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This free flow rate calculator gives you the circulation flow required to deliver a heat load at a chosen design ΔT — the temperature difference between flow and return. It is built for UK heating engineers sizing pumps, balancing systems and checking whether a circuit can actually carry the boiler or heat pump output it is asked to.
Enter the heat load in kW and the design ΔT in °C (the default is 20°C, the ΔT20 commonly used for condensing boiler systems). The calculator uses the specific heat capacity of water — 4.18 kJ/kg·°C — to convert the load into a required flow rate, shown in both litres per minute and litres per second.
Flow (L/s) = kW ÷ (4.18 × ΔT); Flow (L/min) = L/s × 60
kW is the heat the circuit must deliver, 4.18 is the specific heat capacity of water in kJ/kg·°C, and ΔT is the design temperature drop between flow and return in °C. Dividing the load by 4.18 × ΔT gives the mass flow in kg/s, which for water equals litres per second; multiplying by 60 gives the more familiar litres per minute.
Enter the load in kW — the boiler or heat pump output for a whole-system check, or the load on a single zone or circuit if you are balancing at circuit level.
Enter the flow-to-return temperature difference the system is designed to run at. The default is 20°C (ΔT20), typical for condensing boiler systems; heat pump systems are usually designed to a smaller ΔT, which pushes the required flow up.
The calculator shows the required flow in L/min and L/s. Check the pump can deliver this flow at the circuit's resistance, and use the per-circuit figures as balancing targets on lockshields or flow setters.
The relationship between heat, flow and ΔT is fixed physics: power equals mass flow × specific heat × temperature difference. For a given load, halving the ΔT doubles the required flow — which is why heat pump circuits, designed at smaller temperature differences than boilers, need noticeably higher flow rates and often larger pipework for the same output.
Use the whole-system figure to check pump duty: the pump must achieve the required flow against the index circuit's resistance, so read it alongside the pump curve rather than in isolation. Use per-emitter or per-zone figures when balancing, adjusting until each circuit's measured ΔT sits at the design value under steady load.
A measured ΔT well above design means the flow is too low — undersized pump setting, excessive resistance or a blockage; a ΔT well below design means flow is higher than needed, which wastes pump energy and can hurt condensing performance. This calculator covers water; glycol mixtures have a lower specific heat capacity, so they need slightly more flow for the same load.
Divide the heat load in kW by 4.18 times the ΔT in °C to get litres per second, then multiply by 60 for litres per minute. For example, 24 kW at ΔT20 needs 24 ÷ (4.18 × 20) = 0.287 L/s, or about 17.2 L/min.
ΔT20 means the system is designed for a 20°C temperature drop between flow and return — for example 80°C flow and 60°C return, or 70/50. It is the common design ΔT for condensing boiler systems and is this calculator's default, though you can enter any ΔT your design calls for.
Because they run at a smaller ΔT. Flow is inversely proportional to ΔT for a given load, so a system designed at ΔT5 needs four times the flow of the same load at ΔT20. That is why heat pump installations often need larger pipework and more capable circulation pumps than an equivalent boiler system.
Work out each radiator or zone's share of the load, calculate its required flow at the design ΔT, then adjust the lockshield (or flow setter) until the measured flow-to-return ΔT across that emitter sits at the design figure under steady load. Circuits with a bigger-than-design ΔT are being starved of flow; those with a smaller ΔT are taking more than their share.
Approximate heat requirement for a room from its size and use.
Approximate water content of a heating system.
Minimum vessel size for a sealed heating system.
Timed meter test — natural gas or LPG, metric or imperial — with live kW results.
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