Calculator
Plumbing Water Flow Rate (GPM) Calculator
Turn pipe size and velocity into gallons per minute, or time a bucket fill to measure what a service actually delivers. Uses real inside diameters for copper Type L and M, PEX, and Schedule 40, and flags anything past the code velocity limits. Free, no signup.
Free
Fluid & service typeAntifreeze changes how much heat a gallon carries, and the velocity limit depends on whether this is a closed loop or domestic water.
Constant for water: 499.8 = 8.33 lb/gal × 60 min/hr × 1.000 Btu/lb·°F
Flow rate
7.54 GPM
5.00 ft/s in 3/4" Copper Type L
Inside the domestic cold water target band
Flow rate
7.54 GPM5.00 ft/s × 0.00336 ft² × 448.831
Velocity in selected pipe
5.00 ft/s3/4" Copper Type L · 0.785 in ID · target ≤ 6 ft/s
Recommended size
3/4"Smallest Copper Type L that stays at or under 6 ft/s, runs 5.00 ft/s
Capacity at 8 ft/s
12.1 GPMMost 3/4" Copper Type L should ever carry on domestic cold water
Heat carried
75,396 BTU/hr499.8 × 7.54 GPM × 20°F ΔT
Copper Type L, flow at a glance
| Size | Actual ID | At 6 ft/s | At 8 ft/s |
|---|---|---|---|
| 1/2" | 0.545" | 4.4 | 5.8 |
| 3/4" | 0.785" | 9.1 | 12.1 |
| 1" | 1.025" | 15.4 | 20.6 |
| 1-1/4" | 1.265" | 23.5 | 31.3 |
| 1-1/2" | 1.505" | 33.3 | 44.4 |
| 2" | 1.985" | 57.9 | 77.2 |
| 2-1/2" | 2.465" | 89.2 | 119.0 |
| 3" | 2.945" | 127.4 | 169.9 |
| 4" | 3.905" | 224.0 | 298.6 |
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.
Industry benchmarks
- 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
What you'll get
Choose how you know the flow
Start from velocity and pipe size, from a timed bucket fill, or from a heating load in BTU/hr. All three modes feed the same velocity and pipe-sizing checks.
Set the pipe and service type
Pick copper Type L or M, PEX SDR-9, or Schedule 40 steel or PVC, plus the nominal size. Then choose cold water, hot water, or a closed loop so the right velocity limit applies.
Read flow, velocity, and size
Get GPM, the velocity it produces, whether it clears the 8 ft/s cold or 5 ft/s hot limit, the pipe’s capacity at that limit, and the smallest size that stays inside the design target.
How it works
- 1
Choose how you know the flow
Start from velocity and pipe size, from a timed bucket fill, or from a heating load in BTU/hr. All three modes feed the same velocity and pipe-sizing checks.
- 2
Set the pipe and service type
Pick copper Type L or M, PEX SDR-9, or Schedule 40 steel or PVC, plus the nominal size. Then choose cold water, hot water, or a closed loop so the right velocity limit applies.
- 3
Read flow, velocity, and size
Get GPM, the velocity it produces, whether it clears the 8 ft/s cold or 5 ft/s hot limit, the pipe’s capacity at that limit, and the smallest size that stays inside the design target.
Why velocity, not just GPM, decides the pipe size
Flow rate through a pipe is simple continuity: GPM = velocity in feet per second × the pipe’s cross-sectional area in square feet × 448.831, where 448.831 is 7.48 gallons per cubic foot times 60 seconds per minute. The trap is the word "size". A 1-inch pipe is not 1 inch inside. Copper Type L is 1.025 inches, Type M is 1.055, Schedule 40 is 1.049, and 1-inch PEX is only 0.875 inches. That last one matters: swap copper for PEX at the same trade size and velocity jumps by more than a third at identical flow, which is how quiet systems turn noisy.
Velocity is what the code and the copper actually care about. Standard practice caps domestic cold water at 8 ft/s and hot water at 5 ft/s, because hot water erodes copper far faster at the fittings and elbows where flow turns. Designing to 6 ft/s cold and 4 ft/s hot leaves headroom for pressure surges and keeps the system quiet. When you cannot calculate it, measure it, a 5-gallon bucket and a stopwatch gives you real delivered flow at a fixture, and 5 gallons in 30 seconds is exactly 10 GPM.
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Water Flow Rate (GPM) Calculator FAQs
Multiply velocity in ft/s by the pipe’s inside area in square feet, then by 448.831 to convert cubic feet per second to gallons per minute. Area = π × ID² ÷ 576 when the inside diameter is in inches. Use the actual inside diameter, not the nominal trade size.
Open the fixture fully, time how long a container of known volume takes to fill, then divide gallons by minutes. A 5-gallon bucket filled in 30 seconds is 10 GPM; filled in 12 seconds it is 25 GPM. Time it two or three times and average for a reliable reading.
Common practice limits cold water to 8 ft/s and hot water to 5 ft/s. Hot water erodes copper more aggressively, especially at elbows and fittings. Designing to 6 ft/s cold and 4 ft/s hot keeps the system quiet and leaves margin for pressure surges and water hammer.
PEX has a much thicker wall relative to its outside diameter. At SDR-9, 1-inch PEX has a 0.875-inch bore against copper Type L’s 1.025 inches, about 27% less flow area, so velocity runs roughly 37% higher at the same GPM. Plan on upsizing PEX a step on long or high-demand runs.
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