- 1. Create certificates and manage your jobs with Gas Engineer Software
- 2. Other useful heating & plumbing calculators
- 3. Expansion Vessel Sizing Calculator
- 4. Gas Rate Calculator
- 5. Gas Pipe Sizing Calculator
- 6. Heat Loss Calculator
- 7. Installation Volume Calculator
- 8. How to use this heating system volume calculator
- 9. How to work out heating system water volume
- 10. Frequently asked questions
- 1. Create certificates and manage your jobs with Gas Engineer Software
- 2. Other useful heating & plumbing calculators
- 3. Expansion Vessel Sizing Calculator
- 4. Gas Rate Calculator
- 5. Gas Pipe Sizing Calculator
- 6. Heat Loss Calculator
- 7. Installation Volume Calculator
- 8. How to use this heating system volume calculator
- 9. How to work out heating system water volume
- 10. Frequently asked questions
FREE TOOLS
Heating System Volume Calculator
Work out the total water content of a sealed heating system in seconds (pipework, radiators, boiler and extras) ready for expansion vessel sizing, inhibitor dosing or glycol calculations. Got your figure? Size the vessel with our Expansion Vessel Sizing Calculator .
Copper — enter the total length of each size, in metres.
Typical water content each — adjust if you have manufacturer figures.
Show breakdown & method
Method: total = pipework (length × water content per metre) + radiators + boiler + extras. Copper water content per metre:
| 8 mm | 0.03 L/m | 15 mm | 0.145 L/m | 28 mm | 0.54 L/m |
| 10 mm | 0.055 L/m | 22 mm | 0.32 L/m | 35 mm | 0.84 L/m |
Plastic pipe holds slightly less than copper, and radiator figures are typical averages — use manufacturer data for an exact result.
Take this figure to the Expansion Vessel Sizing Calculator to size your vessel.
Guidance only. An estimate of total system water content for expansion vessel sizing, chemical dosing and glycol calculations — cross-check against manufacturer data where accuracy matters.
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How to use this heating system volume calculator
The calculator adds up the total water content of a sealed heating system, so you can size an expansion vessel, dose inhibitor or work out glycol. Enter what’s on the system and read the total.
- Enter your pipework. Put in the total length, in metres, of each copper pipe size. The calculator works the water content from the internal bore.
- Enter your radiators. Add how many of each type you have. The litres-each figures are typical averages, so overwrite them if you have the manufacturer’s water content.
- Add the boiler and extras. Enter the boiler or heat-exchanger content from its manual, plus anything else on the circuit: a buffer vessel, underfloor loops, a low-loss header or a filter.
- Read the result. You get the total system water volume, with a breakdown of where the litres come from. Copy it, or take it straight to the Expansion Vessel Sizing Calculator to size your vessel.
How to work out heating system water volume
The total is everything the sealed circuit holds, added together:
Pipework
Multiply the total length of each pipe size by its water content per metre. Copper figures, worked from the internal bore:
| Copper size | Water content | Copper size | Water content |
|---|---|---|---|
| 8 mm | 0.03 L/m | 22 mm | 0.32 L/m |
| 10 mm | 0.055 L/m | 28 mm | 0.54 L/m |
| 15 mm | 0.145 L/m | 35 mm | 0.84 L/m |
Radiators
Multiply the number of each type by its water content. Manufacturer figures are exact, but typical averages are:
| Radiator type | Typical content each |
|---|---|
| Single panel (Type 11) | ~5 L |
| Double panel (Type 21/22) | ~9 L |
| Column / cast iron | ~15 L (varies widely) |
| Towel rail | ~3 L |
Boiler and extras
Add the boiler or heat-exchanger content (typically 1–3 L, in the manual), then anything else on the circuit: a buffer vessel or volumiser, underfloor heating loops, a low-loss header, or a magnetic filter.
- Pipework: (40 × 0.145) + (15 × 0.32) = 5.8 + 4.8 = 10.6 L
- Radiators: 8 × 9 = 72 L
- Boiler: 2 L
- Total: 10.6 + 72 + 2 = ~85 litres
These are estimates using copper pipe and typical radiator contents — plastic pipe holds slightly less, and radiator content varies with size, so use manufacturer data where accuracy matters (glycol quantities especially).
Frequently asked questions
Does it use the internal bore or the nominal pipe size?
The water content is worked from the internal bore, not the nominal size. The figures you enter against (15, 22, 28 mm, etc.) are the outside diameters pipe is sold as, and the bore each one runs on is shown beneath it. Getting this right matters — using the nominal size would over-read the volume by roughly 20%.
How accurate is estimating volume from boiler kW?
The kW estimate uses the BS 7074-1 rule of thumb (around 12 litres per kW of output) — quick, and close enough for sizing an expansion vessel, since that rounds up to the next stock size anyway. Where the exact figure matters, such as a glycol charge, add the system up component by component instead.
Should I include the boiler, buffer and low-loss header?
Yes — anything holding water on the sealed circuit counts. Add the boiler or heat-exchanger content from its manual, plus any buffer vessel or volumiser, low-loss header and magnetic filter. Individually they're small, but on a larger system they add up.
How do I account for underfloor heating?
UFH holds a surprising amount because the loops are long. Work out the loop volume (total pipe length × the water content per metre for that pipe size) and enter it in the "other" field. Leaving it out will noticeably undersize the total.
Does it work for plastic pipe?
The per-metre figures are for copper, taken from the internal bore. Plastic push-fit has a thicker wall and a slightly smaller bore, so it holds a little less per metre — treat a mostly-plastic system's pipework figure as a small over-estimate, or work it out exactly where accuracy matters.
What do I use the total for?
Sizing an expansion vessel, dosing inhibitor or cleaner (both go by system volume), and working out a glycol charge for frost protection — increasingly relevant on heat pump work. Round dosing and glycol up to whole containers.