HVAC Hydronic Flow Rate (GPM) Calculator

Work out the flow a hydronic loop actually needs. Enter the load in BTU/hr and your design ΔT to get GPM, switch fluids to see what glycol costs you in extra flow, then check velocity and pipe size in copper, PEX, or steel. Free, no signup.

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Hydronic Flow Rate (GPM) Calculator

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Constant for water: 499.8 = 8.33 lb/gal × 60 min/hr × 1.000 Btu/lb·°F

Required loop flow

10.00 GPM

3.89 ft/s in 1" Copper Type L

Inside the closed hydronic loop target band
Flow rate
10.00 GPMLoad ÷ (499.8 × 20°F ΔT)
Velocity in selected pipe
3.89 ft/s1" Copper Type L · 1.025 in ID · target ≤ 4 ft/s
Recommended size
1"Smallest Copper Type L that stays at or under 4 ft/s, runs 3.89 ft/s
Capacity at 8 ft/s
20.6 GPMMost 1" Copper Type L should ever carry on closed hydronic loop
Heat carried
100,000 BTU/hr499.8 × 10.00 GPM × 20°F ΔT
SizeActual IDAt 4 ft/sAt 8 ft/s
1/2"0.545"2.95.8
3/4"0.785"6.012.1
1"1.025"10.320.6
1-1/4"1.265"15.731.3
1-1/2"1.505"22.244.4
2"1.985"38.677.2
2-1/2"2.465"59.5119.0
3"2.945"84.9169.9
4"3.905"149.3298.6

Flow in GPM. Sizes are nominal; the calculation uses the actual inside diameter, which is why Copper Type L differs from other materials at the same trade size.

Water constant (the "500")
8.33 lb/gal × 60 × 1.0 = 499.8
Velocity → flow
ft/s × area (ft²) × 448.831
Hydronic design ΔT
20 °F heating · 10-15 °F chilled
Closed hydronic loop velocity
2-4 ft/s
Copper cold / hot water max
8 / 5 ft/s
30% / 50% propylene glycol
constant 475 / 441

  • Enter the load and ΔT

    Put in boiler net output, coil capacity, or the zone load in BTU/hr and your design temperature drop. GPM = BTU/hr ÷ (500 × ΔT), with 500 = 8.33 lb/gal × 60 min/hr × 1.0 Btu/lb·°F.

  • Pick the fluid and the pipe

    Choose water or 30/50% propylene or ethylene glycol, the constant drops to match the fluid’s density × specific heat. Then select copper Type L or M, PEX, or Schedule 40 and the nominal size.

  • Check velocity and size the pipe

    See the velocity your flow produces, whether it sits in the 2-4 ft/s closed-loop band, the smallest size that stays inside it, and how much heat the flow carries back at your ΔT.

How it works

  1. 1

    Enter the load and ΔT

    Put in boiler net output, coil capacity, or the zone load in BTU/hr and your design temperature drop. GPM = BTU/hr ÷ (500 × ΔT), with 500 = 8.33 lb/gal × 60 min/hr × 1.0 Btu/lb·°F.

  2. 2

    Pick the fluid and the pipe

    Choose water or 30/50% propylene or ethylene glycol, the constant drops to match the fluid’s density × specific heat. Then select copper Type L or M, PEX, or Schedule 40 and the nominal size.

  3. 3

    Check velocity and size the pipe

    See the velocity your flow produces, whether it sits in the 2-4 ft/s closed-loop band, the smallest size that stays inside it, and how much heat the flow carries back at your ΔT.

Why hydronic flow starts with BTU/hr and delta-T

Every hydronic loop is sized by one relationship: BTU/hr = 500 × GPM × ΔT. The 500 is not magic, it is 8.33 pounds per gallon of water times 60 minutes per hour times water’s specific heat of 1.0 Btu per pound per °F, which works out to 499.8. Rearranged, GPM = BTU/hr ÷ (500 × ΔT). A 100,000 BTU/hr load at the classic 20 °F design drop needs 10 GPM; halve the ΔT to 10 °F and the same load needs 20 GPM, a bigger circulator, and a pipe size or two more. Delta-T is the cheapest lever you have on pump power and pipe cost.

Antifreeze changes the arithmetic. Propylene and ethylene glycol are both denser than water but carry noticeably less heat per pound, and density × specific heat is exactly what the constant represents. At 30% propylene glycol the constant falls to about 475 and at 50% to about 441, so the same load needs roughly 5% and 13% more flow respectively. Ethylene glycol runs lower still. Size the circulator and the piping for the glycol number, not the water number, and keep velocity in the 2 to 4 ft/s window, because below 2 ft/s the loop will not carry entrained air to the separator and above 4 ft/s it starts to sing.

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Worth asking before you book

Hydronic Flow Rate (GPM) Calculator FAQs

Divide the load by 500 times the design temperature difference: GPM = BTU/hr ÷ (500 × ΔT). A 100,000 BTU/hr load at a 20 °F ΔT needs 10 GPM. The 500 constant is 8.33 lb/gal × 60 min/hr × 1.0 Btu/lb·°F for water, which is precisely 499.8.
It is the weight of a gallon of water (8.33 lb at 60 °F) multiplied by 60 minutes per hour and by water’s specific heat of 1.0 Btu/lb·°F. That gives 499.8, universally rounded to 500. Change the fluid and the constant changes with it, because it is really just density × specific heat.
Roughly 5% more at 30% propylene glycol and about 13% more at 50%, because the constant drops from 499.8 to about 475 and 441. Ethylene glycol is heavier and carries less heat per pound, so 50% ethylene needs around 17% more flow. The calculator shows the water-equivalent GPM alongside the corrected figure.
Aim for 2 to 4 ft/s. Below 2 ft/s the flow is too slow to sweep entrained air to the air separator; above 4 ft/s you get noise, and ASHRAE’s customary limit for 2-inch and smaller pipe is 4 ft/s. Erosion becomes a concern well before 10 ft/s, so this tool flags anything past 8 ft/s.
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