Find the internal volume of a pipe from its inner diameter and length — and the mass of the liquid it holds. Enter the dimensions in any units, set the liquid density (water by default), and the calculator returns the volume in litres, cubic metres, gallons, and more.
The pipe volume calculator works out how much liquid a pipe can hold, based on its inner diameter and length, and tells you the mass of that liquid too. A pipe is simply a hollow cylinder, so its internal volume is found with the classic cylinder-volume formula. Enter the inner diameter and the length in whatever units you like — millimetres, centimetres, metres, inches, or feet — and the calculator instantly returns the volume in litres, cubic metres, US gallons, cubic feet, and cubic inches, plus the weight of the liquid inside for water or any other fluid you specify.
Knowing a pipe's volume matters for plumbers sizing a system, engineers estimating flow and capacity, pool and aquarium owners, irrigation planners, and anyone filling or draining a length of pipe. This page explains the formula, walks through worked examples you can reproduce, covers units and liquid mass, and answers the questions people ask most about pipe volume.
A pipe is a cylinder, and the volume of a cylinder is the area of its circular cross-section multiplied by its length. The cross-section is a circle whose radius is half the inner diameter, so the formula is:
pipe volume = π × radius² × length = π × (inner diameter / 2)² × length
The key word is inner diameter. Liquid fills only the hollow interior of the pipe, so you must use the internal diameter, not the outer diameter that includes the wall thickness. If you only know the outer diameter, subtract twice the wall thickness to get the inner diameter before calculating. Once you have the volume in a base unit such as cubic metres, it can be converted to litres, gallons, or any other volume unit.
Once you know the volume, finding the mass of the liquid inside is straightforward. Mass equals volume multiplied by density:
liquid mass = volume × liquid density
Density is how much a given volume of a substance weighs. The calculator defaults to the density of water, 997 kg/m³ at room temperature (about 20 °C). You can change it to model other liquids — seawater is around 1,025 kg/m³, milk about 1,030, olive oil roughly 918, and gasoline near 750. Multiplying the internal volume by the chosen density gives the weight of the fluid the pipe contains, which is useful for load, pressure, and support calculations.
Suppose you have a pipe with an inner diameter of 15 cm and a length of 6 m, filled with water. First convert to consistent units: the radius is 7.5 cm = 0.075 m, and the length is 6 m. Then apply the formula:
volume = π × (0.075 m)² × 6 m = π × 0.005625 × 6 ≈ 0.106 m³
That is about 0.106 m³, or 106 litres. To find the mass of the water, multiply by the density of water:
liquid mass = 0.106 m³ × 997 kg/m³ ≈ 105.71 kg
So this pipe holds roughly 106 litres of water weighing about 105.7 kg. These are the calculator's default values, so you can press Calculate to see the full breakdown and then enter your own numbers.
In imperial units, take a pipe with an inner diameter of 2 inches and a length of 6 feet. The radius is 1 inch, which is 1/12 of a foot. Working in feet:
volume = π × (1/12 ft)² × 6 ft ≈ 0.13 ft³ ≈ 0.979 US gallons
So a 2-inch pipe that is 6 feet long holds about 0.13 cubic feet, which is roughly 0.98 US gallons — just under a gallon. The calculator handles the unit conversions for you, so you can enter inches and feet directly and read the answer in gallons.
Pipe volume can be expressed in many units, and this calculator shows the most common ones side by side so you don't have to convert by hand. In the metric system, litres (L) and cubic metres (m³) are typical — one cubic metre equals 1,000 litres. In the imperial and US systems, cubic feet (ft³), cubic inches (in³), and US gallons are common, with one cubic foot holding about 7.48 US gallons. For plumbing and everyday tasks, litres and gallons are the friendliest units, while engineers often prefer cubic metres or cubic feet. Whatever unit you enter the dimensions in, the calculator computes an exact volume and presents it in all of these forms at once.
Getting the diameter right is the most important step, and it is where mistakes most often happen. Pipes are usually described by a "nominal" size or by their outer diameter, but the liquid only fills the hollow inside, whose width is the inner (internal) diameter. The difference is twice the wall thickness: inner diameter = outer diameter − 2 × wall thickness. For a thin-walled pipe the gap is small, but for thick-walled or high-pressure pipe it can be significant, and using the outer diameter would overstate the volume. If a manufacturer lists a nominal size, check the specification sheet for the actual internal bore, because nominal sizes rarely match the true internal measurement exactly. Always feed the calculator the real inner diameter for an accurate result.
Calculating pipe volume has many practical uses. Plumbers and heating engineers need it to size systems, work out how much water a network holds, and estimate how long it takes to fill or drain. In central heating, the total system volume determines the size of the expansion vessel and the amount of inhibitor or antifreeze to add. Pool and aquarium owners use pipe and plumbing volume when calculating turnover rates and dosing chemicals. Irrigation designers estimate the water held in supply lines. In industry, pipe volume feeds into flow-rate, residence-time, and pressure calculations, and knowing the mass of liquid a pipe holds is important for supporting long horizontal runs, which can be surprisingly heavy when full. Even for a simple DIY job like flushing a garden hose or a solar line, knowing the volume tells you how much water you need to push through.
Pipe volume is closely tied to flow rate and the time it takes fluid to move through a pipe. If you know the volume of a length of pipe and the flow rate (volume per unit time), you can find the residence time — how long it takes a given parcel of liquid to travel that length — by dividing volume by flow rate. Conversely, the time to fill an empty pipe equals its volume divided by the fill rate. For example, a pipe holding 106 litres, filled at 20 litres per minute, takes about 5.3 minutes to fill. While this calculator focuses on the static volume, that figure is the starting point for these dynamic calculations, which matter in plumbing, process engineering, and water treatment where contact and residence times affect performance.
It can help to have a feel for how much common pipes hold per unit length, since volume grows with the square of the diameter. A pipe with a 1 cm inner diameter holds only about 0.0785 litres per metre, while a 5 cm pipe holds about 1.96 litres per metre — twenty-five times as much, because five squared is twenty-five. This square relationship is why a modest increase in diameter dramatically increases capacity, and why large water mains hold enormous volumes. When comparing pipes, remember that doubling the inner diameter quadruples the volume for the same length. The calculator makes these comparisons easy: just change the inner diameter and keep the length fixed to see how quickly the volume climbs.
Because pipe volume is the amount of liquid a pipe holds, it directly answers practical timing questions. To find how long an empty pipe takes to fill, divide its volume by the fill rate: a 106-litre pipe filled at 25 litres per minute takes about 4.2 minutes. The same logic applies to draining. This is genuinely useful when priming a pump, bleeding a heating circuit, flushing a new water line before use, or purging air from an irrigation system. If you are dosing a closed system with a chemical — antifreeze in a heating loop, chlorine in a pool return line, or a cleaning solution in process pipework — the pipe volume tells you how much product the pipe alone contains, which you add to the volume of tanks and vessels for the total system charge. Having the volume in both litres and gallons, as this calculator provides, means you can match whatever units your pump, meter, or product label uses without extra conversion.
This calculator models a pipe as an ideal straight cylinder, which is an excellent approximation for the vast majority of runs. In the real world there are a few refinements to keep in mind for exacting work. Bends, elbows, tees, and valves each hold a small amount of extra volume that a straight-pipe formula ignores; for a long simple run these are negligible, but in a compact, fitting-heavy assembly they can add up. Internal deposits such as scale, rust, or biofilm gradually reduce the effective bore over a pipe's life, so an old pipe may hold slightly less than its nominal size suggests. Flexible hose can bulge under pressure, changing its internal diameter. And corrugated or ribbed pipe has an internal profile that is not a smooth circle. For everyday plumbing, DIY, and estimation these effects are minor, but for precise engineering you should measure the true internal geometry and add fitting volumes separately, using this calculator for each straight section.
Treat the pipe as a cylinder: volume = π × (inner diameter / 2)² × length. Use the inner diameter, since the liquid fills only the inside of the pipe.
About 0.13 cubic feet, or roughly 0.979 US gallons. Using the formula: π × (1 inch)² × 72 inches ≈ 226 cubic inches ≈ 0.13 ft³.
Always the inner (internal) diameter. Liquid fills only the hollow bore, so the outer diameter would overstate the volume. Inner diameter = outer diameter − 2 × wall thickness.
Multiply the volume by the density of water (about 997 kg/m³, or 1 kg per litre). For example, 106 litres of water weighs about 105.7 kg.
It depends on the inner diameter. A 1 cm bore holds about 0.0785 L/m, a 2 cm bore about 0.314 L/m, and a 5 cm bore about 1.96 L/m — volume grows with the square of the diameter.
The volume is the same for any contents. For mass, enter the density of your fluid — the calculator defaults to water but accepts any density, so it works for oil, seawater, fuel, and more.
This Pipe Volume Calculator is provided for general informational and educational purposes. It models a pipe as a perfect cylinder using the inner diameter you enter and does not account for fittings, joints, or internal deposits. For critical engineering work, verify results against the pipe's specification and professional standards.