A pipe is a cylinder with a job to do, and how much water it holds decides everything from chemical dosing to how long a system takes to drain. This Pipe Volume Calculator takes the pipe’s inside diameter and length and returns the contained volume in cubic feet, US gallons and litres — the three currencies of plumbing, irrigation and process work.
Worked Example: 100 Feet of 6-Inch Pipe
The bore’s cross-section is a circle of radius 3 inches (0.25 ft): area = π × 0.25² = 0.1963 sq ft. Over 100 feet the volume is 0.1963 × 100 = 19.63 cubic feet, which is 146.9 US gallons or 556.0 litres. Two properties of the geometry deserve attention because they govern every sizing decision downstream. First, volume grows with the square of diameter: step from 6-inch to 12-inch pipe and the same 100 feet holds not double but four times as much — 78.5 cubic feet, 587 gallons — which is why an upsized main transforms flushing times, chemical contact times and the weight a trench crew must drain before a repair (that 6-inch run’s water alone weighs about 1,225 lb at 8.34 lb per gallon). Second, the contained volume is not the flow: a pipe can hold 147 gallons while delivering 20 gallons a minute or 500, depending on pressure and friction — this page answers “how much is inside,” and its companion questions (how fast it moves, how long to empty at a given draw) start from this number: emptying our example at 50 gpm takes 146.9 / 50 ≈ 2.9 minutes, and dosing it to 10 parts per million of a treatment chemical needs 146.9 gallons × 10 / 1,000,000 of product — arithmetic that is trivial once the volume is known and guesswork until it is.
Inside Diameter: The Number That Isn’t Printed on the Pipe
The calculator asks for inside diameter because pipe is sold by a nominal size that is neither the inside nor the outside measurement — the industry’s most durable trap. A “6-inch” Schedule 40 PVC pipe has an outside diameter of 6.625 in and an inside diameter of about 6.065 in; the same nominal 6-inch in Schedule 80 thickens the wall and shrinks the bore to about 5.761 in. Run this calculator at the nominal 6.000 and the volume error is a modest 2% against Schedule 40 — but run it at the outside diameter by mistake and the error jumps past 20%, because the square law amplifies diameter errors: a 10% diameter mistake is a 21% volume mistake. Copper and PEX add their own conventions (copper tube is sized by outside diameter with wall types K, L and M; PEX nominal tracks copper-tube-size outside dimensions with its own wall series), and steel pipe follows the iron-pipe-size tables where a 2-inch pipe’s bore is 2.067 in in Schedule 40. The professional habit this page enforces: look up or measure the actual bore for the material and schedule in front of you — calipers on a cut end settle it in seconds — and enter that. Every downstream figure (gallons, chemical dose, drain time, weight) inherits the bore’s accuracy squared, which makes the inside diameter the highest-leverage number in the whole calculation.
Where Pipe Volume Decides the Work
Water treatment and dosing: disinfection contact time, chlorination of a new main, and chemical feed calibration all begin from the contained volume of the run being treated. Drainage and repair planning: knowing a run holds 147 gallons tells the crew the pump and tank capacity a shutdown needs and how long a gravity drain will run. Irrigation and fertigation: the volume between the fertilizer injector and the farthest head is the lag the schedule must respect — product reaches the crop one pipe-volume after injection starts. Hydronic heating: system water volume sets expansion-tank sizing and the glycol or inhibitor dose for the whole loop. Aquatics and aquaculture: turnover and medication math for piped systems starts here. Testing: hydrostatic pressure tests fill and bleed the run, and the fill volume predicts both the water needed and the air to be vented. In each case the pattern is this page’s pattern: a cylinder, honestly measured at the bore, converted into the unit the next decision speaks. Keep the diameter-squared law in front of every estimate — it is the reason “just one size up” is never a small change.
Volume per Foot: The Pocket Table
Most field questions are per-foot questions — how much does this run hold, roughly, per length — and the diameter-squared law makes a memorable ladder (true bores, US gallons per foot of pipe): a 1-inch bore holds 0.041 gal/ft, a 2-inch 0.163, a 4-inch 0.653, a 6-inch 1.469, an 8-inch 2.61, and a 12-inch 5.88. Read the ladder’s rhythm: each doubling of diameter multiplies the per-foot volume by four, so the jump from a 2-inch branch line to a 4-inch main is not “twice the water” but four times, and a 100-foot 12-inch main holds about 588 gallons — a small tanker truck, lying in a trench. The table also runs the calculator in your head backwards: if a dosing pump must exchange 10 gallons of a run before fresh product arrives, that is 61 feet of 2-inch pipe but only 15 feet of 4-inch — the reason chemical injection points are placed with a tape measure, not optimism. Commit the two middle rungs (2-inch ≈ a sixth of a gallon per foot, 6-inch ≈ a gallon and a half) and most plumbing volumes you will ever estimate are one multiplication away.
Vertical Runs, Traps and Fittings: The Honest Extras
The calculator prices the straight cylinder; systems add three small corrections worth knowing. Vertical risers are just pipe standing up — a three-storey riser of 2-inch bore and 30 feet holds 30 × 0.163 ≈ 4.9 gallons — but their volume matters twice: once for drain-down planning, and again because a tall riser drains with enthusiasm a horizontal run never shows. Fittings add equivalent length, not much volume: an elbow’s contained water is a thimbleful against these run volumes, and even a valve body or a filter housing typically adds less than a gallon — the exception is vessels (expansion tanks, water heaters, pressure tanks), which are volumes in their own right and belong in any system total as themselves, not as pipe. Traps and low loops hold permanent water that drain calculations must not promise to recover: a drained system keeps its trap volumes, which is why freeze protection and winterization deal with the low points individually. Sum the straights with this page, add the vessels by name, forgive the fittings, respect the traps — the estimate that results is the one the drain valve will agree with.
Heat Expands the Answer: Hot-Water Systems
One correction belongs to heating work: water expands as it warms, so the volume in a hot system is a few percent larger than the cold fill this page computes. Between a 50°F fill and a 140°F operating temperature, water’s density falls enough to grow the volume by roughly 2% — the example run’s 146.9 gallons becomes about 150 gallons hot, and those extra 3 gallons per hundred are exactly what the expansion tank exists to absorb; a tank sized from the cold volume alone, with no expansion allowance, is the arithmetic behind relief valves that weep every heating cycle. Glycol mixtures move the figures a little further (and change the weight and heat capacity with them), which is why hydronic designers work from hot volumes and manufacturers’ tables. For cold plumbing, irrigation and drainage, the calculator’s figure stands as printed. For anything with a boiler attached, add the expansion allowance after the calculation, and size the tank for the difference — the pipe’s geometry is fixed, but its contents breathe with temperature.
Common Mistakes
Entering the nominal size or the outside diameter instead of the bore. Forgetting that volume scales with diameter squared when comparing pipe sizes. Mixing US and imperial gallons (the imperial gallon is about 20% larger — this page uses US gallons). Ignoring fittings and vertical runs in long systems (their volumes are small but real, and a tall riser is just more pipe). And treating contained volume as flow capacity — the two meet only through velocity and time, never by identity.
Related Tools
Continue with the Concrete Column Calculator, the Flow Rate Converter, the CFM Calculator, or browse the full Construction Calculators hub.
Frequently Asked Questions
How do I calculate the volume of a pipe?
Volume = π × (inside radius)² × length. A 6-inch bore (3-inch radius) over 100 ft holds π × 0.25² × 100 = 19.63 ft³ = 146.9 US gallons. Always use the inside diameter.
How many gallons does 100 feet of 6-inch pipe hold?
About 146.9 US gallons (556 litres), using the true 6-inch bore. A Schedule 40 bore of 6.065 in holds slightly more — enter the actual inside diameter for your material and schedule.
Why does the inside diameter matter so much?
Because volume scales with diameter squared: a 10% diameter error becomes a 21% volume error. Nominal pipe sizes match neither bore nor outside diameter exactly, so measure or look up the real bore.
Does doubling the diameter double the volume?
No — it quadruples it. Twelve-inch pipe holds four times the water of 6-inch pipe at the same length, which is why upsizing transforms drain times, dosing and weight.
How long will a pipe take to drain?
Divide this volume by the drain rate: 146.9 gallons at 50 gallons per minute takes about 2.9 minutes (ignoring the inflow and the slowing head as it empties — gravity drains taper).
What does the water in a pipe weigh?
US water weighs about 8.34 lb per gallon, so the example run’s 146.9 gallons weigh roughly 1,225 lb — a figure that matters for supports, trench safety and draining plans.