Flow Rate in All Units
Full Calculation Table
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How to Use the GPM Calculator
Use Flow From Load to calculate required flow rate from a BTU/h or kW load and a design temperature difference. Use Unit Converter to convert a flow rate you already have between GPM, L/min, and m³/h.
In Flow From Load, enter the equipment's rated capacity and its design temperature difference, available from the manufacturer's specification sheet. In Unit Converter, enter any value and pick the unit you are converting from.
The result panel shows the flow rate in GPM, L/min, and m³/h at once, so you can use whichever unit your pipe sizing or pump sizing tool expects.
Getting From Load to Flow Rate
Every hydronic pipe and pump sizing calculation starts with a flow rate, but most equipment is rated in BTU/h or kW, not GPM or L/min, a conversion the BTU calculator handles from the load side when you are starting from a raw heat loss or heat gain figure rather than an equipment nameplate rating. Converting between them requires knowing one more number: the design temperature difference between supply and return, often written as delta T. The same load produces very different flow rates depending on delta T, which is why this calculation needs both numbers, not just the load, a distinction that also matters when pulling a cooling load figure from the cooling load calculator for a chilled water loop.
The Imperial Formula
In imperial units, GPM equals BTU/h divided by 500 times delta T in degrees Fahrenheit. The constant 500 comes from water's specific heat and density at typical hydronic operating temperatures, combined with the unit conversions between BTU, gallons, and minutes. This is the formula most commonly taught in North American HVAC programs and used on equipment spec sheets that list BTU/h ratings.
The Metric Formula
In metric units, L/min equals kW divided by 0.0698 times delta T in degrees Celsius. The constant reflects water's volumetric heat capacity in metric units, drawn from the same kind of standard reference property data available in NIST's Chemistry WebBook fluid properties database. Canadian equipment specifications increasingly list both BTU/h and kW, and this calculator handles either system directly so you do not need to convert the load yourself first.
Picking the Right Delta T
Delta T should come from the specific equipment or loop you are sizing, not a generic assumption, since using the wrong delta T changes the calculated flow rate proportionally. Residential hydronic heating systems commonly use a 10 to 20 degree Fahrenheit (roughly 5.5 to 11 degree Celsius) delta T, while chilled water cooling loops often run a tighter 10 to 12 degree Fahrenheit (roughly 5.5 to 6.5 degree Celsius) delta T. The City of Toronto's design guideline for district energy-ready buildings lists specific supply and return temperatures by system type as a real-world example of how these ranges get applied in practice. Always check the manufacturer's rated delta T for the specific boiler, chiller, or coil in the system.
What to Do With the Result
Once you have a flow rate, it becomes the input for the pipe sizing calculator to find a recommended pipe diameter, or the flow velocity calculator to check an existing pipe size against this new flow rate. From there, total circuit head on the pump head calculator before selecting a circulator pump.
A Note on Accuracy
This calculator uses water's standard specific heat and density at typical hydronic temperatures. For glycol mixtures, specific heat is lower than water, so the actual required flow rate is higher than this calculator shows for the same load and delta T. Check the glycol concentration calculator for the flow correction factor at your glycol percentage.
Frequently Asked Questions
The standard formula is GPM equals BTU/h divided by 500 times the design temperature difference in degrees Fahrenheit. The constant 500 comes from water's specific heat and density at typical hydronic temperatures. In metric units, L/min equals kW divided by 0.0698 times the temperature difference in degrees Celsius, using water's volumetric heat capacity.
Use the design supply and return temperature difference for the specific equipment or loop, not a generic assumption. Many residential hydronic heating systems use a 10 to 20 degree Fahrenheit (about 5.5 to 11 degree Celsius) delta T, while chilled water cooling systems often use 10 to 12 degrees Fahrenheit (about 5.5 to 6.5 degree Celsius). Check the equipment manufacturer's specification, since a wrong delta T directly changes the calculated flow rate.
Not directly. This calculator uses water's specific heat, and glycol has a lower specific heat than water, so a glycol system needs more flow than this result to deliver the same capacity. Use the glycol concentration calculator to find the flow penalty percentage at your glycol concentration, then increase this calculator's result by that percentage.
Different tools and equipment spec sheets use different flow units depending on the manufacturer and region. Showing all three at once saves a separate conversion step, since you can read off whichever unit your next calculation or equipment sheet expects without leaving this page.
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