Coolant Flow Rate Calculator
To carry a known heat load out of an engine or component, a liquid cooling system needs enough coolant flow that the temperature rise across it stays within the design limit. This calculator applies the standard sensible-heat energy balance: it takes the heat to be rejected, the coolant specific heat, the coolant density and the allowed temperature rise, then returns the required mass flow rate and volumetric flow in litres per second, litres per minute and US gallons per minute. Coolant properties are inputs because they depend on the fluid and its concentration.
Coolant flow rate formula
Mass flow (kg/s) = heat (kW) / (specific heat * temperature rise)
Volume flow (L/s) = mass flow / density
Volume flow (L/min) = volume flow (L/s) * 60
Volume flow (gal/min) = volume flow (L/min) / 3.785412
Heat in kilowatts equals kilojoules per second, so dividing by the specific heat (kJ per kg per K) and the temperature rise (K) yields mass flow in kg/s. Dividing by density gives volume flow. The factor 3.785412 litres per US gallon is an exact definition.
Coolant flow context
- A smaller allowed temperature rise demands more flow to carry the same heat.
- Water has the highest specific heat of common coolants; glycol mixes carry less heat per kg.
- Pure water specific heat is about 4.18 kJ/kg.K; a 50/50 glycol mix is roughly 3.6 kJ/kg.K.
- The balance assumes single-phase (no boiling) flow; phase-change cooling needs latent-heat analysis.
- Enter measured coolant properties for your fluid and temperature for an accurate result.
Coolant flow rate: frequently asked questions
How is required coolant flow rate calculated?
Mass flow equals the heat load divided by the product of the coolant specific heat and the allowed temperature rise. Dividing mass flow by coolant density gives volumetric flow. This is the standard sensible-heat energy balance for a single-phase coolant.
Why must specific heat and density be inputs?
Coolant properties depend on the fluid and its concentration. A 50/50 ethylene glycol mix has a different specific heat and density from pure water, and both vary with temperature. To stay accurate the calculator takes these as user inputs rather than assuming a value.
What is the allowed temperature rise?
It is the difference between the coolant outlet and inlet temperatures across the component you are cooling. A smaller allowed rise requires more flow to carry the same heat. Enter the temperature rise your system is designed to maintain.
How is the result converted to liters per minute?
The energy balance gives mass flow in kilograms per second; dividing by density in kilograms per litre gives litres per second, and multiplying by 60 gives litres per minute. All conversions are exact unit relations, not estimates.
Does this apply to any liquid coolant?
Yes, as long as the coolant stays liquid (no boiling) and you enter its specific heat and density. The same sensible-heat equation governs water, glycol mixes and oils. For boiling or phase-change cooling a latent-heat analysis is needed instead.
Official sources
- U.S. Department of Energy: Pumping and Cooling Systems.
- U.S. National Institute of Standards and Technology: SI Units and Conversions.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.