Pipe Water Volume Calculator
Knowing how much water a pipe holds is essential for a range of plumbing and hydronic tasks: draining a system before repairs, calculating antifreeze quantities for freeze protection, sizing expansion tanks, estimating fill time, and determining chemical treatment dosages. The calculation is straightforward geometry: the volume of a cylinder equals pi times the square of the internal radius times the pipe length. Enter the inside diameter and length in any consistent unit pair and the calculator returns volume in US gallons, liters, and cubic feet.
Pipe volume formula
V (cubic feet) = π × (diameter / 2 / 12)² × length (ft)
V (gallons) = V (cubic feet) × 7.481
V (liters) = V (cubic feet) × 28.317
Weight (lb) = V (gallons) × 8.34
Diameter is divided by 2 to get radius, then divided by 12 to convert inches to feet, matching the length unit. The result in cubic feet is converted using the exact conversion factors: 1 cubic foot = 7.480519 US gallons = 28.3168 liters. Water weighs 8.34 lb per US gallon at 60 F.
Common pipe inside diameters
- 1/2-inch copper (Type L): 0.545 inch inside diameter.
- 3/4-inch copper (Type L): 0.785 inch inside diameter.
- 1-inch copper (Type L): 1.025 inch inside diameter.
- 1/2-inch PVC Schedule 40: 0.622 inch inside diameter.
- 3/4-inch PVC Schedule 40: 0.824 inch inside diameter.
- 1-inch PVC Schedule 40: 1.049 inch inside diameter.
- Always use actual inside diameter, not nominal size, for accurate volume calculations.
Pipe water volume: frequently asked questions
How do I calculate water volume in a pipe?
Volume equals the cross-sectional area of the pipe times its length. For a round pipe: V = pi x (diameter/2) squared x length. Convert the diameter and length to the same units first. The result is in cubic units, which you convert to gallons (1 cubic foot = 7.481 gallons) or liters (1 cubic foot = 28.317 liters).
What is the difference between nominal pipe size and actual inside diameter?
Nominal pipe size (NPS) is a label, not an actual measurement. A 1-inch NPS copper pipe (Type L) has an actual inside diameter of about 1.025 inches. A 1-inch NPS PVC schedule 40 pipe has an inside diameter of 1.049 inches. Always use the actual inside diameter for volume calculations.
Why does pipe water volume matter for hydronic systems?
In hydronic heating or cooling systems, knowing the total water volume in the piping helps you determine glycol antifreeze quantities, expansion tank sizing, and the amount of water treatment chemicals needed. Large systems with miles of tubing can hold thousands of gallons.
How much does a gallon of water weigh?
One US gallon of water weighs approximately 8.34 pounds (3.78 kg) at room temperature. If you need to calculate the weight of water in a pipe for structural loading, multiply volume in gallons by 8.34 lb/gal.
What is the volume of a 1-inch pipe per foot of length?
A 1-inch inside diameter pipe holds pi x (0.5 in)^2 x 12 in = 9.42 cubic inches per foot, which equals about 0.041 gallons per foot, or about 0.154 liters per foot. A 100-foot run holds approximately 4.1 gallons.
Official sources
- NIST: NIST Handbook 44 (units and measurements).
- EPA WaterSense: Water Efficiency Resources.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.