and movement joints are handled separately.