About this calculator
This calculator helps estimate how much pipe a water (hydronic) underfloor heating installation needs, how many circuits a room must be split into, how many manifold ports to allow for, and how much screed and edge insulation strip go with it — enter the room shape, the pipe spacing and outer diameter, where the manifold sits, and the maximum length you're willing to run a single circuit. Works for any water underfloor heating pipe (PEX, PE-RT, or multilayer) laid in a screed.
Rectangular, L, T, U and alcove rooms are supported, as is a custom outline built from rectangles. The coil is laid on the room's own true outline, never on its bounding box, so it never runs through space the room doesn't have — and a room of any of these shapes can be split into several circuits.
Each circuit is drawn as a bifilar (counterflow) spiral: supply and return run side by side into the centre, turn round, and come back out between each other, so warm and cooler runs alternate across the whole floor. Every corner is a real bend at the pipe's minimum bend radius, and the pipe length is measured from exactly the shape that's drawn.
Formula
All lengths are handled in whole nanometres internally; s is the pipe spacing, e the wall clearance, R the minimum bend radius.
R = bend radius factor × pipe outer diameter (spacing must be ≥ 2R)
e = max(wall clearance, R)
the two pipes of a circuit travel as one "ribbon": supply on the wall side,
return on the inside, s apart; the ribbon's centre line:
ring k = the room (or zone) shrunk by e + s/2 + 2s·k on every side
lap k = once round ring k; it steps in to ring k+1 with one 2s step,
just before getting back to where it started
centre = the last lap ends with a short run into what's left inside,
then a half-circle turn (radius s/2) joins supply to return
every corner: the inner pipe bends at R, the outer at R + s (same centre)
rule: any two runs of the ribbon that aren't neighbours stay ≥ 2s apart,
so every pipe is ≥ s from every other pipe
lead-in = the same ribbon from the manifold's ports to the spiral's start
circuit length = spiral (field) length + lead-in (connection) length
if a circuit would be longer than the maximum circuit length:
split the room into N zones of equal area, N = 2, 3, … (at most 12),
as strips across or along the manifold wall, each with its own spiral
and its own pair of manifold ports; lead-ins to far zones run in lanes
next to the manifold wall (or up a side lane) that nearer zones leave free
use the smallest N that keeps every circuit within the maximum
total pipe length = sum of every circuit's length
waste pipe length = total pipe length × waste% / 100 (rounded up)
total pipe length (incl. waste) = total pipe length + waste pipe length
manifold port pairs = number of circuits
edge insulation strip length = room perimeter
screed volume = room area × screed thickness (above the pipe)
If the pipe spacing is narrower than 2 × R, the calculator stops with an error: the pipe on
the inside of a bend would have to bend tighter than its minimum radius. If the room is too
small for even one full turn of the ribbon at the spacing and wall clearance entered, it also
stops with an error rather than drawing a partial coil.
Worked example
The defaults — rectangle 5 × 4 m, pipe spacing 200 mm, 16 mm pipe, bend radius factor 5 (an 80 mm bend radius), wall clearance 150 mm, maximum circuit length 100 m, waste 5% — with the manifold moved to the middle of wall 1 (2,500 mm along it).
- Ring 0 is the room shrunk by 150 + 100 = 250 mm: 4,500 × 3,500 mm. Each further ring is 2s = 400 mm further in on every side, so a full lap fits on rings 0–3 (the short side is 3,500 − 800k ≥ 400 mm for k ≤ 3), and ring 4 (1,300 × 300 mm) takes the centre run. That is 4 full laps — 8 pipe rings — plus the centre.
- The ribbon's centre line, from the manifold to the centre turn, is 46.0 m long; the two pipes together are twice that, less 154.5 mm for each of its 22 rounded corners ((2 − π/2) × (80 + 280) mm), plus the half-circle centre turn (π × 100 mm): 92.0 − 3.40 + 0.31 = 88.91 m for the one circuit, of which 0.71 m is the lead-in.
- 5% waste adds 4.45 m: 93.36 m in total, 1 manifold port pair.
FAQ
Does the pipe spacing tell me how much heat the floor will output (W/m²)? No. Heat output depends on the flow temperature, the floor covering, and the insulation below the pipe — none of which this calculator asks for. This calculator only computes how much pipe, how many circuits, and how much screed/edge strip a given spacing and layout need — it is not a heating design tool.
Why was my room split into more than one circuit? Each circuit — its spiral plus its lead-in from the manifold — can't exceed the maximum circuit length you entered. If one circuit can't cover the room within that limit, the room is split into equal-area zones, each with its own spiral and its own pair of ports on the manifold, using as few circuits as will fit. Zones far from the manifold have longer lead-ins, so their circuits are longer even at equal area; each circuit is reported separately.
Why does the manifold's position matter? Every circuit starts and ends at the manifold, so where it sits decides where each spiral starts and how long each lead-in is. The loops' ports are placed side by side on the manifold wall around the position you entered (moved only as far as needed to keep the pipes the wall clearance away from the walls at the ends of that wall).
What's a "manifold port pair"? Every circuit needs its own supply and return connection at the manifold — one pair of ports. Two circuits need two pairs (four ports total), and so on.
Why does pipe outer diameter matter if it isn't in the pipe length formula? It sets the bend radius (bend radius factor × diameter), which rounds every corner and so shortens the pipe slightly, and decides the smallest spacing that works. It's also used to check that the edge insulation strip is tall enough: it has to cover the screed above the pipe plus the pipe's own diameter.
Why is supply always the outer pipe? Supply runs in along the wall side of the spiral and return comes back out between its laps, so the hottest water meets the room's edges — where most heat is lost — first, and every warm run has a cooler run beside it. The only place two runs of the same kind sit side by side is at the very centre, where the circuit turns back.
Is the drawing exactly how I should lay the pipe on site? It's a realistic layout, not a construction drawing: zone boundaries are an even split of the floor, while a real installer places them around furniture, cold walls and the manifold. The circuit count and every length are real, measured from exactly the drawing shown. The dashed runs are each circuit's lead-in from the manifold; the solid runs are its heating field.
Why did part of my room get no spiral, with a note on it? A single spiral can't reach every pocket of some shapes — for example a U whose connecting part is narrower than one lap of pipe. The calculator first tries one more circuit to reach it; if that doesn't work either, it keeps the layout and adds a note that part of the zone is not covered.
What does marking a wall as exterior change? Exterior walls are the coldest, so along them the hot supply pipe should be the one nearest the wall. Every loop already runs supply on the outside of its own spiral; what can put the return pipe outside is a lead-in or a lane running beside a wall on its way from the manifold. For the walls you mark, the calculator chooses, among the layouts it can build, the spiral direction and the lead-in (where and how it joins the spiral, and so which port is supply) that keep supply on the wall side along the most exterior-wall length; the rings, spacing and zones stay the same. Where that still isn't possible, the layout is kept and a warning names the wall. Marking no walls gives exactly the layout you'd get without this option. Denser pipe spacing in an edge zone along exterior walls is not modelled.
The manifold is on a short step wall of an L-shaped room. Is the part behind it heated? Yes: the circuit covers the whole room, including the part of the floor behind the line of the manifold's wall (the other arm of the L). Only if no layout of the whole room works — for example when one circuit would be too long and the manifold's wall is too short for a second pair of ports — does the calculator fall back to the part in front of that wall, and then it warns that the floor behind the wall's line has no pipe. Moving the manifold to a longer wall usually avoids this.
Why did I get an error that the room couldn't be split? When one circuit would be longer than the maximum circuit length, the room is split into zones, each with its own pair of ports on the manifold. If no split into 2–12 circuits can be laid out, the error says why: the manifold's wall is too short for that many port pairs (with the length needed and the length available), the zones come out too narrow for a full turn, or the pipes to the farther zones can't be routed past the nearer ones. Putting the manifold on a longer wall is usually the fix; raising the maximum circuit length is the other.
Why did I get an error about pipe spacing and bend radius? Two neighbouring runs turn a corner around a common centre, so the inner one can only bend at the minimum radius if the spacing leaves room for it: the spacing must be at least twice the bend radius. Increase the spacing, lower the bend radius factor, or choose a smaller pipe diameter.
Where do the maximum circuit length, pipe spacing, and bend radius factor defaults come from? They're common practical values, not figures from a cited standard — see Assumptions and limits.
Assumptions and limits
meta.standards is deliberately empty. No single citable standard was found for maximum
single-circuit pipe length, pipe spacing, or the bend radius factor — all three depend on the
pipe material, the circulator's available head, and the specific underfloor heating system's
own technical approval, none of which this calculator asks for or models. The 200 mm spacing
default, 100 m maximum circuit length default, and bend radius factor default of 5 (a common
trade rule-of-thumb for a firm, supported PEX/PE-RT bend, pieced together from several
manufacturers' own installation guidance rather than one cited source) are common practical
values, not values derived from a cited standard.
If the wall clearance you enter is smaller than the bend radius, the bend radius is used instead: the outermost pipe bends at that radius round inside corners, and would otherwise come closer to the wall than the clearance. Where two circuits' zones meet, each keeps its pipes at least half a spacing (or the wall clearance, if larger) from the boundary, so neighbouring circuits are a full spacing apart.
The manifold is drawn as a row of port pairs on its wall, one pair per circuit, a spacing apart — real manifolds have their ports closer together and the pipes fan out to spacing just in front of them. A manifold wall too short for that row, a manifold behind a gap narrower than the pipes need, or a room whose zones would fall apart into separate pieces when split (possible for some U-shaped and alcove rooms that need several circuits) produces an error rather than a wrong layout.
This calculator does not compute or imply a heat output (W/m²). Spacing affects comfort and output, but translating it into a real figure needs the flow temperature, floor covering, and insulation below the pipe — none of which is asked for here. Treat this calculator's result as a quantity take-off, not a heating design.
The edge-strip-height warning (comparing your entered strip height against screed thickness plus pipe diameter) is general installation guidance, not a cited standard — an edge strip shorter than the full floor build-up can let the screed pour bridge to the wall.