Rule-of-thumb estimate for 16mm pipe. Confirm loop lengths and spacing against the manifold and system design.
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This calculator estimates how much underfloor heating pipe a room needs and how many manifold loops it splits into, from the floor area and the pipe spacing. Enter the area to be heated, choose the pipe centres and add the run back to the manifold, and it returns the pipe laid in the floor, the total including tails, and the number of loops.
It is built for UK heating engineers and plumbers pricing and setting out wet underfloor heating. Ordering the right amount of pipe and knowing the loop count up front avoids both running short mid-install and buying whole extra coils you don't need, and it feeds straight into manifold selection.
The method is the standard one: pipe laid per square metre is set by the spacing between pipe centres, so tighter spacing means more pipe and more output. The calculator adds an allowance for the tails running back to the manifold and splits the total into loops kept under about 100 m each to keep the pressure drop and flow balance manageable.
Pipe in floor = area ÷ spacing (m); Total = pipe in floor + (2 × tail run); Loops = Total ÷ 100 (rounded up)
Dividing the floor area by the pipe spacing in metres gives the length laid in the floor — 200 mm centres is 5 m of pipe per m², 150 mm is about 6.7 m/m², 100 mm is 10 m/m². Two tails run from the manifold to each loop and back, so twice the manifold distance is added per loop. The total is then divided by a practical maximum loop length of around 100 m for 16 mm pipe and rounded up to give the number of loops, each fed from its own manifold port.
Enter the actual heated floor area in square metres — the area the pipe covers, excluding fixed units, walls and any zones left uncovered.
Select the pipe centres. 200 mm is typical for a well-insulated room, 150 mm for higher output or perimeter zones, and 100 mm for rooms needing maximum output such as bathrooms or areas with high heat loss.
Enter the distance from the manifold to the room. The calculator allows two tails per loop over that distance, since each loop leaves and returns to the manifold.
The calculator shows the pipe laid in the floor, the total including tails, and the number of loops needed with each kept under about 100 m. Use the loop count to select the manifold and the total to order pipe.
The per-square-metre figures follow standard practice: 5 m/m² at 200 mm centres, roughly 6.7 m/m² at 150 mm and 10 m/m² at 100 mm. Tighter spacing puts more pipe in the floor and lifts the output, which is how perimeter zones and high-loss rooms get boosted — often by running the edges at 100 mm and the field at 200 mm.
Keeping loops under about 100 m for 16 mm pipe limits the pressure drop so the circulator can push adequate flow and the loops balance at the manifold. Larger-bore pipe can run longer loops; very long single loops starve of flow and leave cold ends. Aim to keep loop lengths on a manifold reasonably similar so balancing is straightforward.
This is an estimate for ordering and setting out, not a full UFH design. The real output depends on floor construction, covering, flow temperature and insulation, and the definitive loop layout comes from the manifold schedule and a proper heat loss and emitter design. Add a sensible margin when ordering pipe. Guidance only — confirm against the system design.
Rule-of-thumb estimate for 16mm pipe. Confirm loop lengths and spacing against the manifold and system design.
It is set by the pipe spacing: about 5 m of pipe per m² at 200 mm centres, 6.7 m/m² at 150 mm, and 10 m/m² at 100 mm. So a 20 m² room at 200 mm centres needs roughly 100 m of pipe in the floor, plus the tails back to the manifold.
For 16 mm pipe, loops are normally kept to around 100 m to keep the pressure drop manageable and the flow adequate. Larger 18 mm or 20 mm pipe can run longer loops. Exceeding the practical maximum leaves cold ends and makes balancing difficult, so rooms bigger than one loop are split across several loops on the manifold.
200 mm centres suit a typical well-insulated room, 150 mm gives more output for higher heat loss, and 100 mm is used where maximum output is needed such as bathrooms or against cold external walls. Many installs mix spacings — tighter at the perimeter, wider across the middle — to match the heat loss.
Divide the total pipe length (floor pipe plus tails) by the maximum loop length — about 100 m for 16 mm pipe — and round up. A room needing 180 m of pipe therefore splits into two loops. Each loop takes its own flow and return port on the manifold, so the loop count also tells you the manifold size.