Central Heating System Volume Calculator — Total Water Content

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.