About this calculator
How much water does a heating system hold? You need the figure to size the expansion vessel, to know how much antifreeze to buy for a system that may freeze, and to plan filling and flushing.
The calculator adds up the water in every pipe run — by material, size and length — and the water in the boiler, radiators, buffer tank and anything else you enter from data sheets. If the fill is a water–glycol mixture, it also splits the total into antifreeze concentrate and water.
Formula
d_i = OD − 2 × e
V_pipe = π / 4 × d_i² × L
V_total = Σ V_pipe + V_boiler + V_emitters + V_buffer + V_other
V_glycol = V_total × c / 100
V_water = V_total − V_glycol
OD × e— nominal outside diameter × wall thickness, as printed on the pipe.L— length of the run;c— antifreeze share of the fill, % by volume.
The pipe sizes offered, and where their dimensions come from:
| Pipe | Sizes (OD × wall, mm) | Read in |
|---|---|---|
| Copper (EN 1057) | 10×1, 12×1, 15×1, 18×1, 22×1, 28×1, 35×1.2, 42×1.2, 54×1.5 | HME SANCO data sheet |
| PE-X / PE-RT (EN ISO 15875 / 22391) | 14×2.0, 16×1.5, 16×2.0, 17×2.0, 20×2.0, 25×2.3, 32×2.9 | REHAU RAUTHERM, Uponor Comfort Pipe PLUS |
| Multilayer (EN ISO 21003) | 16×2.0, 20×2.0, 20×2.25, 25×2.5, 32×3.0, 40×4.0, 50×4.5, 63×6.0 | REHAU, Uponor MLC, Cappellotto |
| Steel, medium series (EN 10255) | DN15 21.3×2.6 … DN50 60.3×3.6 | suppliers' EN 10255 tables |
For every pipe the manufacturer also lists the water content per metre, the calculator's figures match them (for example 22×1 copper 0.314 l/m, 16×2 multilayer 0.113 l/m, 32×2.9 PE-X 0.539 l/m). A size that is not listed can be entered as Other pipe with its inner diameter.
Worked example
A small radiator system: 60 m of 16×2 multilayer to the radiators, a 20 m main in 20×2 multilayer, 6 m of 22×1 copper in the boiler room; the boiler holds 5 l and the eight radiators 40 l together.
16×2: d = 12 mm → 0.1131 l/m × 60 m = 6.79 l
20×2: d = 16 mm → 0.2011 l/m × 20 m = 4.02 l
22×1: d = 20 mm → 0.3142 l/m × 6 m = 1.88 l
pipes 12.69 l
+ boiler 5 l + radiators 40 l = 57.69 l
The pipes hold barely a fifth of the water — the radiators hold most of it. With 35 % antifreeze the fill needs 57.69 × 0.35 = 20.2 l of concentrate and 37.5 l of water.
FAQ
Where do I find the radiators' and boiler's water content? On their data sheets: radiator manufacturers list litres per radiator or per metre of length for each type and height; boilers and heat pumps state the water content of their heat exchanger. A buffer tank holds its nominal volume.
What do I do with the result? Enter it as the system volume in the expansion vessel calculator — the vessel size grows directly with it. Underfloor heating and buffer tanks make the total much larger than a "litres per kW" rule of thumb suggests.
How much antifreeze do I need? The calculator gives the litres for the percentage you enter. How cold that mixture can get without freezing depends on the product, ethylene or propylene glycol — use the dilution table on its packaging. A pre-mixed fluid is already diluted: if you fill with one, the whole system volume is that fluid.
Why is my multilayer pipe a different size? Multilayer wall thicknesses differ between brands (20×2.0 and 20×2.25 are both sold as "20 mm"). Check the marking printed on your pipe; if your size is not in the list, enter its inner diameter as Other pipe.
Assumptions and limits
- Inner diameter = nominal outside diameter − 2 × nominal wall thickness. Manufacturing tolerances change the real volume by a few per cent at most.
- The pipe length is the length of pipe, not of the route: count flow and return separately.
- Fittings, valves and manifolds are not included unless entered under "Anything else".
- Antifreeze is a volume share of the whole fill; mixing and freeze protection follow the product's instructions.