Estimate how many litres of water a radiator holds from its type, length and height, or the total water volume of a whole heating system.

Typical water content per metre of length at 60 cm high; the result scales with height. Check the maker’s data sheet for an exact figure.

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Radiator Water Capacity Formula

The calculator uses typical water content per metre for each radiator type. A simple rectangular-volume approximation can be used as a cross-check if you have a reasonable estimate of the effective internal water depth.

WRC ≈ (L × H × D) / 1000
  • Where WRC is the estimated water volume (liters)
  • L is the length (cm)
  • H is the height (cm)
  • D is the effective internal water depth used for the approximation (cm)

To calculate the rectangular-approximation water volume, multiply the dimensions in centimeters to get cm³, then divide by 1000 to convert cm³ to liters. For most real radiators, manufacturer water-content data (or a radiator-type-specific estimate such as the calculator’s radiator-type presets) is more reliable than a simple box approximation.

What is a Radiator Water Capacity?

Definition:

Radiator water capacity (also called water content) is the amount of water/coolant the radiator holds internally when filled. It depends on the radiator’s internal channels and construction, so it is typically best obtained from the manufacturer’s specifications or estimated using radiator-type data (for example, liters per section for cast iron radiators).

How to Calculate Radiator Water Capacity?

Example Problem:

The following example outlines the steps and information needed to calculate a rectangular-approximation water volume by hand.

First, determine the length. In this example, the length is 120 cm.

Next, determine the height. In this example, the height is 50 cm.

Then, choose an effective internal water depth for the approximation. In this example, D = 3 cm.

Finally, calculate the water volume using the formula above:

WRC ≈ (L × H × D) / 1000

WRC ≈ (120 × 50 × 3) / 1000

WRC ≈ 18 liters

FAQ

Why does the Basic tab use an “effective depth” input?

A true radiator’s water capacity depends on internal channels/tubes and cannot be determined reliably from external height and length alone. A simple rectangular-volume approximation requires an assumed effective internal depth. The calculator instead uses typical water content for the radiator type you choose; check manufacturer specifications for an exact figure.

How accurate is the calculated water capacity?

Accuracy depends on the method. The calculator’s radiator types use typical water-content estimates for those categories (still not universal). A box approximation is a rough geometric estimate and can be significantly different from a real radiator’s internal water content.

Can this formula be used for all types of radiators?

No. The rectangular-approximation formula is not a general radiator water-content formula. Different radiator designs (panel, column/cast iron, convectors, fan coils, etc.) have different internal volumes, so manufacturer water-content data or a type-specific estimate is recommended.