Pipe Volume Calculator
Find how many litres or gallons a pipe holds from its inside diameter and length, with the water weight, time to fill, flow speed and the weight of the pipe itself.
A nominal size is a name, not the inside size. Picking one only puts its inch figure in as a starting guess: check your pipe's actual inner diameter (printed on the pipe or on the maker's sheet).
For a pipe lying flat and only part full, like a drain. Depth from the bottom inside.
Time a 10 L bucket: 10 ÷ seconds × 60 = litres a minute.
Typical figures vary by grade and maker. Type the maker's figure if you have it. Wall thickness is needed for this.
How your pipe volume is worked out
The round inside of the pipe, times how long it is.
- 1
Inner diameter
d 50 mm = 0.05 mradius 0.025 m
- 2
Round area
π × 0.025²= 0.0019635 m²
- 3
Times the length
0.0019635 × 10 m= 0.019635 m³
- 4
Turn into litres
0.019635 × 1,000= 19.635 L
Read more: the formula, a worked example, partly full pipes, flow speed and the pipe wall
A pipe is a long cylinder, so its volume is the round area inside it times its length. Always use the inner diameter: the outer size includes the wall and gives too much. Work in metres, so the answer comes out in cubic metres (m³), then times 1,000 for litres.
The formula
inner diameter d = outer diameter − 2 × wall thickness (if you know the outside)
radius r = d ÷ 2 · all sizes in metres
inside area = π × r²
volume = π × r² × length · add up every run
litres = m³ × 1,000 · US gallons = litres ÷ 3.785411784 · UK gallons = litres ÷ 4.54609 · ft³ = m³ ÷ 0.0283168
water weight ≈ 1 kg per litre
part full, lying flat, water depth y = (r² × acos((r − y) ÷ r) − (r − y) × √(2 × r × y − y²)) × length
time to fill = litres ÷ litres a minute
flow speed (m/s) = flow (m³ a second) ÷ inside area (m²)
wall volume = π ÷ 4 × (outer diameter² − inner diameter²) × length · wall weight = wall volume × density
Worked example
Simple. A pipe 50 mm across inside and 10 m long. In metres the diameter is 0.05 m, so the radius is 0.05 ÷ 2 = 0.025 m. The inside area is π × 0.025² = π × 0.000625 = 0.0019635 m². Times the length, 0.0019635 × 10 = 0.019635 m³, which is 0.019635 × 1,000 = 19.635 litres (19.635 ÷ 3.785411784 = 5.187 US gallons, or 19.635 ÷ 4.54609 = 4.319 UK gallons). That is 1.9635 litres per metre of pipe, and full of water it weighs about 19.6 kg (43.3 lb).
Advanced. With a 3 mm wall, the outer diameter is 50 + 2 × 3 = 56 mm. The wall's area is π ÷ 4 × (0.056² − 0.05²) = π ÷ 4 × 0.000636 = 0.00049951 m², so 10 m of it is 0.0049951 m³ of material. At a typical PVC density of 1,400 kg/m³ that is 0.0049951 × 1,400 = 6.99 kg of pipe. At 20 litres a minute it fills in 19.635 ÷ 20 = 0.98 minutes, about 59 seconds, and the water moves at 20 ÷ 1,000 ÷ 60 = 0.00033333 m³ a second ÷ 0.0019635 m² = 0.17 m/s (0.56 ft/s). If it lay flat with water half way up (50% of the diameter), it would hold half: 9.817 litres.
Litres per metre for round inside sizes
| Inner diameter | Litres per metre | US gal per 100 ft |
|---|---|---|
| 15 mm | 0.1767 | 1.42 |
| 20 mm | 0.3142 | 2.53 |
| 25 mm | 0.4909 | 3.95 |
| 32 mm | 0.8042 | 6.48 |
| 40 mm | 1.2566 | 10.12 |
| 50 mm | 1.9635 | 15.81 |
| 75 mm | 4.4179 | 35.57 |
| 100 mm | 7.854 | 63.24 |
| 150 mm | 17.6715 | 142.29 |
| 200 mm | 31.4159 | 252.96 |
| 300 mm | 70.6858 | 569.16 |
| Inner diameter | US gal per foot | Litres per metre |
|---|---|---|
| 0.5 in | 0.0102 | 0.1267 |
| 0.75 in | 0.0229 | 0.285 |
| 1 in | 0.0408 | 0.5067 |
| 1.5 in | 0.0918 | 1.1401 |
| 2 in | 0.1632 | 2.0268 |
| 3 in | 0.3672 | 4.5604 |
| 4 in | 0.6528 | 8.1073 |
| 6 in | 1.4688 | 18.2415 |
| 8 in | 2.6112 | 32.4293 |
| 12 in | 5.8752 | 72.9659 |
These tables are pure geometry for a round inside of exactly that size. They are not the inside sizes of real pipes: a pipe sold as "1 inch" or "DN25" can be bigger or smaller inside, depending on its standard, schedule or pressure class and material. Check your pipe's actual inner diameter.
Tips
- To flush or fill a heating, solar or glycol loop, add every run of each size, then add the volume of the boiler, tank or radiators from their labels.
- A part-full drain is not a straight line: a quarter of the depth holds only about 19.6% of the water, while half the depth holds exactly half.
- Water in home pipes often moves at about 1–2 m/s. Much faster can be noisy; if the speed here is high, a wider pipe may suit better. Check your local plumbing rules for the limits.
- Water is about 1 kg a litre, so a full pipe weighs its litres in kg plus the pipe itself. That matters for long runs hung from brackets.
Common questions
How do I work out the volume of a pipe?
Measure the inside (inner) diameter and the length. Halve the diameter to get the radius, then work out π × radius × radius × length with every size in metres. That gives cubic metres; times 1,000 gives litres. A pipe 50 mm across inside and 10 m long holds π × 0.025 × 0.025 × 10 = 0.019635 m³, which is 19.635 litres.
How many gallons are in 100 feet of 1 inch pipe?
With an inner diameter of exactly 1 inch, the radius is 0.0127 m and 100 ft is 30.48 m, so the pipe holds π × 0.0127² × 30.48 = 0.015444 m³, which is 15.444 litres or about 4.08 US gallons (3.40 UK gallons). Real "1 inch" pipe often has an inner diameter a little different from 1 inch, so check yours.
Is a 1 inch pipe 1 inch across inside?
Not always. Nominal sizes such as ½ in, 1 in or DN25 are names, not the inner diameter. The real inside size depends on the standard, the schedule or pressure class and the material, so two "1 inch" pipes can hold different amounts. Measure the inside, or read it from the pipe or the maker's sheet.
How do I get the inner diameter from the outer diameter?
Take away the wall thickness twice: inner diameter = outer diameter − 2 × wall. A pipe 60 mm across outside with a 5 mm wall is 60 − 2 × 5 = 50 mm across inside.
How much water is in a pipe that is only part full?
For a pipe lying flat, measure the water depth y. With radius r, the water fills a slice of the circle with area r² × acos((r − y) ÷ r) − (r − y) × √(2 × r × y − y²). Times the length gives the volume. Half the depth is half the water, but a quarter of the depth holds only about 19.6% of the water.
How fast does water flow in my pipe?
Flow speed = flow rate ÷ the inside area. 20 litres a minute is 0.000333 m³ a second; in a 50 mm pipe with an area of 0.0019635 m², that is 0.000333 ÷ 0.0019635 = 0.17 m/s.
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