Skip to content
Calcapedia
Menu
Trades

Pipe Volume Calculator

Find how many gallons or litres of water a run of copper, PEX, PVC or steel pipe holds, from its trade size or inside diameter and length.

Level: Grades 7–8

  • Common Core standard 8.G.C.9Know the formulas for the volumes of cones, cylinders, and spheres and use them to solve real-world and mathematical problems.
  • Common Core standard 7.G.B.4Know the formulas for the area and circumference of a circle and use them to solve problems; give an informal derivation of the relationship between the circumference and area of a circle.
Printable worksheet

Pipe Volume Calculator

Know the answer you need? Pick an input to work backward.

Inputs

Add it to see how long the water in the pipe takes to flush out, such as the wait for hot water.

Result

Water in the pipe

1.257 US gal

50 ft of 3/4 in Type L copper holds about 1.257 US gal (4.759 L) of water.

Water in the pipe
4.759 L
Volume
0.168 ft³
Per foot of pipe
0.02514 gal/ft
Per meter of pipe
0.3122 L/m
Weight of that water (20 °C)
10.47 lb
Weight of that water (20 °C)
4.75 kg

See the step-by-step solution ↓

Step-by-step solution

  1. Find the area of the open circle inside 3/4 in Type L copper (A), using its inside diameter d of 0.785 in

    A=π4×0.7852=0.483982 in²

  2. Change the length to inches so it matches the area (1 ft = 12 in)

    50×12=600 in

  3. Multiply the area by the length to get the volume of water inside

    0.483982×600=290.3892 in³

  4. Turn cubic inches into US gallons (1 gal = 231 in³)

    290.3892231=1.257096 gal

  5. Turn gallons into litres (1 gal = 3.785411784 L)

    1.257096×3.785411784=4.758626 L

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

Find out how much water sits inside a run of pipe. That one number answers several everyday plumbing questions. How much antifreeze or treatment chemical does a heating loop need? How much water must you drain before a repair? How heavy is a filled pipe run? And why does hot water take so long to reach a far bathroom? The answer depends on the pipe's inside diameter, the width of the hole the water flows through. That's not the same as its trade size, sometimes called the nominal size. It also differs between copper types, PEX and plastic pipe schedules (wall-thickness grades).

This calculator already knows the published inside diameters for Type K, L and M copper tube and for PEX tubing. It also knows Schedule 40 and 80 PVC and Schedule 40 steel pipe. Pick the material and trade size, or type any inside diameter, then enter the length. You'll get the volume in US gallons, litres and cubic feet. You'll also see the volume per foot and per meter, and the weight of the water. Add the flow rate at a tap to see how many seconds the water in the line takes to flush through.

Where it's used

Why this math matters outside the classroom:

  • Draining for a repair. A plumber expects about 1.26 gallons to come out of 50 ft of 3/4 in Type L copper before cutting in a valve.
  • Hydronic antifreeze. Heating installers add up the pipe volume in a boiler loop to buy the right amount of glycol mix.
  • Hot water wait. At 1.5 gpm, the 1.26 gallons in a 50 ft run take about 50 seconds to flush through to the tap.
  • Comparing PEX and copper. Half-inch PEX holds less water per foot than half-inch Type L copper, so hot water arrives sooner through it.
  • Cylinder volume. Students find the volume of a long thin cylinder, πr²h, and convert cubic inches to gallons using 231 in³ per gallon.

Grade level and standards: Grades 7–8; Common Core State Standards for Mathematics 8.G.C.9, 7.G.B.4. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Choose the pipe or tube type, then its trade size. Or choose a custom inside diameter in inches or millimeters.
  2. Enter the length of the run in feet or meters.
  3. If you like, enter the flow rate at the tap. You'll see how long the pipe takes to empty once.
  4. Read the volume in gallons and litres, the volume per foot, and the weight of the water.

What the inputs mean

Pipe or tube
Trade sizes are just names; the hole inside depends on how thick the wall is. A 3/4 in Type L copper tube has a 0.785 in bore (inside width). A 3/4 in PEX tube is about 0.681 in, and 3/4 in Schedule 40 PVC is about 0.804 in.
Inside diameter
Use the inside diameter from the maker's data sheet, or measure the bore of a cut end.
Flow rate at the tap (optional)
Kitchen and bathroom faucets often run at 1 to 2.2 gpm (gallons per minute); showerheads at up to 2.5 gpm. The time shown is how long it takes to push out the water already in the pipe. If the pipe has cooled, that's roughly your wait for hot water.

Formula and method

Area A = π ÷ 4 × d²            (d = inside diameter, in)
Volume (in³) = A × length (in)
US gallons = in³ ÷ 231
litres = gallons × 3.785411784
Flush time (s) = gallons ÷ flow (gpm) × 60

The water in a full pipe is a long cylinder as wide as the pipe's bore. d is that inside diameter in inches, and π (pi) is about 3.1416. A is the area of the circle you'd see looking into the cut end. Multiply that area by the length, and you have the volume. A US gallon is exactly 231 cubic inches, and the litre conversion is exact too.

The bores come from published data. Copper sizes come from the Copper Development Association's Copper Tube Handbook. PEX sizes follow the SDR 9 standard, and PVC and steel come from pipe reference charts. Real pipe varies a little within its manufacturing tolerance. Fittings, valves and fixtures add a little more volume too.

Worked example

A 50 ft run of 3/4 in Type L copper from a water heater to a bathroom, with a faucet running 1.5 gpm.

  1. Inside diameter 0.785 in, so the area is π ÷ 4 × 0.785² = 0.483982 in².
  2. Length: 50 × 12 = 600 in, so the volume is 0.483982 × 600 = 290.3892 in³.
  3. Gallons: 290.3892 ÷ 231 = 1.257096 gal, which is 4.758626 L (0.02514 gal per foot).
  4. Flush time at 1.5 gpm: 1.257096 ÷ 1.5 × 60 = 50.28 s.

Result: The run holds about 1.26 gallons (4.76 L), or about 10.47 lb of water. It takes roughly 50 seconds to flush at 1.5 gpm.

Understanding the result

The volume tells you how much to drain, fill or treat. For hot water, the flush time is the shortest wait at a tap if the line has cooled. Heat lost to the cold pipe walls makes the real wait a little longer.

Comparing materials shows how much the bore matters. PEX has a smaller bore for the same trade size, so it holds less water. That shortens the wait for hot water. But it also raises friction loss, the pressure lost as water rubs along the walls, at the same flow.

Assumptions and limitations

  • The pipe is full, and its bore equals the published trade-size or average inside diameter.
  • Water weight uses a density of 0.9982 kg per litre (20 °C).
  • Fittings, valves, fixtures and tanks aren't included.
  • Inside diameters vary between makers and within tolerance, especially for plastic pipe. For precise work, enter a measured bore.
  • Only US gallons are used; one imperial gallon is about 1.2 US gallons.
  • It isn't a design tool for sizing supply lines. Plumbing work should follow local code and be done or approved by a licensed plumber.

Frequently asked questions

How many gallons are in 100 ft of 3/4 in pipe?

It depends on the pipe, because their bores differ. 3/4 in Type L copper holds about 2.51 gal per 100 ft. 3/4 in PEX holds about 1.89 gal, and 3/4 in Schedule 40 PVC about 2.64 gal.

Why is the inside diameter different from the pipe size?

Trade sizes are names that fix the outside diameter. The wall thickness sets the bore, and it differs between Type K, L and M copper or Schedule 40 and 80 PVC.

How long should I wait for hot water?

At least the flush time this calculator shows. That's the time to push out the cooled water already in the line. A long run of large pipe at a low-flow faucet can take a minute or more.

How much does water in a pipe weigh?

About 8.33 lb per US gallon, or 1 kg per litre. A 50 ft run of 3/4 in Type L copper holds about 10.5 lb of water. That matters when you plan supports for larger pipes.

Sources

How this calculator is checked

Method: Cylinder volume π/4 × d² × L; 1 US gal = 231 in³ = 3.785411784 L. Bores from CDA Copper Tube Handbook Tables 14.2a–c, PEXUniverse PEX (SDR 9) table, and Rain Bird PVC Schedule 40/80 and steel charts. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

Embed this calculator

Put this Pipe Volume Calculator on your own website or blog for free. Paste this code where you want it to appear: