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Central Heating System Volume Calculator — Total Water Content
Calculate the total water volume in your central heating system. Essential for dosing inhibitor, antifreeze, and sizing expansion vessels.
Count all radiators in the system
Total flow + return pipework
Enter 0 if combi boiler (no cylinder)
Typically 2-5 litres for domestic boilers
F&E header tank water volume if open-vented system (default 0)
Litres of inhibitor per 100L system water (default 1.0)
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How We Calculate This
This calculator adds up the water content of every component in your central heating system to give a total system volume in litres.
The formula
Total volume = Radiator volume + Pipe volume + Cylinder volume + Boiler volume + Header/F&E tank volume (open-vented systems)
The inhibitor figure is then calculated as system volume × dose rate ÷ 100 (default 1 litre of inhibitor per 100 litres of system water).
Component volumes used
The radiator figures are deliberately conservative allowances, not measured per-dimension content. Measured manufacturer content (Stelrad Compact, BS EN 442, 600mm height: Type 11 = 3.25 L/m, Type 22 = 6.60 L/m) is lower. Over-estimating system volume is the safe direction for inhibitor dosing and expansion-vessel sizing, though.
- Small single panel: 5 litre allowance (≈2 L measured for a 600x600)
- Medium double panel: 10 litre allowance (≈6–7 L measured for a 600x1000)
- Large double convector: 15 litre allowance (≈9–12 L measured for a 600x1400)
- 10mm pipe: 0.06 litres/metre
- 15mm pipe: 0.14 litres/metre
- 22mm pipe: 0.32 litres/metre
- 28mm pipe: 0.54 litres/metre
Pipe figures use the BS EN 1057 Table X copper bore (8.8 / 13.6 / 20.2 / 26.2 mm internal diameter). For exact volumes, check individual radiator manufacturer data sheets. The resulting system volume is what you need for dosing inhibitor (per BS 7593, the standard for treatment of water in domestic central heating systems). It's also what you need for sizing an expansion vessel.
Frequently Asked Questions
Knowing your total system water volume is essential for correctly dosing inhibitor (like Fernox F1 or Sentinel X100), sizing expansion vessels, and calculating antifreeze quantities. Under-dosing inhibitor can lead to corrosion and sludge, while over-dosing wastes money. Use our Inhibitor Dosing Calculator and Expansion Vessel Sizing Calculator with the result from this tool.
A typical UK home with 8-10 radiators and a combi boiler holds approximately 80-120 litres of water. A system with a hot water cylinder may hold 150-250+ litres depending on cylinder size. Larger properties with more radiators hold proportionally more.
As a rough guide, measure the distance from the boiler to the furthest radiator and multiply by 2.5 to account for flow, return, and branch runs. For a typical 3-bed semi, pipe runs are often 40-60 metres total. For more accuracy, measure each floor and add runs to each radiator.
Measured manufacturer content is lower than people expect. Stelrad Compact data (tested to BS EN 442) gives 3.25 litres per metre for a Type 11 single panel and 6.60 litres per metre for a Type 22 double panel, both at 600mm height. So a 600x600 single panel holds only about 2 litres, a 600x1000 double about 6-7 litres, and a 600x1400 double convector about 9-12 litres. This calculator deliberately uses larger round allowances (5, 10 and 15 litres) because over-estimating system volume is the safe direction for inhibitor dosing and expansion-vessel sizing. You will never under-dose. Use manufacturer datasheets if you need exact content.
Yes. Combi boilers typically hold 2-4 litres of water in the heat exchanger. System boilers may hold 3-5 litres. Regular boilers with an external heat exchanger hold similar amounts. We default to 3 litres which is typical for most domestic boilers.
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Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.