Pipe Water Volume Calculator
Enter the pipe’s actual inside diameter and length to get the water volume held in the line — useful for draindown, flushing, and chemical-treatment estimates. The result updates as you type.
Pipe Water Volume Calculator
Use the actual inside diameter, not the nominal pipe size — it varies by material and schedule.
Formula: gallons = π × (ID ÷ 2)² × (length × 12) ÷ 231. Always use the actual inside diameter for your specific pipe material and schedule, not the nominal size — a nominal 3/4 in pipe can have an actual ID anywhere from about 0.545 in to 0.811 in depending on type.
The formula, worked example
A pipe is a cylinder, so its water volume is the cross-sectional area times length, converted from cubic inches to gallons (231 cubic inches per gallon).
| Step | Example: 0.75 in ID, 100 ft |
|---|---|
| Length to inches | 100 × 12 = 1,200 in |
| Cross-section area | π × (0.75 ÷ 2)² = 0.442 in² |
| Volume in cubic inches | 0.442 × 1,200 = 530.1 in³ |
| Volume in gallons | 530.1 ÷ 231 = 2.29 gal |
Nominal size is not actual inside diameter
Pipe is sold by nominal size, but the actual inside diameter depends on material and wall thickness. A nominal 3/4 in line runs roughly 0.785–0.811 in ID in type L copper, and noticeably different in PEX or PVC schedule 40. For an accurate volume, pull the actual ID from the manufacturer’s spec sheet for the specific pipe installed.
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Frequently asked questions
How much water does a 100 ft pipe hold?
It depends on inside diameter. At a 0.75 in actual ID, 100 ft of pipe holds about 2.29 gallons. Larger diameter pipe holds far more — volume scales with the square of the radius.
Why use actual inside diameter instead of nominal size?
Nominal size is a trade label, not a measurement. Wall thickness varies by material and schedule, so the true inside diameter — and therefore the true volume — can differ meaningfully from the nominal number.
What is this calculation used for?
Common uses include estimating draindown volume before repairs, sizing chemical treatment or flush volumes, and rough water-loss or capacity checks on a line.
Is this pipe volume calculator free?
Yes — free, no sign-up. Enter inside diameter and length and read the volume in gallons and cubic inches. Estimates only; confirm actual ID with your pipe manufacturer’s spec.