How this instrument works
A straight section of pipe is a long, thin cylinder, so the liquid it can hold follows the ordinary cylinder-volume formula: the circular cross-sectional area, pi times the radius squared, multiplied by the length the pipe runs. What makes pipe volume worth its own instrument rather than a generic cylinder calculator is the units involved — pipe is specified by inside diameter in inches and run in feet, while the answer that actually matters on the job, capacity, is wanted in US gallons, the unit water utilities, well systems and irrigation flow rates are metered in.
This instrument converts the pipe's length from feet to inches, computes the cylinder's volume in cubic inches, and divides by 231 — the exact number of cubic inches in one US gallon by legal definition — to hand back a gallon figure directly. That single conversion factor is why the formula looks more involved than a plain cylinder volume: length is converted to matching units first, then the whole cubic-inch result is rescaled once at the end.
Knowing a pipe run's internal capacity matters in several everyday plumbing and irrigation situations: estimating how long a hose or line takes to fill or drain at a known flow rate, sizing how much standing water needs to be flushed out of a long feeder line before a fresh sample is drawn, or working out how much a section of pipe will add to a system's total water volume for chemical dosing or expansion-tank sizing.
- Enter Pipe inside diameter (in) — the pipe's inner bore, not its outer or nominal size, since wall thickness reduces how much a pipe can actually hold.
- Enter Pipe length (ft) — the run's total length in feet.
- Read Volume (US gallons) beneath both fields; it recalculates the instant either measurement changes.
- For a run with fittings, elbows or a tank at one end, add their volumes separately — this instrument covers straight pipe only.
- If your pipe's spec sheet gives an outside diameter and wall thickness instead, subtract twice the wall thickness from the outside diameter to get the inside diameter this field expects.
Worked example — a 4 in diameter, 100 ft pipe run
Enter 4 into Pipe inside diameter (in) and 100 into Pipe length (ft) — a 4-inch line running the length of a typical yard for an irrigation main. Radius is 2 in, length converts to 100 × 12 = 1,200 in, so volume works out to π × 2² × 1,200 = π × 4,800 ≈ 15,079.6 cubic inches.
Volume (US gallons) reads 65.280 — that figure divided by 231, the exact number of cubic inches in a US gallon. That is how much standing water this single run holds and would need to be flushed, drained, or accounted for before the line does anything else.
Questions
Should I use the pipe's inside diameter or its nominal size?
Inside diameter — the actual bore the water travels through. Nominal pipe sizes, like '2-inch PVC', are a naming convention and often do not match the true inside dimension once wall thickness is accounted for; check the pipe's spec sheet or measure the bore directly for an accurate volume.
Why divide by 231 specifically?
Because one US gallon is legally defined as exactly 231 cubic inches — not an approximation, but the definition itself. Once the pipe's volume is computed in cubic inches, dividing by 231 converts it straight to US gallons with no rounding introduced by the conversion factor itself.
Does this include fittings, valves or a connected tank?
No — this instrument sizes a straight run of pipe only. Elbows, tees, valve bodies and any connected tank each add their own volume on top of this figure and need to be measured or estimated separately if a total system volume is what you're after.
How is this different from the site's tank volume instrument?
The underlying formula is the same cylinder-volume math, but the shape it describes is different in practice: a pipe is long and thin, entered by length in feet and answered per run, while a tank is short and wide relative to its diameter, entered by height and answered as a single standing vessel's capacity. Both report US gallons because both hold liquid.
Can I use this for gas or air lines instead of water?
The volume calculation itself is purely geometric and works for any fluid filling the pipe's bore, gas or liquid alike. What changes for gas is that the answer represents an internal air volume rather than a liquid quantity you would drain or flush — useful for compressed-air line sizing, but not directly comparable to a water flush-out time.
What if my pipe has a slight taper or varies in diameter along its length?
This formula assumes a constant inside diameter for the full length entered. A pipe that tapers or changes size partway through needs to be split into separate constant-diameter sections, each run through this instrument on its own, with the resulting volumes added together.