In short: a 4-inch pipe 100 feet long holds about 65.3 US gallons (247 liters) of water, weighing roughly 544 lb (247 kg) at room temperature. Volume scales with the square of the diameter but only linearly with length, so pipe size matters far more than pipe run — use the calculator above for your exact size.
These figures are planning aids based on straight-pipe geometry. For plumbing code compliance or structural loading, confirm actual pipe dimensions with your installer or a licensed engineer.
How to calculate pipe volume
A straight pipe is a cylinder, so its volume is just the cross-sectional area of the bore multiplied by its length. The area of a circle is π ÷ 4 times the diameter squared, so:
Worked example. Take a 4-inch pipe, 100 feet long. Convert 4 inches to feet: 4 ÷ 12 = 0.333 ft. The cross-sectional area is (π ÷ 4) × 0.333² = 0.0873 ft². Multiply by the length: 0.0873 × 100 ft = 8.73 ft³. One cubic foot is 7.4805 US gallons, so 8.73 × 7.4805 = 65.3 gallons (247 liters). That single multiplication, done in consistent units, is all the calculator is doing.
Sizing a new pipe instead? This tool assumes you already know the diameter and just want the capacity. If you're working the other direction — picking a diameter to carry a target flow — use the Pipe Size Calculator instead.
Why diameter matters more than length
Because diameter is squared in the formula and length isn't, doubling the diameter has a far bigger effect than doubling the length. Double the diameter of our 4-inch, 100-foot example to 8 inches and the volume quadruples — from 65.3 gallons to 261.1 gallons — even though the pipe is the same length. Double the length instead, to 200 feet, and the volume merely doubles, to 130.6 gallons.
That's why getting the diameter right matters so much for an accurate answer — and why "nominal" pipe sizes can mislead you. A pipe's nominal size (like "1-inch pipe") is a naming convention from Nominal Pipe Size (NPS) standards, not a direct measurement of the bore. The actual inside diameter is usually somewhat different, especially for smaller pipe. The table below shows the gap for common Schedule 40 sizes.
Water volume per foot of pipe
Using the diameter as entered (treated as the actual bore). Multiply by your pipe's length in feet, or use the "per 100 ft" column directly for a longer run.
| Diameter | Liters per foot | US gal per foot | US gal per 100 ft |
|---|---|---|---|
| 1/2 in | 0.0386 | 0.0102 | 1.02 |
| 3/4 in | 0.0869 | 0.0229 | 2.29 |
| 1 in | 0.1545 | 0.0408 | 4.08 |
| 1¼ in | 0.2414 | 0.0638 | 6.38 |
| 1½ in | 0.3476 | 0.0918 | 9.18 |
| 2 in | 0.6178 | 0.1632 | 16.32 |
| 3 in | 1.3901 | 0.3672 | 36.72 |
| 4 in (default) | 2.4713 | 0.6528 | 65.28 |
| 6 in | 5.5604 | 1.4689 | 146.89 |
| 8 in | 9.8848 | 2.6113 | 261.13 |
Nominal size vs. actual inside diameter (Schedule 40)
"Nominal" pipe size is a label, not a measurement. Schedule 40 is the most common wall thickness for steel and PVC water pipe; a heavier schedule (like Schedule 80) has a thicker wall and a smaller actual bore at the same nominal size, so it holds less water. Always enter the actual inside diameter for a precise result.
| Nominal size | Actual ID (Sch. 40) | US gal per foot |
|---|---|---|
| 1/2 in | 0.622 in | 0.0158 |
| 3/4 in | 0.824 in | 0.0277 |
| 1 in | 1.049 in | 0.0449 |
| 1¼ in | 1.380 in | 0.0777 |
| 1½ in | 1.610 in | 0.1058 |
| 2 in | 2.067 in | 0.1743 |
| 3 in | 3.068 in | 0.3841 |
| 4 in | 4.026 in | 0.6613 |
| 6 in | 6.065 in | 1.5008 |
| 8 in | 7.981 in | 2.5990 |
Notice that a "1/2-inch" pipe actually holds about 55% more water than a naive 0.5-inch-diameter calculation would suggest, because its real bore (0.622 in) is larger than its nominal name. The gap narrows at bigger sizes. Use the diameter presets above to load these Schedule 40 values directly.
Worked examples
3/4″ garden supply line, 50 ft
1.15 gal
A 50-foot run of 3/4-inch pipe holds about 1.15 US gallons (4.34 L) of water, weighing roughly 9.6 lb (4.3 kg) at room temperature.
4″ water main, 200 ft
130.6 gal
A 200-foot length of 4-inch water main holds about 130.6 US gallons (494 L), weighing about 1,088 lb (493 kg).
6″ fire line, 500 ft
734.4 gal
A 500-foot run of 6-inch fire main holds about 734.4 US gallons (2,780 L) — roughly 6,118 lb (2,775 kg) of water, which matters when planning supports or drain-down time.
Once you know the volume, pair it with the Pipe Fill Time Calculator to see how long it takes to fill or drain that pipe at a given flow rate.
What else affects the result
Wall thickness and schedule. A higher Schedule number means a thicker wall and a smaller actual bore at the same nominal size — see the Schedule 40 table above, and check your pipe's actual rating if it isn't Schedule 40.
Fittings and valves. Elbows, tees and valves add a small amount of internal volume beyond straight pipe, and their bore can differ slightly from the pipe itself. For most estimating purposes this is small enough to ignore; for precise dosing or draw-down calculations, add the fitting manufacturer's internal volume.
Liners and scale. Older pipe with mineral scale or a corrosion-resistant liner has a smaller effective inside diameter than its as-built spec. Measuring the actual bore, where practical, gives the most accurate volume.
Elevation and slope. This calculator assumes the pipe is completely full. A sloped or partially-drained pipe holds less water than its full capacity — the geometry here gives you the maximum, not necessarily the current contents.
The water-weight figure uses the same Kell-equation density behind our Water Weight Calculator, so temperature and salinity affect it the same way they would there.
Frequently asked questions
How do I calculate the volume of water in a pipe?
Multiply the pipe's cross-sectional area by its length: volume = (π ÷ 4) × diameter² × length, using consistent units throughout. For a 4-inch (0.333 ft) diameter pipe 100 feet long, the area is about 0.0873 ft², so the volume is 0.0873 × 100 = 8.73 ft³, which is 65.3 US gallons (247 liters).
How much water is in 100 feet of 4-inch pipe?
About 65.3 US gallons (247 liters), which weighs roughly 544 lb (247 kg) at room temperature. That figure uses the nominal 4-inch size; the actual inside diameter of real Schedule 40 pipe is slightly different, so check the nominal-versus-actual table above for a more precise number.
Does the calculator use the nominal pipe size or the actual inside diameter?
Whatever diameter you enter. Nominal pipe size (NPS) is a naming convention, not a measurement — a "1-inch" pipe does not have a 1.000-inch bore. For an accurate result, enter the actual inside diameter, which you can look up for Schedule 40 pipe in the reference table above.
How much does the water inside a pipe weigh?
Weight equals volume multiplied by the density of water at that temperature. At room temperature (20 °C / 68 °F), water weighs about 8.33 lb per US gallon, so 100 gallons of water weighs about 833 lb (378 kg). The calculator computes this automatically alongside the volume.
Does pipe wall thickness change the volume?
Yes. A thicker wall (a higher Schedule number) leaves less room inside the pipe even when the nominal size and outside diameter stay the same. Schedule 80 pipe holds noticeably less water than Schedule 40 pipe of the same nominal size, because its actual inside diameter is smaller.
What's the difference between pipe volume and flow rate?
Pipe volume is a fixed amount — how much water the pipe holds when full, regardless of time. Flow rate is how much water moves through the pipe per minute or second, such as gallons per minute (GPM). You need the volume to know how long a pipe takes to fill or drain at a given flow rate.
Sources & references
- ASME B36.10M / B36.19M. Welded and Seamless Wrought Steel Pipe / Stainless Steel Pipe — defines nominal pipe size and Schedule wall-thickness dimensions.
- Standard solid geometry: volume of a right circular cylinder, V = πr²h.
- Kell, G. S. (1975). Density, thermal expansivity, and compressibility of liquid water from 0° to 150 °C. Journal of Chemical & Engineering Data, 20(1), 97–105.
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)