Underfloor Heating Calculator (Output, Pipe Length, Loops & Electric Mats)

How it works

output needed = room heat loss ÷ heated area · floor output (EN 1264 basic curve) q = 8.92 × (surface − room)^1.1 W/m², capped at the 29 °C (27 °C timber/vinyl) surface limit · mean water = surface + q × (R_finish + R_construction + R_spacing); flow = mean + ΔT/2, capped by the heat source · pipe = heated area ÷ spacing × 1.1 + 2 × tail per loop; loops = floor pipe ÷ (max loop − 2 × tail) · flow = W ÷ (4,180 × ΔT) L/s · electric: kW = area × W/m²; cost = heat loss × hours × price ÷ efficiency

Underfloor heating turns the whole floor into a low-temperature radiator, and the physics that makes it comfortable also caps it. The European standard EN 1264 gives the heat a floor surface passes to the room as 8.92 × (surface − room temperature)^1.1 W/m², about eleven watts per square metre per degree at normal temperatures. Surfaces above about 29 °C are uncomfortable underfoot and can damage timber, so a floor can only deliver so many watts. The room’s heat loss divided by the area actually heated must fit under that ceiling — easy in an insulated house, impossible in a leaky conservatory. The water must also be warm enough to push that heat up through the screed, boards and floor finish: each layer adds thermal resistance, so carpet or thick timber needs warmer water than tile, and wide pipe spacing needs warmer water than close spacing. The calculator checks the flow temperature against your heat source, since a heat pump works best at 45 °C or less. Loops are limited to about 70, 100 or 120 metres for 12, 16 or 20 mm pipe so the pump can push water through them, including the tails to the manifold. Electric mats skip the plumbing at a higher running cost, which suits small rooms and bathrooms.

Worked example

A 20 m² living room of average insulation loses about 1,200 W; with 10% of the floor under fixed units, the 18 m² of heated floor must deliver 67 W/m². A tiled floor at 21 °C can give up to 8.92 × 8^1.1 ≈ 88 W/m², so it works with the surface near 27 °C. In screed at 150 mm centres the water must average about 32 °C, so the flow is about 36 °C at a 7 K drop — comfortable for a heat pump. The floor takes 132 m of 16 mm pipe; with the manifold 3 m away that is two loops and 144 m in total, a flow of 2.5 L/min. At a SCOP of 4 and $0.25/kWh, six hours at full output cost about $0.45 a day.

Sources: Warmup — guide to underfloor heating temperature and heat output · The Underfloor Heating Store — design and layout guide (pipe sizes, spacing, surface limits)

Frequently asked questions