Pipe Volume Calculator
Pipe capacity in gallons or liters with water weight.
What it does
Pipe capacity is a cylinder: bore area times run length. Plumbers use it to size drains, dose treatments, and judge how much water a run holds before first draw. This page returns volume, liquid capacity, and water weight in the active unit set.
The default is a 2 in bore over 10 ft. That run holds 0.22 ft3, which is 1.63 gal, and the water inside weighs 13.6 lb. Metric defaults convert the same pipe to 5.08 cm by 3.05 m and give 0.0062 m3, 6.18 L, and 6.2 kg.
Diameter choice decides accuracy. Inside diameter gives true capacity, while outside diameter adds wall thickness and overstates small pipe badly. Measure the bore or read the ID print on the pipe wall.
Straight math only. Elbows, tees, valves, slope, and trapped air change real fills, so add a small margin on long runs with many fittings.
How to use
- Pick imperial or metric first.
- Enter inside diameter (in or cm).
- Enter straight length (ft or m).
- Read volume first, then capacity and water weight.
- Add runs separately where sizes change.
Formula
- d
- Inside diameter across the bore.
- L
- Straight pipe run length.
- pi
- 3.1416, circle constant.
Diameter must be inside diameter in the same length units after conversion. The tool converts inches to feet behind the scenes for imperial and cm to meters for metric, then applies the cylinder formula once.
Worked example
Example: 2 in pipe, 10 ft
2 in diameter, 10 ft long.
- Radius: 2 / 2 / 12 = 0.0833 ft.
- Area: 3.1416 x 0.0833 x 0.0833 = 0.0218 sq ft.
- Volume: 0.0218 x 10 = 0.22 ft3.
- Gallons: 0.22 x 7.48052 = 1.63 gal.
- Water: 1.63 x 8.34 = 13.6 lb.
- Tool shows 0.22 ft3, 1.63 gal, 13.6 lb.
Reference table
| Bore | Per 10 ft | Water |
|---|---|---|
| 1 in | 0.41 gal | 3.4 lb |
| 2 in | 1.63 gal | 13.6 lb |
| 3 in | 3.68 gal | 30.7 lb |
| 4 in | 6.53 gal | 54.5 lb |
| 6 in | 14.70 gal | 122.6 lb |
Limits
- Straight pipe only. Fittings add volume.
- Assumes full bore. Air pockets lower real fills.
- Ignores pressure and temperature stretch.
- One size per pass. Mixed runs need adding.
Common mistakes
- Using outside diameter for capacity.
- Mixing inches and feet without conversion.
- Forgetting gallons to liters is 3.78541.
- Reading weight as pipe weight (this is water only).
- Averaging mixed sizes in one pass.
Try it yourself
Quick checks. Answers sit tucked under each question.
4 in bore, 20 ft long?
Volume 1.75 ft3, 13.06 gal, 108.9 lb.
Metric defaults 5.08 cm by 3.05 m?
Volume 0.0062 m3, 6.18 L, 6.2 kg.
Questions
How do you find pipe volume?
Pi times radius squared times length. A 2 in pipe over 10 ft holds 1.63 gal.
How do cubic feet become gallons?
7.48052 gallons per cubic foot. Liters follow at 3.78541 per gallon.
How heavy is the water inside?
About 8.34 lb per gallon. Here 1.63 gal weighs 13.6 lb.
Does it work in metric?
Yes. Switch to metric and enter cm and meters for m3, liters, and kg.
Inside or outside diameter?
Use inside diameter for capacity. Outside size overstates flow volume.
Does it include fittings?
No. Fittings, slope, and air pockets sit outside straight pipe math.