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Work out how much pipe you need for wet underfloor heating. Choose the pipe spacing and size, enter the room and the distance to the manifold, and press Calculate to see the pipe in the floor, the number of loops, the length of each loop and the total pipe to buy.
| Spacing | Pipe In Floor | Loops | Pipe To Buy |
|---|
The same room, pipe size and manifold distance at other spacings.
You get the pipe in the floor, the number of loops and the length of each loop including tails, the total pipe to buy, the manifold ports and the edge insulation strip. A plan shows the loops running from the manifold, a table compares other spacings, and you can copy the result or save it as a PDF.
The pipe in the floor depends on the heated area and the spacing:
Pipe in the floor (m) = heated area (m²) × 1000 ÷ spacing (mm)
Then each loop needs a flow and a return tail between the manifold and the room, about twice the distance plus a metre to go up and down the wall at the manifold. Add a little extra, about 5%, for cutting and mistakes. Some suppliers quote slightly less pipe per square metre than the simple sum because the pipe stops short of the walls, so the calculator's figure is on the safe side.
| Spacing | Pipe Per m² | Typical Use |
|---|---|---|
| 100 mm | 10 m | High heat loss rooms, conservatories, edges by large glazing |
| 150 mm | 6.7 m | Average heat loss, or floor finishes that resist heat such as thick carpet |
| 200 mm | 5 m | Well-insulated rooms; the most common spacing |
| 300 mm | 3.3 m | Low heat loss areas, usually with larger pipe |
Closer pipes give more heat from the same water temperature, which matters for rooms with high heat loss or when running at the low flow temperatures used with heat pumps. Wider spacing uses less pipe but gives less heat. The right spacing comes from a heat loss calculation, not from guesswork.
Each loop, or circuit, is a single unjoined pipe that leaves the manifold, runs through the floor and comes back. Long loops cause too much pressure drop and uneven heat. Common UK guidance is a maximum of about 80 m for 12 mm pipe, 100 m for 16 mm and 120 m for 20 mm, including the tails; for example, Polypipe's guidance for its 15 mm pipe is not to exceed 100 m per circuit including tails. If a room needs more pipe, split it into equal loops on the manifold. Keeping loops at similar lengths makes the flow meters easier to balance.
Tails are often forgotten but add up quickly. A manifold 4 m from the room adds about 9 m to every loop, and a manifold 15 m away adds over 30 m, which can mean splitting a room into more loops. Place the manifold centrally, keep tails short, and insulate tails that pass through other rooms so they do not overheat them. Where several tails cross a doorway, the pipes get close together, which can make that spot warmer.
Pipe types include PE-RT, PEX and multilayer, usually with an oxygen barrier. Pipe is sold in coils; buy coils long enough to run each loop without a joint in the floor.
In a screeded floor, the pipe is clipped or stapled to insulation boards and covered with screed, which spreads the heat. In timber floors, the pipe sits in aluminium spreader plates between the joists or in grooved boards. Low-profile overlay boards go on top of an existing floor. Fit edge insulation strip around the room before screeding so the screed can expand. If you are levelling the floor before the finish goes down, our floor levelling compound calculator works out the bags, and our sand calculator helps with screed materials.
This calculator estimates pipe quantities; it does not design the system. A proper design starts with a room-by-room heat loss calculation, then picks the pipe spacing, flow temperature and number of loops to meet it, allowing for the floor finish. Many suppliers offer a free design with their kit. For radiators in rooms without underfloor heating, our radiator BTU calculator gives a rough heat requirement. Always pressure test the pipework before it is covered.
Kitchen: 5 × 4 m with 2 m² of fixed units, 16 mm pipe at 200 mm centres, manifold 4 m away.
Open-plan room: 7 × 5 m, 16 mm pipe at 150 mm centres, manifold 6 m away.
The calculator takes the room area, subtracts any area not heated, and multiplies by 1000 ÷ spacing for the pipe in the floor. Each loop's tails are twice the distance to the manifold plus 1 m. It starts with one loop and adds loops until each loop, its share of the floor pipe plus its tails, is within the maximum for the pipe size. The total is the floor pipe plus all the tails, plus 5%, and the coil figure rounds that up to the next 50 m. If the tails alone would use most of a loop, it warns that the manifold is too far away. The table repeats the sum at the other spacings.
About 10 m at 100 mm centres, 6.7 m at 150 mm, 5 m at 200 mm and 3.3 m at 300 mm, plus tails to the manifold.
Common UK guidance is about 100 m for 16 mm pipe, 80 m for 12 mm and 120 m for 20 mm, including tails.
Enough that each loop, including its tails, stays within the maximum length. A 20 m² room at 200 mm centres often needs one or two.
200 mm is common in well-insulated rooms; 100 or 150 mm suits higher heat loss. A heat loss calculation decides.
Yes. The flow and return pipes between the manifold and the room are part of the loop.
Loops should be one unjoined length in the floor. Buy coils long enough for each loop.
16 mm is the most common. 12 mm is used for low-profile systems and 20 mm for large areas.
Yes, always pressure test the pipework before the screed or floor covers it.
Pipe per m² is the simple 1 ÷ spacing figure. Maximum loop lengths are common UK guidance, including supplier guidance such as Polypipe's. This is a quantity estimate, not a heating design. Reviewed November 2026.