System water volume calculator
Estimate total system capacity from radiators and pipework, and how much inhibitor to dose. Set the radiators and pipe run — the rest updates as you go.
The system
Count every radiator and towel rail on the system.
Total run of flow and return pipe, roughly.
Total system volume
0.96L of inhibitor — a single 1L bottle covers it.
Illustrative only. Based on average radiator water content and standard pipe fill rates, plus a general 1L-per-100L inhibitor dosing guideline. Check the exact dose on your inhibitor's own datasheet.
Inhibitor only works if there's enough of it in the system. Dose too little and corrosion, sludge and boiler noise creep back in within a year or two. Dose randomly — "a bottle should do it" — and you're either wasting product or leaving the system under-protected. This calculator estimates total system water content from radiators and pipework, then applies the standard dosing guideline so you know how much to actually put in.
Where the volume comes from
Most of a system's water sits in the radiators, not the pipes — a single large double panel radiator can hold more water than several metres of pipework combined. Pipe diameter matters too: 28mm pipe holds more than three times as much water per metre as 15mm, so a system with wide primary runs adds up faster than the radiator count alone suggests. A fixed allowance for the boiler's own heat exchanger and smaller components rounds out the total.
The dosing guideline
The figure used here — roughly 1 litre of inhibitor per 100 litres of system water — is the general rule most UK inhibitor manufacturers publish, and it's a reasonable planning figure for working out how much stock to bring to a job. Different products dose slightly differently, and some brands quote dosing by radiator count instead of volume. Always check the datasheet on the actual product you're using before you dose a real system.
Worked example
Eight medium radiators (10L each = 80L), 40 metres of 15mm pipework (5.8L) and the fixed boiler allowance (10L) comes to just under 96 litres — call it a single 1L bottle of inhibitor, with a little margin either way.
When to measure instead of estimate
On a full system flush or a new install, draining and measuring the actual volume as it refills is more accurate than any calculator — do that where precision genuinely matters. This tool is for quoting and stocking the van, not for replacing a proper fill-and-measure on a job where getting the dose exactly right counts.
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