Rule of thumb: ~6L per single, ~10L per double, plus boiler and pipework.
The TradePlanr app puts this calculator — plus gas & electrical certificates, quoting, invoicing and job sign-offs — on your phone, working even with no signal. Free to start — works in your browser, no download needed.
This free system volume calculator estimates the total water content of a wet central heating system in litres. It is built for UK heating engineers and plumbers who need a quick volume figure for inhibitor and cleaner dosing, expansion vessel sizing, or estimating fill and drain-down times.
Count the single and double panel radiators on the system and enter them along with an allowance for the boiler and pipework (25 litres by default). The calculator uses the standard rule of thumb of roughly 6 litres per single panel radiator and 10 litres per double, then adds the boiler and pipework allowance to give the estimated total.
System volume (L) = (singles × 6) + (doubles × 10) + boiler & pipework allowance
Each single panel radiator holds roughly 6 litres of water and each double panel roughly 10 litres — sensible averages across typical domestic radiator sizes. The boiler and pipework allowance (default 25 litres) covers the heat exchanger, any low-loss header or cylinder coil, and the pipe runs. The sum is the estimated total water content of the system.
Enter the number of single panel radiators on the system, including towel rails as singles. Each one is counted as roughly 6 litres of water content.
Enter the number of double panel radiators — each counts as roughly 10 litres. If the system has a lot of unusually large or small radiators, adjust your counts to compensate.
The default 25 litres covers a typical domestic boiler and pipe runs. Increase it for larger properties, long pipe runs in bigger bore pipework, or systems with a cylinder coil or low-loss header.
The calculator shows the radiator water content and the estimated total system volume in litres. Use the total to dose inhibitor, size an expansion vessel or estimate refill quantities.
This is a rule-of-thumb estimate: roughly 6 litres per single panel radiator, 10 litres per double, plus a boiler and pipework allowance. It is accurate enough for chemical dosing and vessel sizing on typical domestic systems, where a litre or two either way makes no practical difference.
Systems that stray from the typical domestic pattern need more care. Underfloor heating loops, oversized period radiators, cast iron column radiators, buffer stores and extensive 28 mm primaries all add significant volume that the standard per-radiator figures miss — raise the allowance or work out those elements separately.
Getting the volume roughly right matters most for inhibitor dosing, since underdosed inhibitor protects poorly, and for expansion vessel sizing, where the vessel calculation scales directly with system volume. If you need a precise figure, meter the water in on refill.
Rule of thumb: ~6L per single, ~10L per double, plus boiler and pipework.
As a rule of thumb, allow about 6 litres per single panel radiator and 10 litres per double, plus around 25 litres for the boiler and pipework. A typical 3-bed house with 4 singles and 4 doubles comes out at roughly 89 litres. For a precise figure, meter the water in when refilling the system.
Dose to the manufacturer's stated coverage for your estimated system volume. Most standard bottles of inhibitor treat around 100 litres — roughly a typical domestic system — but always check the label, and add another dose if your calculated volume exceeds the bottle's coverage. Underdosing leaves the system poorly protected against corrosion and scale.
Three main reasons: dosing inhibitor and cleaner correctly, sizing an expansion vessel (the calculation scales directly with water content), and estimating fill, flush and drain-down quantities. It also feeds into glycol dosing on systems that need frost protection.
No — the per-radiator figures only cover panel radiators. Underfloor heating loops hold significant extra water depending on pipe diameter and total loop length, so calculate their content separately from the pipe specification and add it to the boiler and pipework allowance.
TradePlanr Agency builds custom-coded, SEO-optimised websites and runs Google Ads for UK trades — from £375, by the team behind TradePlanr.
Explore TradePlanr Agency