How Much Water Does a Radiator Hold?
Quick Answer
About 7 litres for a standard 600mm x 1200mm double-panel radiator
Type 21 (double panel): 7.1–7.2 litres. Type 22 (double panel, double convector): 7.2–7.3 litres. Type 11 (single panel): about 3.7 litres. Figures from manufacturer water-content data at 600mm high x 1200mm long.
Where These Figures Come From
Radiator water content depends on the internal waterway design, not just the overall size, so manufacturers publish it separately from the height and length on their technical data sheets. Kudox’s published steel panel radiator technical data lists water content in litres for every model across its full height and length range. Cross-checking against Screwfix’s own published specification for a Flomasta-branded 600mm x 1200mm radiator gives 7.10 litres for the Type 21 (single convector) version and 7.16 litres for the Type 22 (double convector) version, both close to Kudox’s 7.22 and 7.28 litre figures for the same nominal size. The small gap between manufacturers reflects differences in panel pressing and waterway design, not a measurement error.
| Radiator type (600 x 1200mm) | Water content |
|---|---|
| Type 11 — single panel | about 3.7 litres |
| Type 21 — double panel, single convector | 7.1–7.2 litres |
| Type 22 — double panel, double convector | 7.2–7.3 litres |
Water Content Changes with Length
For a 600mm-high Type 21 double-panel radiator, one of the most common sizes sold in the UK, Kudox’s published data shows:
- 600 x 600mm: 3.61 litres
- 600 x 900mm: 5.42 litres
- 600 x 1200mm: 7.22 litres
- 600 x 1600mm: 9.63 litres
- 600 x 1800mm: 10.84 litres
Water content scales roughly with length, since the extra length simply adds more of the same waterway cross-section.
Estimating Your Whole-System Water Volume
To size an inhibitor dose or check an expansion vessel, add up the radiators and allow for the pipework, boiler, and cylinder coil as well.
Example: an 8-radiator house, mostly 600mm x 1200mm double-panel radiators
- 8 radiators x 7.2 litres = 57.6 litres in the radiators alone
- Add roughly 20–30 litres for 15mm and 22mm pipe runs, the boiler heat exchanger, and (if fitted) a hot water cylinder coil
- Estimated total system volume: approximately 80–90 litres
That sits inside the 80–120 litre range our inhibitor dosing guide uses for a typical 3-bed semi with 8–12 radiators, which also puts the average radiator at 8–12 litres, consistent with the double-panel figures above.
Single vs Double Panel: Why the Gap Is Nearly 2x
A double panel (Type 21) is two steel panels welded together with a water gap between them, so it holds close to twice the water of a single panel (Type 11) of the same size: 7.22 litres versus 3.72 litres at 600 x 1200mm. Adding the second set of convector fins to make a Type 22 barely changes the water content (7.28 litres versus 7.22 litres), because the fins sit in the airflow behind the panels, not in the waterway.
Column and Towel Radiators
The figures above are for standard flat steel panel radiators, the most common type in UK homes. Column radiators and towel/ladder rails use a different internal structure of multiple tubes or bars rather than a flat panel, and their water content varies a lot between models and column counts. Check the specific product’s technical datasheet before using it to size an inhibitor dose or expansion vessel.
Why It Matters
- Inhibitor dosing: most corrosion inhibitors are dosed at 1 litre per 100 litres of system water
- Expansion or pressure vessel sizing: unvented systems need the vessel matched to the total system volume
- Feed and expansion tank capacity: on older open-vented systems
- Draining down: knowing roughly how much water will come out when removing a radiator for decorating or replacement
Figures cross-checked against Kudox and Screwfix manufacturer technical data, July 2026. Column and towel radiator water content varies by model and is not covered by the panel-radiator figures above. Last updated: July 2026.