Pipe Volume & Capacity Calculator
Find how much water a pipe actually holds — gallons per foot and total fill volume — from the published average inside diameter of the material you installed, not the nominal trade size. Copper Type L, PEX, PVC Schedule 40, and steel Schedule 40 all carry different IDs at the same nominal size, and the difference compounds over a long run. Use the result for flush and purge volumes, system fill and drain-down estimates, glycol charges, and chlorination dosing. Free, no login.
Pipe
Inside diameter: 0.545" — published average ID for Copper Type L
Capacity
Enter pipe length to calculate
How this calculator works
A pipe is a cylinder, so the water it holds is its cross-section times its length. With the inside diameter ID in inches and the run L in feet:
Vgal = π × (ID/2)² × 12 × L ÷ 231 (231 in³ per US gallon)
The whole trick is using the right inside diameter. Nominal size is a trade label, not a measurement: 3/4" copper Type L has a 0.785" ID while 3/4" PEX is only 0.671" — about 27% less water per foot. This calculator carries published average IDs per material — copper tube standard Type L, steel Schedule 40 (ASME B36.10), PVC Schedule 40 (ASTM D1785 average IDs), and PEX (ASTM F876 CTS SDR-9 average IDs) — so the gallons-per-foot figure reflects the pipe on your truck, not a generic cylinder.
Worked example
How much water is in 100 ft of 3/4" copper Type L (ID 0.785")?
- Cross-section: π × (0.785/2)² = 0.4840 in².
- Per foot of run: 0.4840 × 12 = 5.808 in³, and 5.808 ÷ 231 = 0.0251 gal/ft.
- Over 100 ft: 0.0251… × 100 = 2.51 gal (about 0.336 ft³).
Enter the same pipe above (Copper Type L, 3/4", 100 ft) to reproduce it. Swap the material to PEX and the same run holds only 1.84 gal — the ID difference in action.
Reference: average inside diameters (inches)
The verified average IDs behind the calculator. PVC values are ASTM D1785 average IDs (average OD minus twice the average wall) — which is why they run slightly smaller than steel Schedule 40’s at the same nominal size. PEX is copper-tube-size OD with an SDR-9 wall per ASTM F876, so its bore is the smallest of the four at every size.
| Nominal | Copper Type L | Steel Sch 40 | PVC Sch 40 | PEX (SDR-9) |
|---|---|---|---|---|
| 1/2" | 0.545 | 0.622 | 0.602 | 0.475 |
| 3/4" | 0.785 | 0.824 | 0.804 | 0.671 |
| 1" | 1.025 | 1.049 | 1.029 | 0.862 |
| 1-1/4" | 1.265 | 1.380 | 1.360 | 1.054 |
| 1-1/2" | 1.505 | 1.610 | 1.590 | 1.244 |
| 2" | 1.985 | 2.067 | 2.047 | 1.629 |
| 3" | 2.945 | 3.068 | 3.042 | — |
Frequently asked questions
How many gallons are in 100 feet of 2 inch pipe?
It depends on the actual inside diameter, not the nominal size. 100 ft of 2" copper Type L (1.985" ID) holds about 16.1 gallons, 2" Schedule 40 steel or PVC (2.067" ID) about 17.4 gallons, and 2" PEX (roughly 1.63" ID) only about 10.8 gallons. That spread is why the calculator asks for the material before it gives you a number.
How do I calculate gallons per foot of pipe?
Gallons per foot = inside diameter in inches, squared, times 0.0408. That comes straight from cylinder volume — π/4 × d² square inches of bore times 12 inches per foot, divided by 231 cubic inches per US gallon. This calculator runs that math using a typical inside diameter for the material and nominal size you pick; if you have the exact ID from a spec sheet, the same formula applies.
Why does the material matter if the nominal size is the same?
Nominal size is a trade label, not a measurement. 3/4" copper Type L has a 0.785" ID while 3/4" PEX runs about 0.671" — roughly 27% less cross-sectional area, and volume scales with diameter squared, so small ID differences move the answer more than you would guess. Actual ID also varies by schedule, class, and manufacturer, so treat the result as a close estimate rather than a certified figure.
How much water do I need to flush or chlorinate a new water line?
Start from the fill volume this calculator gives you. Disinfection dosing works from line volume — the gallons in the pipe determine how much chlorine solution reaches the target concentration — and flushing specs are usually written as some multiple of line-volume turnovers. Concentrations and contact times come from your utility or the specification on the job, not from a rule of thumb, so use the volume here and take the dosing procedure from the governing spec.
Does pipe volume affect hot water wait time?
Directly — everything between the heater (or recirc loop) and the fixture has to be purged before hot water arrives. 50 ft of 3/4" copper Type L holds about 1.26 gallons; the same run in 1/2" copper holds about 0.61 gallons, so at a 1.5 GPM lavatory faucet that is roughly a 50-second wait versus 24. Where fixture demand allows it, downsizing the final legs of a hot-water run is often the cheapest wait-time fix on the table.
Method: cylinder geometry on published average inside diameters at 231 in³ per US gallon — copper Type L and steel Schedule 40 per their standard dimension tables (ASME B36.10 for steel), PVC Schedule 40 per ASTM D1785 average IDs, PEX per ASTM F876 CTS SDR-9 average IDs. This is a physical-volume figure, not a code value: actual ID varies slightly by manufacturer and tolerance class, fittings and valves add or subtract a little, and other schedules (Sch 80, Type K/M copper, SDR series) differ. For dosing or drain-down where the margin matters, round up.
Pair with the Tank Volume Calculator for vessel capacity on the same job, or the GPM Flow Rate Calculator and Water Pressure Loss Calculator to work the hydraulics of the run you just measured.