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.
Formula
The calculator converts every length to whole millimetres, then:
manifold point = a point along the chosen wall, at the entered distance from that wall's
own start corner
for a candidate circuit count n = 1, 2, 3, ...:
split the room's bounding box into n equal-width zones
for each zone:
trace a bifilar (double) spiral coil over the zone, at the entered pipe spacing,
entering/leaving at the zone's own corner nearest the manifold point
coil length = the sum of that trace's own segment lengths
manifold leg (flow) = distance from the manifold point to the coil's own entry
manifold leg (return) = distance from the manifold point to the coil's own exit
zone total = coil length + manifold leg (flow) + manifold leg (return)
stop at the first n where every zone's own total fits the maximum circuit length
total pipe length = sum of every zone's own total
waste pipe length = ceil(total pipe length × waste% / 100)
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)
Each circuit's own pipe length is the actual traced spiral, not a shared area-based estimate — a zone's own coil, plus its own real distance to the manifold, both come from the manifold's entered position. That is why splitting a room into more circuits doesn't just divide one number by two: a farther zone's own circuit is longer than a nearer zone's, even at equal area, and the calculator reports each one separately rather than an average.
Worked example
Defaults: room shape — rectangle 5 × 4 m, pipe spacing 150 mm, pipe outer diameter 16 mm, manifold on wall 1 at 500 mm from its start corner, maximum circuit length 100 m, screed thickness 50 mm, edge strip height 80 mm, waste 5%.
For the calculator's own default inputs, the room fits in a single circuit: the manifold sits close to the room (500 mm along wall 1), so that one circuit's own coil plus its two short manifold legs stays under the 100 m maximum, and no split is needed. A larger room, a tighter pipe spacing, or a shorter maximum circuit length is what pushes the result to more than one circuit — see the next section and the FAQ below for what changes when that happens.
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's own length — the coil needed to cover its share of the floor, plus its own real distance to and from the manifold — can't exceed the maximum circuit length you entered. If one circuit can't cover the whole room within that limit, the calculator splits the room into as many zones as needed, traces a separate spiral for each, and reports each circuit's own pipe length rather than one average.
Why does the manifold's position matter, instead of just entering one distance? Because every circuit sits at a different point in the room, each one's own real distance to the manifold is different too — a zone across the room from the manifold needs more pipe just to reach it than a zone right beside it. Entering where the manifold actually sits (which wall, and how far along it) lets the calculator work out each circuit's own distance instead of applying one guessed number to every circuit alike.
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's used for one check: the edge insulation strip has to be tall enough to cover the full build-up above the subfloor — the screed thickness above the pipe, plus the pipe's own diameter. If your entered strip height is shorter than that, the calculator warns you.
Is the pattern in the drawing exactly how I should route the pipe on site? No. Each circuit's own zone boundary is schematic — a real installer decides where that boundary actually falls based on furniture, cold walls, and the manifold's own position, not an even split of the room. The circuit count and each circuit's own pipe length are real, computed from the actual manifold position and room geometry, not from the drawing's own exact shape. The drawing uses a bifilar (double) spiral for each circuit — flow and return pipe running side by side, spiralling in and back out — because that gives a more even floor surface temperature than a back-and-forth serpentine, whose far end always runs cooler than its near end. A serpentine is the other common real-world pattern; this calculator draws the spiral because it's the one more installers reach for, but doesn't distinguish between the two for the pipe-length math, since both cover the same area at the same spacing.
Where do the maximum circuit length and pipe spacing 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 or pipe spacing — both 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 150 mm spacing default and
100 m maximum circuit length default are common practical values, not values derived from a
cited source.
Each circuit's own zone is one equal-width strip of the room's own bounding box, not the true outline — a schematic simplification for a notched room (L, T, U, alcove, or a custom outline), same as floor-tiles'/laminate-floor's own grid math. Zone area sums to the bounding box's own area, which can exceed the room's true area near a notch; the room area and edge strip length reported separately still come from the true outline.
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.
Each circuit's own zone boundary and spiral are schematic — a real installer decides where a zone boundary actually falls, based on furniture, cold walls, and the manifold's own position, not an even split of the bounding box. The circuit count and each circuit's own reported pipe length are real, traced from the entered spacing and the manifold's own actual position, not approximated from area alone. For a zone with many spiral rings at the chosen spacing, the drawing decimates the ring count so the SVG stays legible; this does not affect the pipe-length numbers, which are always traced at the true spacing.