Pipe Volume Calculator
66.1 US gallons.
| Measure | Value |
|---|---|
| Cubic feet | 8.84 ft³ |
| Cubic metres | 0.2503 m³ |
| US gallons | 66.13 |
| Litres | 250.3 |
| Imperial gallons | 55.07 |
| Weight if full of water 8.33 lb per US gallon | 550 lb |
Pipe volume uses the inside diameter, which is not the nominal size — 4 in schedule 40 PVC has a 4.026 in bore, and thicker-walled schedule 80 is narrower still. Nominal pipe size is a name, not a measurement. Water weighs 8.33 lb per US gallon, so the weight figure is worth reading before putting a tank on a deck, a shelf, or a vehicle: 100 gallons is over 830 lb. For a cylinder lying on its side, volume is emphatically not proportional to depth — the cross-section is widest at the middle, so a tank filled to a quarter of its diameter holds under 20% of its capacity, and half depth is exactly half volume. That calculation uses the circular-segment area, which is why a dipstick reading needs a chart rather than arithmetic.
Calculate the volume of a pipe or tank in gallons and litres — including partly filled horizontal tanks — plus the weight of water it holds.
How to use this calculator
- Choose pipe or tank.
- For a pipe, pick the nominal size — the real schedule 40 bore is shown — or type an exact inside diameter to override it.
- For a tank, pick the shape. Cylinder on its side has an optional fill depth for working out a partly full tank.
- Read off gallons, litres, and the weight of water, which is often the number that actually matters.
The formula
Cylinder = π × r² × length · Cone = π × r² × h ÷ 3 · Sphere = 4/3 × π × r³ · Box = L × W × H
Horizontal cylinder, partly full = [ r²·acos((r−h)/r) − (r−h)·√(2rh − h²) ] × length · 1 ft³ = 7.48 US gal
Worked example — 100 ft of 4 in pipe
- Inside diameter: 4.026 in (schedule 40), so radius 2.013 in
- Cross-section: π × 2.013² = 12.73 in²
- Volume: 12.73 × 1,200 in = 15,276 in³
- Gallons: 15,276 ÷ 231 = 66.1 US gal
- Weight of water: 551 lb
Use the nominal 4 inches instead of the real 4.026 and you get 65.3 gallons — about 1% out. On larger or thicker-walled pipe the gap widens considerably, which is why the bore matters.
Nominal pipe size is a name, not a measurement
A 4-inch pipe is not 4 inches anywhere. Schedule 40 PVC in that size has an outside diameter of 4.5 inches and an inside diameter of 4.026. Neither number is 4.
The designation is historical: it once approximated the bore of wrought iron pipe with the wall thicknesses of the day, and it stuck as a naming convention long after the dimensions drifted. What matters practically is that the outside diameter is fixed for a given nominal size so fittings interchange, and heavier schedules thicken the wall inward. Schedule 80 in the same nominal 4 inches has a 3.826 inch bore — about 10% less cross-section and correspondingly less volume and flow.
| Nominal | Sch 40 inside diameter | Gallons per 100 ft |
|---|---|---|
| 1/2 in | 0.602 in | 1.5 |
| 3/4 in | 0.804 in | 2.6 |
| 1 in | 1.029 in | 4.3 |
| 1-1/2 in | 1.590 in | 10.3 |
| 2 in | 2.047 in | 17.1 |
| 3 in | 3.042 in | 37.8 |
| 4 in | 4.026 in | 66.1 |
| 6 in | 6.065 in | 150.1 |
The horizontal tank problem
A cylinder standing upright is easy: half the height is half the volume. Lay the same cylinder on its side and that stops being true, because the cross-section is widest through the middle and narrows toward the top and bottom.
| Filled to | Percentage of capacity |
|---|---|
| 10% of diameter | 5.2% |
| 25% of diameter | 19.6% |
| 50% of diameter | 50.0% |
| 75% of diameter | 80.4% |
| 90% of diameter | 94.8% |
The relationship is symmetric about the middle — the empty space above a three-quarter-full tank exactly mirrors the liquid in a quarter-full one — but it is emphatically not linear. This is why fuel and water tanks come with a calibration chart rather than a proportional gauge, and why estimating a horizontal tank by eye consistently overestimates when it is low.
Water is heavier than it looks
A US gallon of water weighs about 8.33 pounds. That makes:
- A 50-gallon water heater about 420 lb when full
- A 100-gallon stock tank 833 lb
- A 275-gallon IBC tote nearly 2,300 lb — more than most vehicles should carry
- A modest 6 × 4 × 2 ft pond about 2,990 lb
Run this calculation before putting a tank on a deck, a mezzanine floor, a shelf, or a trailer. Water loads are one of the more common ways people accidentally exceed a structural limit, precisely because the volume looks modest.
What the number gets used for
- Chlorinating or dosing a system. Shock chlorination of a well or a new line needs the volume to get the concentration right.
- Flushing. Knowing the line holds 66 gallons tells you how much has to run before what comes out is what went in.
- Pool and pond chemistry, where every dose is per volume of water.
- Expansion tank sizing and heat loss in a recirculating hot water loop.
- Structural load, as above.
Note that this calculates volume, not flow. How much a pipe carries per minute depends on pressure, slope, and friction, not just its bore.
How we calculate this
Pipe and upright tank volume is π × radius² × length, from the inside diameter — nominal pipe size is a name, not a measurement, so the schedule 40 bore is used unless you enter an exact diameter. A cone is one third of its cylinder, a sphere is 4/3 × π × r³, and a rectangular tank is length × width × height. For a cylinder lying on its side and partly full, the liquid cross-section is a circular segment: r²·acos((r−h) ÷ r) − (r−h)·√(2rh − h²), multiplied by the tank length. Conversions are exact: a US gallon is 3.785411784 litres by definition, and water is taken at 997 kg/m³.
Sources
Frequently asked questions
How many gallons are in a 100 foot run of 4 inch pipe?
About 66 US gallons. Schedule 40 PVC in the 4-inch nominal size has an actual inside diameter of 4.026 inches, giving a cross-section of 12.73 square inches, which over 1,200 inches of length is 15,276 cubic inches — 66.1 gallons.
Why is the inside diameter different from the pipe size?
Because nominal pipe size is a name, not a measurement. A 4-inch schedule 40 pipe has a 4.026-inch bore and a 4.5-inch outside diameter — neither is 4 inches. The nominal designation dates back to when it approximated the inside diameter of iron pipe with the wall thicknesses of the era. Heavier schedules keep the same outside diameter and thicken the wall inward, so schedule 80 has a noticeably smaller bore than schedule 40 at the same nominal size.
How do I calculate the volume of a cylindrical tank?
For an upright tank, π times the radius squared times the height. A 4-foot diameter tank 6 feet tall has a 2-foot radius, so π × 4 × 6 = 75.4 cubic feet, or 564 gallons. For a tank lying on its side that is only true when it is completely full — a partly filled horizontal tank needs the circular segment formula.
How do I work out how full a horizontal tank is?
Not by proportion of depth, which is the intuitive but wrong approach. A cylinder on its side is widest at the middle, so the relationship between depth and volume is a curve. A tank filled to a quarter of its diameter holds under 20% of its capacity, half depth is exactly half volume, and three-quarters depth is over 80%. This calculator uses the circular segment area, which is why dipstick readings need a chart.
How much does a gallon of water weigh?
About 8.33 pounds for a US gallon at room temperature, so 100 gallons is 833 pounds. Worth calculating before putting a tank on a deck, a mezzanine, a shelf, or in a vehicle — water is much heavier than people intuitively expect, and a modest-looking tank can be a serious structural load.
How many gallons is a cubic foot?
7.48 US gallons, or 28.3 litres. Going the other way, a US gallon is 231 cubic inches exactly, and an imperial gallon is about 1.2 US gallons — a distinction that matters when reading specifications from different countries.
How do I calculate how much water is in my pool?
For a rectangular pool, length times width times average depth gives cubic feet; multiply by 7.48 for gallons. Use the tank mode here with the rectangular shape. If the pool has a shallow and a deep end, use the average of the two depths — that is accurate enough for dosing chemicals, which is usually why the number is wanted.
Why do I need to know pipe volume?
Most often for flushing, chlorinating, or dosing a system — you need to know how much water sits in the line to work out the chemical quantity or how long to run it before the line is cleared. It also matters for calculating how long it takes water to reach a fixture, sizing expansion tanks, and estimating heat loss in a recirculating hot water loop.