How do I work out the volume of water in a pipe?

A pipe is a cylinder, so its volume is the circle area of the bore times the length. Use the inside diameter, not the outside — the wall does not hold water. A 2-inch inside diameter pipe 100 feet long holds about 16 gallons. This tool takes the bore and the length and gives you gallons and litres.

Should I use the inside or outside diameter?

Always the inside (the bore). Pipe is often named by a nominal or outside size, and the wall thickness can be a real fraction of a small pipe, so using the outside diameter overstates the volume. If you only know the nominal size, look up the actual inside diameter for that pipe and schedule and enter that.

What is this useful for on the job?

Sizing how much water a line holds for flushing or chlorinating, how much a loop of tubing carries, how long a purge takes, or how much a drain-down will spill into a bucket. It is the same cylinder volume behind a sonotube or a round tank, just expressed in gallons and litres for a fluid.

Pipe volume calculator

This is the geometric volume of a plain cylindrical bore. Fittings, valves and a part-full line all change what a pipe actually holds, so read it as the full-bore capacity and allow for the real run on site.

Enter the inside diameter and the length and the tool works out how much the pipe holds, in gallons and litres at once. The maths is the area of the bore circle multiplied by the length, converted from cubic feet or metres into fluid measure.

Ejemplo resuelto

A pipe with a 2-inch bore has a radius of 1 inch, or 1/12 foot, so the bore area is π × (1/12)² ≈ 0.0218 sq ft. Over 100 feet that is 2.18 cubic feet, and a cubic foot is about 7.48 gallons, so the line holds roughly 16.3 gallons — close to 62 litres.

In metric: a 50 mm bore has a radius of 0.025 m, so the bore area is π × 0.025² ≈ 0.00196 m². Over 40 m that is 0.0785 m³, and a cubic metre is 1,000 litres, so the line holds 78.5 litres — about 20.7 US gallons.

En la obra

Chlorinating a new water service. 300 ft of 1 in line holds 0.041 gal per foot × 300 = 12.2 gallons (in metric, 90 m of 25 mm bore is 0.49 L/m × 90 = 44 litres). The dose is whatever your procedure says per gallon — this page only tells you how many gallons are in the pipe, and it is less than most people guess. Add the volume of the meter pit and any tank on the end; they hold more than the pipe does.

Water for a hydrostatic test. 500 ft of 6 in main: 1.47 gal/ft × 500 = 734 gallons, plus whatever it takes to purge air at the high point. A 4 in line the same length is 327; an 8 in is 1,305. It scales with the square of the bore — twice the pipe, four times the water.

Glycol for a hydronic loop. Total the pipe by size: 200 ft of ¾ in (0.023 gal/ft = 4.6 gal) + 60 ft of 1¼ in (0.064 gal/ft = 3.8 gal) = 8.4 gal in the pipe. Add the boiler and the expansion tank from their nameplates, and the mix ratio the glycol maker specifies does the rest. Use the inside diameter of the actual tubing; nominal sizes are not bores.

What went wrong?

Quick referencefor pipe

Gallons per foot, by bore: ½ in 0.010 · ¾ in 0.023 · 1 in 0.041 · 1¼ in 0.064 · 1½ in 0.092 · 2 in 0.163 · 3 in 0.367 · 4 in 0.653 · 6 in 1.47 · 8 in 2.61
Shortcut: gal/ft = 0.0408 × bore² (bore in inches)
Litres per metre, by bore: 25 mm 0.49 · 50 mm 1.96 · 100 mm 7.85 · 150 mm 17.7
Shortcut: L/m = 0.000785 × bore² (bore in mm)
Unidades: 1 ft³ = 7.48 US gal · 1 gal = 231 in³ = 3.785 L · 1 m³ = 1,000 L

Mistakes to catchbefore you trust the number

Using the nominal pipe size (2 in) as the bore — a 2 in schedule 40 pipe has a bore a little over 2 in, and a 2 in PEX has one well under; measure or read the inside diameter.
Typing the outside diameter for copper tube, which is sized by OD; the water only fills the inside.
Forgetting that a run has fittings and a loop — the length is the pipe, not the straight-line distance.

La fórmula

volume = π × (bore ÷ 2)² × length

The bore area is π times the radius squared; multiply it by the length for the volume, keeping the diameter and length in the same unit first. One cubic foot is 7.48 US gallons and one cubic metre is 1000 litres, which is how the volume becomes a fluid figure.

Preguntas

How do I work out the volume of water in a pipe?: A pipe is a cylinder, so its volume is the circle area of the bore times the length. Use the inside diameter, not the outside — the wall does not hold water. A 2-inch inside diameter pipe 100 feet long holds about 16 gallons. This tool takes the bore and the length and gives you gallons and litres.
Should I use the inside or outside diameter?: Always the inside (the bore). Pipe is often named by a nominal or outside size, and the wall thickness can be a real fraction of a small pipe, so using the outside diameter overstates the volume. If you only know the nominal size, look up the actual inside diameter for that pipe and schedule and enter that.
What is this useful for on the job?: Sizing how much water a line holds for flushing or chlorinating, how much a loop of tubing carries, how long a purge takes, or how much a drain-down will spill into a bucket. It is the same cylinder volume behind a sonotube or a round tank, just expressed in gallons and litres for a fluid.

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