Pipe Volume & Water-Supply Sizing Calculator
Two tools in one for US plumbing. Find how much water your pipe runs hold — in gallons and pounds — for copper, PEX, CPVC, or PVC, and size a supply line from your design flow using pipe velocity (ft/s). Includes a size-vs-velocity table and a downloadable PDF.
| Run | Length | Inside dia. | Volume |
|---|---|---|---|
| Add a pipe run to see volume | |||
| Nominal | Inside dia. | Velocity | Status |
|---|---|---|---|
| Enter a design flow to size the supply | |||
How to use the pipe volume & water-supply sizing calculator
Four quick steps turn your pipe material, runs, and design flow into water volume in gallons and pounds, a velocity check for every size, and a recommended supply-line size — with a downloadable PDF report.
Pick the pipe material
Choose Type L copper, PEX, CPVC, or Schedule 40 PVC. It sets the inside diameter used for every result.
Add your pipe runs
Enter each run by nominal size (½”–2″) and length in feet. Use Qty for identical runs, like repeated branches.
Set flow & velocity limit
Enter the peak design flow in GPM and a maximum velocity — about 8 ft/s for cold, 5 ft/s for hot or quiet lines.
Read results & save a PDF
See total gallons, water weight, fill time, the recommended size, and a velocity table — then download the report.
How pipe volume and supply size are calculated
Volume is the cylinder of water inside the pipe, based on the material’s actual inside diameter (ID). Sizing uses the velocity method: the smallest size whose water speed stays at or below your limit. Here is the exact math.
Worked example — Type L copper, 40 ft of ¾” main + 60 ft of ½” branch, 10 GPM, 8 ft/s limit
¾” run (ID 0.785″): 0.0408 × 0.785² × 40 ≈ 1.01 gal
½” run (ID 0.545″): 0.0408 × 0.545² × 60 ≈ 0.73 gal
Total: 1.73 gal · weight 1.73 × 8.34 ≈ 14.5 lb · fill time 1.73 ÷ 10 ≈ 10 sec
Velocity at 10 GPM: ½” 0.4085 × 10 ÷ 0.545² ≈ 13.8 ft/s (too fast) · ¾” ≈ 6.6 ft/s (OK)
≈ 1.73 gallons in the runs, and a ¾” copper supply for 10 GPM. At a 5 ft/s limit, it steps up to 1″ (≈ 3.9 ft/s).
Pipe volume & sizing charts
Handy lookups for inside diameters, gallons per 100 feet, and the maximum flow each size can carry. Values use the same typical inside diameters as the calculator — actual IDs vary slightly by schedule and brand.
| Nominal size | Copper Type L | PEX (F876) | CPVC (CTS) | PVC Sch 40 |
|---|---|---|---|---|
| ½” | 0.545 | 0.475 | 0.469 | 0.602 |
| ¾” | 0.785 | 0.671 | 0.695 | 0.804 |
| 1″ | 1.025 | 0.863 | 0.901 | 1.029 |
| 1¼” | 1.265 | 1.053 | 1.101 | 1.360 |
| 1½” | 1.505 | 1.243 | 1.309 | 1.590 |
| 2″ | 1.985 | 1.629 | 1.716 | 2.047 |
Nominal size is a trade name, not the bore. PEX and CPVC have thicker walls, so they carry less water than copper or PVC at the same nominal size.
| Nominal size | Copper Type L | PEX | CPVC | PVC Sch 40 |
|---|---|---|---|---|
| ½” | 1.21 | 0.92 | 0.90 | 1.48 |
| ¾” | 2.51 | 1.84 | 1.97 | 2.64 |
| 1″ | 4.29 | 3.04 | 3.31 | 4.32 |
| 1¼” | 6.53 | 4.52 | 4.95 | 7.55 |
| 1½” | 9.24 | 6.30 | 6.99 | 10.31 |
| 2″ | 16.08 | 10.83 | 12.01 | 17.10 |
Multiply by 8.34 for pounds of water per 100 ft — e.g. 2″ copper ≈ 134 lb full. Divide by 100 for gallons per foot.
| Nominal size | Copper Type L | PEX | CPVC | PVC Sch 40 |
|---|---|---|---|---|
| ½” | 5.8 | 4.4 | 4.3 | 7.1 |
| ¾” | 12.1 | 8.8 | 9.5 | 12.7 |
| 1″ | 20.6 | 14.6 | 15.9 | 20.7 |
| 1¼” | 31.3 | 21.7 | 23.7 | 36.2 |
| 1½” | 44.4 | 30.3 | 33.6 | 49.5 |
| 2″ | 77.2 | 52.0 | 57.7 | 82.1 |
GPM = 8 × ID² ÷ 0.4085. For a 5 ft/s hot-water limit, multiply any value by 0.625 (e.g. ¾” copper ≈ 7.5 GPM). Velocity only — not pressure loss.
| Fixture | Typical flow |
|---|---|
| Bathroom faucet | 1.0–1.5 GPM |
| Kitchen faucet | 1.5–2.2 GPM |
| Showerhead | 1.8–2.5 GPM |
| Tub filler | 4–6 GPM |
| Toilet (tank refill) | 2–3 GPM |
| Dishwasher | 1.5–2 GPM |
| Washing machine | 3–5 GPM |
| Hose bib | 5 GPM or more |
Add up the fixtures likely to run at the same time to get a design flow — not every fixture in the house at once.
| Max velocity | Typical use |
|---|---|
| 8 ft/s | cold-water supply (common upper limit) |
| 6 ft/s | general design, mixed hot & cold |
| 5 ft/s | hot water, recirculation, quiet runs |
| 4 ft/s | conservative — low noise, long life |
Faster water means more noise, water hammer, and erosion at copper fittings. Check your code and the pipe maker’s guidance.
Everything the calculator works out
A material, a few pipe runs, and a design flow give you the water your system holds and the size it needs — side by side for every nominal size.
Key figures behind the math
Built for planning a plumbing job
From a DIY repipe to a quick check on a contractor’s plan, the same calculator turns pipe runs and flow into gallons and a size.
Planning a repipe, a new bathroom, or a longer run and wants to know what size line makes sense before buying pipe.
- Compare PEX vs copper
- Check velocity at your GPM
- Bring the PDF to your plumber
Wants a quick velocity check and water volume for flushing, disinfection, or freeze protection on a job.
- Enter each run & qty
- Get gallons to flush or treat
- Confirm with full code sizing
Dealing with long hot-water waits or winterizing a vacant unit and wants to know how much water sits in the lines.
- Estimate volume to drain
- See wait time at fixture flow
- Weigh recirculation vs repipe
7 tips for sizing and measuring pipe
Small diameter changes make big differences in both volume and velocity. These points keep your numbers realistic — and remember this is an estimate, not a code design.
Pipe volume & sizing calculator FAQ
The volume, flow, and sizing questions people ask most about water-supply pipe.
It depends on the inside diameter and the length. Volume per foot is π ÷ 4 × ID² × 12 ÷ 231, which simplifies to about 0.0408 × ID² gallons per foot, with ID in inches.
A 100 ft run of ¾” Type L copper holds about 2.51 gallons, while the same length of ¾” PEX holds about 1.84 gallons because PEX has a smaller inside diameter.
Nominal size is not the inside diameter, so always use the material’s actual ID.
Square the inside diameter in inches and multiply by 0.0408. That comes from the cylinder volume (π ÷ 4 × ID²) times 12 inches per foot, divided by 231 cubic inches per US gallon.
For ½” Type L copper (ID 0.545″): 0.545² × 0.0408 ≈ 0.0121 gal/ft, or about 1.21 gallons per 100 ft.
Multiply by total length for the run’s volume, and by 8.34 for the water weight in pounds.
Using the velocity method, pick the smallest size that keeps water speed at or under your limit. Velocity in ft/s is 0.4085 × GPM ÷ ID².
At 10 GPM in Type L copper, ½” runs about 13.8 ft/s (too fast), ¾” about 6.6 ft/s, and 1″ about 3.9 ft/s. With an 8 ft/s cold-water limit, ¾” works; with a 5 ft/s limit, you’d step up to 1″.
This is a velocity check only, not a full pressure-loss or fixture-unit design.
A common rule of thumb is about 8 ft/s for cold water and about 5 ft/s for hot water, and many designers cap everything at 5–6 ft/s for quiet operation.
Faster water causes noise, water hammer, and, in copper, erosion-corrosion at fittings, which is worse with hot water. The calculator lets you choose 8, 6, 5, or 4 ft/s.
Check your local code and the pipe manufacturer’s guidance for your project.
Nominal size is a trade name, not the inside diameter. Wall thickness and sizing standards differ by material, so a ¾” pipe has an ID of about 0.785″ in Type L copper, 0.671″ in PEX, 0.695″ in CTS CPVC, and 0.804″ in Schedule 40 PVC.
Because both volume and velocity depend on ID squared, those small differences matter.
PEX at the same nominal size holds less water and runs faster, which is why PEX lines are sometimes upsized.
Water weighs about 8.34 pounds per US gallon, so multiply the pipe volume by 8.34.
A 100 ft run of ¾” Type L copper holds about 2.51 gallons, or roughly 21 lb of water. A 100 ft run of 2″ copper holds about 16.1 gallons, or roughly 134 lb.
The figure is the water only; add the weight of the pipe itself when planning hangers and supports.
At minimum, the time it takes to push the cooled water out of the pipe between the heater and the fixture. That is pipe volume ÷ flow rate.
60 ft of ½” copper holds about 0.73 gallons, so a 1.5 GPM faucet needs about 29 seconds just to clear it — and real waits run longer because the pipe itself absorbs heat. The calculator’s fill-time figure uses the same volume ÷ flow math.
Shorter or smaller hot runs, insulation, or a recirculation system shorten the wait.
No. It sizes by velocity only, from a design flow you enter.
A code-compliant water-supply design also accounts for water supply fixture units (WSFU), available street or well pressure, elevation, pressure loss through pipe, fittings, meter, and backflow devices, and the minimum pressure required at each fixture under the IPC or UPC.
Use this tool to estimate and compare, then have a licensed plumber or designer confirm sizes for your project.
Estimate Only — Not a Plumbing Design: Inside diameters are typical published values for US nominal pipe and vary by schedule, SDR, and manufacturer. Pipe sizing here uses the velocity method (v = 0.4085 × GPM ÷ ID²) from a design flow you enter; it does not calculate pressure loss, fixture units, available supply pressure, elevation, or minimum fixture pressure, and it is not a substitute for a code-compliant water-distribution design. Water-supply piping must meet your local plumbing code (IPC, UPC, or local amendments) and material listings. Confirm sizes and materials with a licensed plumber or your local building department before buying or installing. For planning purposes only.