GPM Flow Rate Calculator
Convert between flow and velocity for water in a pipe, in either direction. Give it a flow in GPM and an inside diameter and it returns the velocity in feet per second — with a warning when you cross the 8 fps design ceiling — or give it a target velocity and it returns the flow that pipe carries. Pipe presets fill in the verified average ID for copper Type L, PEX, PVC, and steel, or type any custom ID for tubing and specialty pipe. Free, no login.
Inputs
Result
Enter values to calculate
How this calculator works
This is pure continuity math (Q = V × A) — no code table, and the page cites none. Converting a flow in gallons per minute (231 in³ each) over a circular bore of diameter d inches into feet per second collapses every unit factor into one constant:
v (fps) = 0.4085 × GPM ÷ d² · GPM = v × d² ÷ 0.4085
Both directions are the same identity solved for different unknowns, so the calculator is exact — the only judgment call is which d you feed it. Use the actual inside diameter, not the nominal size: the presets carry published average IDs, and the ID box stays editable for anything else. The 8 fps ceiling the result is checked against is design practice for supply piping (erosion, noise, and water-hammer intensity all climb with velocity), not a code limit; drainage design cares about the other end of the scale, keeping roughly 2 fps so solids stay in suspension.
Worked example
Both directions on 3/4" copper Type L (ID 0.785"):
- Velocity from flow: at 10 GPM, v = 0.4085 × 10 ÷ 0.785² = 4.085 ÷ 0.6162 = 6.63 fps — under the 8 fps ceiling.
- Flow from velocity: at the 8 fps limit, GPM = 8 × 0.785² ÷ 0.4085 = 4.9298 ÷ 0.4085 = 12.07 GPM — the most that size should carry under the design ceiling.
Reproduce it above: the pipe preset defaults to Copper Type L 3/4" — enter 10 GPM in velocity mode, then switch to flow mode and enter 8 fps.
Reference: copper Type L capacity by velocity
Flow at three common design velocities, computed from the verified copper Type L inside diameters — 4 fps is a conservative hot-recirculation ceiling, 8 fps the usual cold-supply limit.
| Nominal | ID (in) | GPM @ 4 fps | GPM @ 6 fps | GPM @ 8 fps |
|---|---|---|---|---|
| 1/2" | 0.545 | 2.9 | 4.4 | 5.8 |
| 3/4" | 0.785 | 6.0 | 9.1 | 12.1 |
| 1" | 1.025 | 10.3 | 15.4 | 20.6 |
| 1-1/4" | 1.265 | 15.7 | 23.5 | 31.3 |
| 1-1/2" | 1.505 | 22.2 | 33.3 | 44.4 |
| 2" | 1.985 | 38.6 | 57.9 | 77.2 |
Frequently asked questions
How do I calculate water velocity in a pipe from GPM?
Velocity in feet per second is v = 0.4085 × GPM ÷ d², where d is the inside diameter in inches — the constant just converts gallons per minute through the pipe’s cross-sectional area. For example, 10 GPM through 3/4" Type L copper (0.785" ID) moves at about 6.6 fps. The calculator also runs it in reverse: enter a target velocity and it returns the flow that produces it.
What is a good maximum water velocity for supply piping?
About 8 fps is the common ceiling for domestic supply — above that, erosion, noise, and water hammer intensity climb quickly, which is why the result turns cautionary above 8 fps and flags anything over 10 fps as erosion risk. Hot water is usually held lower: copper manufacturers’ guidance commonly limits hot lines to around 5 fps, and recirculating loops lower still. These are design-practice numbers rather than a single code rule, so verify any velocity limits in the plumbing code and product listings that apply in your jurisdiction.
Do I use nominal pipe size or actual inside diameter?
Inside diameter — and it differs from the nominal name by more than you might expect. Nominal 3/4" pipe has an ID of about 0.785" in Type L copper, 0.824" in Schedule 40 steel, and only 0.671" in PEX. Since velocity scales inversely with d², the same 10 GPM runs roughly 37% faster in 3/4" PEX than in 3/4" copper. Pick the material and size to auto-fill a typical ID, or type the manufacturer’s actual ID for anything unusual.
How many GPM can a 3/4-inch pipe carry?
It depends on the ID and the velocity you will accept. At the common 8 fps ceiling, 3/4" Type L copper (0.785" ID) carries about 12 GPM, while 3/4" PEX (0.671" ID) carries about 8.8 GPM. Those are velocity limits, not code capacities — on long runs, friction loss usually becomes the binding constraint well before erosion velocity does, so check pressure loss over the actual run too.
Does this calculator tell me pressure loss?
No — it converts between flow and velocity only. Velocity is the wear-and-noise screen; pressure loss also depends on run length and pipe roughness, so two runs at identical velocity can lose very different amounts of pressure. Use the pressure loss calculator for the friction side, and treat the two checks together: velocity for erosion and hammer, friction for whether the fixture still has enough pressure.
Method: continuity (Q = V × A) reduced to v = 0.4085 × GPM ÷ d² with d the inside diameter in inches — pure geometry with no code citation. Pipe presets use published average inside diameters (copper Type L, steel Schedule 40 per ASME B36.10, PVC Schedule 40 per ASTM D1785, PEX per ASTM F876 CTS SDR-9). Velocity ceilings are design practice, not code limits. This tool converts flow you already know; establishing the design flow itself is a separate step — fixture-unit demand converts to GPM through the tabulated Hunter’s curve (IPC Table E103.3(3)) — and it says nothing about pressure, which needs the friction-loss check.
Pair with the Water Pressure Loss Calculator and Pipe Sizing Calculator to finish the hydraulic picture, or the WSFU to GPM chart and fixture water demand chart to establish the GPM you feed in.