Radiator Sizing Calculator

Heat load
Operating temperatures
Catalogue data
Sectional radiator

Catalogue outputs converted from 75/65/20 °C, where the logarithmic mean temperature difference is 49.83 K. Check which condition your catalogue uses — older catalogues give 90/70/20 °C, which makes the same radiator look bigger.

Exponent n = 1.3. Panel radiators are typically about 1.28–1.36; each model has its own value on its data sheet. The further your temperatures are from the catalogue condition, the more n matters.

Catalogue output needed (each radiator)1,958W
Output factor at your temperatures0.511
Mean temperature difference ΔT29.72K
Temperatures
Your ΔT = (t₁ − t₂) / ln((t₁ − tᵣ)/(t₂ − tᵣ))29.72 K
Catalogue ΔT at the rating condition49.83 K
Flow − return t₁ − t₂10.0 K
Output
Room design heat load1,000 W
Load per radiator1,000 W
Output factor f = (ΔT / ΔT_catalogue)ⁿ0.511
Catalogue output needed, each = load / f1,958 W
Catalogue output needed, all radiators1,958 W
Water flow per radiator86.0 kg/h

Standards applied: LST EN 442-2

About this calculator

Radiator catalogues list each model's output at one fixed rating condition — today almost always 75/65/20 °C (flow 75 °C, return 65 °C, room 20 °C) from EN 442 (in Lithuania LST EN 442). Your system rarely runs at exactly those temperatures. A condensing boiler often runs at 55/45 °C, a heat pump lower still, and at those temperatures the same radiator gives far less heat.

This calculator takes the room's design heat load — for example from the room heat loss calculator — and your actual flow, return and room temperatures, and tells you which catalogue output to look for. It also gives the correction factor, the water flow each radiator needs and, for a sectional radiator, the number of sections.

Formula

EN 442-2 describes a radiator's output with its characteristic equation Φ = K_m × ΔTⁿ, where ΔT is the logarithmic mean temperature difference between the water and the room:

ΔT      = (t₁ − t₂) / ln((t₁ − tᵣ) / (t₂ − tᵣ))
f       = (ΔT / ΔT_catalogue)ⁿ
Φ_cat   = Φ_design / f
m       = Φ / (1.163 × (t₁ − t₂))          water flow, kg/h
  • t₁, t₂, tᵣ — flow, return and room temperature, °C.
  • ΔT_catalogue — the same formula at the catalogue condition: 49.83 K at 75/65/20 °C (the "ΔT 50 K" of EN 442), 42.06 K at 70/55/20 °C and 59.44 K at 90/70/20 °C.
  • n — the radiator exponent from its data sheet. The default 1.3 is the value Purmo's own correction table is prepared for; Purmo's panel radiators measure 1.28–1.36 depending on type and height.
  • 1.163 Wh/(kg·K) — the specific heat of water (4.187 kJ/(kg·K)).

The logarithmic mean is the one EN 442 uses. The simpler arithmetic mean ((t₁ + t₂)/2 − tᵣ) overstates the output when the return is cool, which is exactly the case of low-temperature systems.

For a sectional radiator, the number of sections is the catalogue output divided by the output of one section at the same catalogue condition, rounded up.

Worked example

A room needs 1000 W. The system runs at 55/45 °C, the room is kept at 20 °C, and the catalogue gives outputs at 75/65/20 °C with n = 1.3:

ΔT          = (55 − 45) / ln(35 / 25) = 10 / 0.3365 = 29.72 K
ΔT_catalogue = (75 − 65) / ln(55 / 45) = 10 / 0.2007 = 49.83 K
f           = (29.72 / 49.83)^1.3 = 0.5107
Φ_cat       = 1000 / 0.5107 = 1958 W
m           = 1000 / (1.163 × 10) = 86 kg/h

Choose a radiator listed at about 1960 W in the 75/65/20 °C column. The same radiator on a 70/55 °C system would need only 1247 W (factor 0.80). These results match Purmo's published correction table for n = 1.3 (1,96 at 55/45/20, 1,25 at 70/55/20).

FAQ

Where do I get the room's heat load? From the heating design, or from the room heat loss calculator on this site. Do not size radiators from floor area alone — a well-insulated room and an old corner room of the same size can differ several times over.

My catalogue gives 90/70/20 °C outputs. What changes? Choose 90/70/20 °C as the rating condition. A radiator gives about 20 % less heat at 75/65/20 °C than at 90/70/20 °C (with n = 1.3), so outputs from older catalogues look bigger than the same radiator in a modern one.

Why does a heat pump need such big radiators? Because output falls faster than the temperature difference: with n = 1.3, halving ΔT leaves only about 40 % of the output. At 45/35/20 °C a radiator gives under a third of its catalogue output. The calculator warns when the catalogue output must be more than twice the room's load.

Does the flow rate matter? EN 442's characteristic equation holds at constant water flow. If the real flow differs a lot from the one that gives your t₂, the return temperature changes and so does the output. The water flow shown is what your chosen t₁ − t₂ requires — balance the system to it.

Assumptions and limits

  • The load is split evenly between identical radiators in the room.
  • Output conversion uses the EN 442-2 characteristic equation with one exponent n. The real exponent depends on the model and slightly on the water flow; the default is a typical value, not your radiator's.
  • The rating conditions offered are 75/65/20, 70/55/20 and 90/70/20 °C.
  • Not included: covers, niches, paint and furniture in front of the radiator, connection type (side/bottom/same-side), and the extra output a radiator may need to heat the room up after a setback. Manufacturers give their own correction factors for these.
  • The "more than twice the load" warning is this calculator's own flag, not a limit from any standard.