Before you request a quotation for underfloor-heating pipe, a contractor has to settle on one number: total metres. Guessing is expensive in both directions — too much coil ties up cash on the warehouse floor, too little stops the job and forces a costly top-up shipment. The formula and guideline tables below cover the vast majority of standard tenders.
The core formula
The 1.10–1.15 factor is the waste allowance for cutting, wall-edge bends and jump-over loops at doorways. Use 1.15 for awkward room shapes, 1.10 for clean rectangles. The lead-in is the run from the edge of the floor area to the manifold; since supply and return each need a leg, budget roughly double the distance to the manifold per loop.
Laying pitch: metres per m²
The tighter the pitch, the more pipe per square metre. Standard pitches are 100, 150 and 200 mm:
- 100 mm — about 10 m of pipe per m² (cold zones, corner rooms, rooms with large glazing).
- 150 mm — about 6.7 m per m² (the most common choice for living spaces).
- 200 mm — about 5 m per m² (well-insulated floors, low-heat-loss corridors).
These are estimation guidelines; confirm the exact pitch and metre count against the pipe's specification sheet and the project heat-loss calculation.
Loop length limits
A single loop is limited by hydraulic resistance. As a rule of thumb: for 16×2 mm keep a loop to about 60–80 m, for 20×2 mm about 80–100 m. Bigger zones should be split into several loops rather than stretched into one long circuit.
Which area to count
Run the calculation on the floor area that will actually be piped, not the gross building area on the plan. Deduct:
- space under fixed furniture and kitchen units — typically 10–20 %;
- wet zones, risers, cable ducts and column bases;
- the cold strip along walls and door thresholds (50–100 mm in from every wall).
For a 90 m² flat the real piped area is usually 55–65 m². Getting this step wrong is exactly where the "extra hundred metres" in an order come from, so ask the designer for a layout drawing with the piping boundaries marked before you price the coil.
Worked example: a 90 m² apartment
Rounding up to whole coils
Pipe ships in factory coils of standard length — 100, 120, 200, 240 or 300 m. Always round the final figure up to whole coils: a calculated 486 m becomes five 100 m coils. The tail of one coil is used as the lead-in of the next circuit, so almost nothing is wasted. Never split a loop into joined pieces: a circuit buried in the screed must be one continuous length with no couplings.
Budget the manifold set and the edge-insulation strip separately — they are counted by number of circuits and room perimeter, not by pipe metres. When you request a quotation, also check whether sleeve protection at the manifold is included in the price.
Guideline table: area → metres
| Covered area (m²) | Pitch 100 mm | Pitch 150 mm | Pitch 200 mm |
|---|---|---|---|
| 30 | ≈ 330 m | ≈ 225 m | ≈ 170 m |
| 50 | ≈ 550 m | ≈ 375 m | ≈ 280 m |
| 70 | ≈ 770 m | ≈ 525 m | ≈ 395 m |
| 90 | ≈ 990 m | ≈ 675 m | ≈ 505 m |
| 120 | ≈ 1 320 m | ≈ 900 m | ≈ 675 m |
The table already includes a ~10 % allowance and is rounded. Treat it as an example guideline: the real loop layout and lead-in lengths must be verified against your drawings and the supplier's pipe data sheet.
Once the metre count is fixed, the next step is logistics — how many of those metres fit in a container and what that does to your landed price. That is the subject of the next articles in this module.