How much water a pipe holds
A full pipe is a cylinder, so its capacity is the cross-sectional area of the bore multiplied by the length:
Keep the diameter and length in the same units, then convert the result to gallons or litres. There's a tidy US shortcut that bakes in the conversion: gallons per foot = ID(in)² × 0.0408, so a 2 inch bore holds about 0.163 gallons per foot, and 100 ft of a 1 inch bore comes to roughly 0.545 cubic feet, about 4.08 US gallons. The full-pipe tab does the geometry and every unit conversion at once.
Use the inside diameter, not the pipe size
The number printed on a pipe is its nominal trade size, which is not the same as the bore that actually holds the water. Depending on material and wall thickness, a pipe sold as “2 inch” might have an inside diameter a little above or below two inches — and because volume depends on the diameter squared, getting the bore wrong scales straight through to the answer. That's why this calculator asks for the measured or looked-up inside diameter and does not bundle a built-in size table; you supply the real bore for your own pipe.
Partially full pipe — the circular-segment area
A pipe that's only part full is the question generic cylinder calculators get wrong. The volume is not the full volume times the fill fraction, because the cross-section is a circle: a half-full pipe holds exactly half, but a pipe filled to a quarter of its depth holds far less than a quarter of its capacity. You need the area of the circular segment the liquid occupies:
The partially-full tab takes the liquid depth measured from the bottom of a horizontal pipe and applies this directly, so the awkward geometry is handled for you. It's the figure you want for a drain running part full, a horizontal tank-like run, or estimating what's left in a line.
Cross-sectional and surface area
Alongside the volume the calculator reports two areas worth having. The cross-sectional area of the bore, π × (ID ÷ 2)², is the same area the pipe flow calculator uses to turn flow into velocity. The internal wetted surface area, π × ID × L for a full pipe, is what you'd use for lining, coating or heat-transfer estimates.
Gallons and litres — getting the conversion right
Volume conversions trip people up mostly because a US gallon and a UK gallon are different sizes — the imperial gallon is about 20 % larger — so this tool always shows both. The factors are exact:
| From | Equals |
|---|---|
| 1 cubic foot | 7.48052 US gallons |
| 1 cubic foot | 28.3168 litres |
| 1 US gallon | 3.78541 litres |
| 1 UK (imperial) gallon | 4.54609 litres |
| 1 cubic metre | 1000 litres |
Several runs at once
For a whole system you rarely have just one pipe. The “several runs” tab lets you add each length at its own bore and totals the volume across all of them — the quick way to find the holding capacity of a loop for filling, draining, pressure testing, chlorination dosing or working out a glycol charge.
Worked examples
Example 1 — 100 ft of 1 inch bore
V = π × (1 ÷ 2)² × (100 × 12) in³ ≈ 942 in³ = 0.545 ft³ ≈ 4.08 US gallons (about 15.4 litres, 3.40 UK gallons). Load it with ?id=1&idunit=in&len=100&lenunit=ft&fill=full&out=usgal.
Example 2 — a 2 inch bore, full
Per foot, a 2 inch bore holds 2² × 0.0408 ≈ 0.163 US gallons, so 50 ft holds about 8.2 US gallons.
Pipe volume FAQ
How do I calculate the volume of water in a pipe?
A full pipe is just a cylinder: volume = π × (inside diameter ÷ 2)² × length. Work in consistent units, then convert to whatever capacity unit you want. In US units a handy shortcut is gallons per foot = inside diameter in inches squared × 0.0408, so a 2 inch bore holds about 0.163 gallons per foot. The calculator does the geometry and every unit conversion for you, and it asks for the inside diameter because the bore is what holds the water, not the nominal pipe size.
Why does it ask for the inside diameter instead of the pipe size?
Because the nominal size printed on a pipe is a trade label, not the actual bore. A pipe sold as 2 inch may have an inside diameter that is a little more or less than two inches depending on material and wall thickness, and since volume depends on the diameter squared, using the nominal number instead of the real bore introduces a noticeable error. This tool deliberately takes the measured or looked-up inside diameter rather than shipping a built-in size table, so read the real bore for your pipe and enter that.
How do I work out the volume in a partially full horizontal pipe?
A part-full pipe is not simply a fraction of the full volume — the relationship between depth and volume is non-linear because the cross-section is a circle. You find the area of the circular segment the liquid fills, then multiply by the length. The segment area is A = r² × arccos((r − h) ÷ r) − (r − h) × √(2rh − h²), where r is the inside radius and h is the liquid depth. The partially-full mode does this for you when you enter the fill depth; a half-full pipe holds exactly half the full volume, but a quarter-deep pipe holds far less than a quarter.
How do I convert pipe volume to gallons or litres?
Once you have the volume in cubic feet or cubic metres, the conversions are exact: 1 cubic foot = 7.48052 US gallons = 28.3168 litres, 1 US gallon = 3.78541 litres, and 1 UK (imperial) gallon = 4.54609 litres. US and UK gallons are not the same — a UK gallon is about 20 % larger — so the calculator shows both separately to avoid the usual mix-up, along with litres, cubic feet and cubic metres.
Can I add up several pipe runs of different sizes?
Yes. Use the add-a-pipe button to enter each run with its own inside diameter and length, and the calculator totals the volume across all of them. That is the quick way to find the whole holding capacity of a system for filling, draining, pressure testing, chlorination dosing or a glycol charge, rather than summing each run by hand.
Is this pipe volume calculator free and private?
Yes. It is free, needs no signup or download, and runs entirely in your browser, so nothing you enter leaves your device. You can copy a shareable link that reopens the calculator with your inside diameter, length and chosen output unit already filled in.