Pipe Velocity Calculator — GPM to FT/S
Water moving too fast is the cause of two problems that show up years apart: it is audible straight away, and it erodes copper from the inside slowly. Both are governed by one number — velocity in feet per second — which depends only on the flow and the real bore of the pipe. Enter both and see where you sit against the cold and hot design limits.
Check your velocity
Changes the available sizes and the real bore behind each one.
Water velocity
Inside the 8 ft/s cold design limit, with 1.37 ft/s to spare.
Max flow at the limit
12.1 gpm
Actual bore
0.785 in
Over the design limit
Expect audible flow noise now, and erosion-corrosion in copper over years — worst just downstream of elbows and tees. Go up a size or reduce the flow.
See the breakdown
The 8 and 5 ft/s figures are ASPE and tube-manufacturer design practice, not a numeric IPC limit — the code requires a system free of excessive noise and erosion without naming a value. A licensed plumber and your AHJ have final say.
The formula, explained in plain English
Velocity is flow divided by area. Everything else is unit conversion — and the whole constant falls out of it.
It goes as diameter squared
Doubling the bore quarters the velocity at the same flow. That is why going up one size is such an effective fix — from 3/4 to 1 inch copper cuts 6.63 ft/s to 3.89 ft/s, a 41% drop.
Nominal size lies
Nominal 3/4 inch means 0.785 in of bore in Type L copper, 0.824 in Schedule 40, and 0.681 in in PEX. Velocity uses the real number, which is computed here from outside diameter and wall thickness rather than transcribed.
Length is irrelevant
Unlike friction loss, velocity does not accumulate. A 6-foot run and a 600-foot run at the same flow and bore have identical velocity — which is exactly why the two checks bind on different jobs.
Where erosion actually happens
Not evenly along the pipe. Turbulence just past an elbow or tee strips the oxide film locally, so pinholes in old copper appear a few inches downstream of fittings, not in the middle of a straight run.
Worked examples
The defaults, the same pipe carrying hot water, and the PEX substitution that quietly breaks the limit.
10 gpm through 3/4 inch copper — cold
V = 0.4085 × 10 ÷ 0.785² = 4.085 ÷ 0.6162
→ 6.63 ft/s against an 8 ft/s limit ✓
max cold flow = 8 × 0.6162 ÷ 0.4085 = 12.07 gpm
Result: fine for cold water with 1.37 ft/s of headroom, and the pipe would take 12.1 gpm before it hit the limit.
The same pipe on hot water
→ 33% over the hot limit ✗
max hot flow = 5 × 0.6162 ÷ 0.4085 = 7.54 gpm
1" copper at 10 gpm = 3.89 ft/s ✓
Result: nothing about the pipe changed and it now fails. A 3/4-inch hot line tops out at 7.54 gpm — which is why hot mains on multi-fixture branches go to 1 inch far earlier than cold ones.
Swapping copper for PEX, same nominal size
V = 0.4085 × 10 ÷ 0.681² = 4.085 ÷ 0.4638
→ 8.81 ft/s — over the 8 ft/s cold limit ✗
same nominal size, 33% more velocity than copper
Result: a like-for-like PEX substitution breaks a limit that copper met. PEX is outside-diameter controlled at SDR-9, so the wall eats into the bore — 0.681 in against copper's 0.785 in, and velocity goes as the square of that.
Maximum flow per pipe size
Type L copper, computed from the same code the calculator runs. The hot column is always 62.5% of the cold column — because 5 ÷ 8 is a constant, and velocity is linear in flow.
| Nominal size | Actual bore | Max cold (8 ft/s) | Max hot (5 ft/s) |
|---|---|---|---|
| 1/2" | 0.545 in | 5.8 gpm | 3.6 gpm |
| 3/4" | 0.785 in | 12.1 gpm | 7.5 gpm |
| 1" | 1.025 in | 20.6 gpm | 12.9 gpm |
| 1-1/4" | 1.265 in | 31.3 gpm | 19.6 gpm |
| 1-1/2" | 1.505 in | 44.4 gpm | 27.7 gpm |
| 2" | 1.985 in | 77.2 gpm | 48.2 gpm |
| 2-1/2" | 2.465 in | 119.0 gpm | 74.4 gpm |
| 3" | 2.945 in | 169.9 gpm | 106.2 gpm |
| 4" | 3.905 in | 298.6 gpm | 186.6 gpm |
10 gpm in 3/4 inch — every material
Same flow, same nominal size, six different answers. Bore is what velocity actually depends on.
| Material | Bore at 3/4" | Velocity at 10 gpm |
|---|---|---|
| Copper, Type L | 0.785 in | 6.63 ft/s |
| Copper, Type M | 0.811 in | 6.21 ft/s |
| PEX (SDR-9) | 0.681 in | 8.82 ft/s over 8 ft/s |
| CPVC, Schedule 40 | 0.824 in | 6.02 ft/s |
| PVC, Schedule 40 | 0.824 in | 6.02 ft/s |
| Galvanized steel, Schedule 40 | 0.824 in | 6.02 ft/s |
Sources & standards: velocity limits per ASPE design guidance and the Copper Development Association erosion-corrosion guidance — the IPC requires a system free of excessive noise and erosion without printing a figure. Pipe dimensions from ASTM B88, F876 and D1785, with every bore computed from outside diameter and wall thickness. Local amendments override the model code.
Frequently asked questions
Common questions about water velocity, noise, and erosion in supply piping.
How fast is water moving in a 3/4 inch pipe?
At 10 gpm through 3/4-inch Type L copper (0.785-inch bore) the answer is 6.63 ft/s — comfortably inside the 8 ft/s cold-water limit but well over the 5 ft/s hot-water limit. That same pipe maxes out at 12.07 gpm cold and only 7.54 gpm hot.
What is the maximum water velocity in a pipe?
The long-standing design values are 8 ft/s for cold water and 5 ft/s for hot. Recirculating loops that run continuously are often held lower still, around 4 ft/s. These come from ASPE and the tube manufacturers — the IPC requires a system free of excessive noise and erosion but does not print a number, so this is design practice rather than a code limit you can be cited for.
Why is the hot water limit lower than cold?
Erosion-corrosion. Fast-moving water strips the protective oxide film off the inside of a copper tube, and the rate of attack rises sharply with temperature — so the same velocity that is harmless at 55 °F will thin a hot line over years. The damage concentrates just downstream of elbows and tees where the flow is turbulent, which is why pinhole leaks in old copper appear there first.
What does too-fast water actually sound like?
A rushing hiss while a fixture runs, and often a crack or bang when it shuts — velocity and water hammer are related problems, because fast-moving water carries more momentum to dissipate. Anything much above 8 ft/s is audible through a wall. A whistle usually means a restriction or a partly closed valve rather than the pipe itself.
Does the pipe material change the velocity?
Not directly — velocity depends only on flow and bore. But bore depends heavily on material at the same nominal size: nominal 3/4-inch is 0.785 in in Type L copper, 0.824 in Schedule 40, and only 0.681 in in PEX. At 10 gpm that is 6.63 ft/s in copper against 8.81 ft/s in PEX — over the limit purely because the hole is smaller.
Is velocity or friction loss the thing that limits my pipe size?
Both, and which one binds depends on length. Velocity does not care how long the run is; friction accumulates with every foot. So short runs are usually velocity-governed and long runs friction-governed. The Water Pipe Size Calculator applies both and tells you which one decided.
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