Why do I need to know my system volume?

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.

How much water does a typical UK central heating system hold?

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.

How do I estimate my pipe run length?

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.

What is the water volume of a radiator?

Radiator water volume varies by size and type. A small single panel radiator (600x600mm) holds about 4-6 litres. A medium double panel (600x1000mm) holds 8-12 litres. A large double convector (600x1400mm+) holds 13-18 litres. Exact figures depend on the manufacturer.

Does the boiler hold water?

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.

TradeCalcs

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.

Number of Radiators

Count all radiators in the system

Average Radiator Size

Total Pipe Run (estimate)

Total flow + return pipework

Main Pipe Diameter

Hot Water Cylinder Volume

Enter 0 if combi boiler (no cylinder)

Boiler Water Volume

Typically 2-5 litres for domestic boilers

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

Component volumes used

Small radiator (600x600 single): ~5 litres
Medium radiator (600x1000 double): ~10 litres
Large radiator (600x1400+ double): ~15 litres
10mm pipe: 0.05 litres/metre
15mm pipe: 0.13 litres/metre
22mm pipe: 0.30 litres/metre
28mm pipe: 0.53 litres/metre

These are typical values based on UK standard radiator and pipe sizes. For precise volumes, check individual radiator manufacturer data sheets. This total system volume is used for dosing inhibitor, sizing expansion vessels, and calculating system requirements per BS 7593 (Treatment of water in domestic hot water central heating systems).

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.