PEX Pipe Volume Calculator
Same volume in every unit
Fluid weight
Estimated fill / drain time
Your recent calculations (saved on this device)
| When | ID | Length | Result |
|---|
PEX is preloaded at 1/2". Pick your size โ the bore is well below the nominal figure.
How to use this calculator
- Choose the unit system โ US, Imperial or metric โ at the top of the calculator.
- Pick the pipe type and schedule, then the nominal size, and the real inside diameter loads for you. Or choose Custom and type any bore.
- Enter the pipe length. Open Advanced to set fill level, number of identical runs, a fittings allowance, the fluid, or a flow rate.
- Press Calculate to see the volume in every unit, plus fluid weight, cross-section area and fill or drain time.
- Copy, share or print the result. Recent calculations are saved on your own device so you can reopen them later.
Tip: not sure where to start? Use the Fill example data button to load a complete worked example โ a 2-inch Schedule 40 line, 50 ft long, at 10 gal/min โ then adjust the numbers to match your job.
Real-world uses, with numbers
Charging a radiant-heat loop
How much water sits in a 300 ft run of ยฝ" PEX.
โ 2.9 US galSizing a well pressure tank
Line-fill volume in 100 ft of 1" Schedule 40.
โ 4.5 US galFlushing a main
Water to displace in 100 ft of 4" Schedule 40.
โ 66 gal ยท 250 LWater weight on hangers
Load from a full 100 ft run of 2" pipe.
โ 145 lb ยท 66 kgMetric heating fill
Charge volume for 50 m of 28 mm bore.
โ 30.8 LFuel or oil line contents
Diesel held in 20 m of 50 mm bore.
โ 39 L ยท 33 kgMore questions, answered
How do I convert between gallons, litres and cubic metres?
1 US gallon = 3.785 litres; 1 Imperial gallon = 4.546 litres; 1 mยณ = 1,000 litres = 264.2 US gallons. The calculator shows all of them from a single entry, so you never convert by hand.
Does the pipe material change the volume?
Only through the bore. Two pipes with the same inside diameter hold the same volume whether they are steel, PVC, copper or PEX. Material and schedule matter because they set the real inside diameter for a given nominal size.
How much does the water inside a pipe weigh?
About 8.34 lb per US gallon, or 1 kg per litre. A full 100 ft run of 2" Schedule 40 holds roughly 17.4 gallons, about 145 lb. Set the fluid in Advanced for hot water, glycol, diesel or seawater.
What if I only know the outside diameter?
Switch the diameter to OD mode and enter the wall thickness; the tool subtracts two wall thicknesses to reach the bore. For a listed standard, just pick the size and the published inside diameter loads automatically.
Can I use it for a part-full or sloped drain?
Yes for a straight, round pipe: open Advanced and set the fill level below 100%, and the volume scales with it. Truly partial gravity flow in a sloped line follows a curved free surface, so treat the part-full figure as an approximation.
Notes and derived figures
- Volume scales with the square of the bore: doubling the inside diameter roughly quadruples the capacity, while doubling the length only doubles it.
- Per-foot and per-metre capacity are constant for a given bore, so any length is a single multiplication โ handy for quick field estimates.
- Nominal size is a label. A "2-inch" Schedule 40 pipe has a 2.067-inch bore; using "2" overstates the volume by more than a third.
- Fill and drain time is volume divided by flow rate. Enter a flow rate in Advanced to turn any result into a charge, flush or drain time.
Why PEX holds less than you expect
PEX tubing is measured by nominal size, but its wall is proportionally thick, so the bore is noticeably smaller than the label. A "1/2-inch" PEX tube has a bore of only about 0.485 in โ meaningfully less than 1/2-inch copper or CPVC. On a home-run system with long individual loops to each fixture, that small bore adds up to real differences in stored water and in how quickly hot water arrives.
| Nominal PEX | Inside diameter | US gal / 100 ft |
|---|---|---|
| 3/8" | 0.350" | 0.500 |
| 1/2" | 0.485" | 0.960 |
| 3/4" | 0.681" | 1.892 |
| 1" | 0.875" | 3.124 |
1/2-inch PEX volume, the common question
The most-searched PEX size is 1/2 inch. Each foot holds about 0.0096 US gallons, so a 50 ft home-run loop stores roughly 0.48 gallons โ a bit under two litres. That is the water that has to clear the line before hot water reaches the tap, which is why loop length is a comfort issue, not just a plumbing detail. Enter your own length above for the exact figure.
Where PEX volume matters
- Home-run manifolds โ total the loops to understand system water and hot-water wait.
- Radiant floor loops โ loop volume feeds glycol charge and balancing.
- Winterizing โ knowing line contents helps plan blow-out and antifreeze volumes.
- Pressure testing โ small volumes mean small make-up water reveals leaks quickly.
Totalling several PEX loops
Home-run systems have many loops. Rather than adding them one by one, calculate one representative loop and use the "number of identical runs" field to multiply, or run each length and sum the results โ the device history keeps your recent calculations so you can tally them up. For a radiant manifold, that total is your starting glycol volume.
Accuracy notes for PEX
The built-in figures are ASTM F876 SDR-9, which covers the vast majority of PEX-a, PEX-b and PEX-c tubing. A few specialty products differ slightly; if in doubt, measure the bore and use Custom mode. Note that PEX is sized differently from PEX-AL-PEX and from copper, so do not reuse copper bores for PEX.
Reading and trusting your result
A volume figure is only useful if you know what it assumes โ so a quick word on reading your pex pipe volume calculator result.
How to interpret the result
The headline value is the internal capacity of a round, uniform pipe running full โ the fluid the bore can hold along the length you entered. Every other unit on the panel is the same physical quantity re-expressed, so you can quote whichever your paperwork needs without recalculating.
The full-pipe assumption
By default the figure assumes the pipe is completely full and perfectly cylindrical. For gravity drains and vented lines that run part full, set a fill level below 100% and the result scales with it. Bends, valves and fittings add a little capacity the straight-length model does not see โ use the fittings allowance to approximate them.
Capacity reference tables & a worked example
Concrete numbers for this page, calculated the same way the tool above does โ use them as a quick check or a lookup.
PEX (SDR-9) 2" pipe: volume by run length
How much a 2" PEX (SDR-9) bore holds as the run gets longer, in US gal. The per-foot figure is constant, so any length is one multiplication away.
| Length (ft) | Volume (US gal) | Per foot |
|---|---|---|
| 10 ft | 1.09 | 0.1088 |
| 25 ft | 2.72 | 0.1088 |
| 50 ft | 5.44 | 0.1088 |
| 100 ft | 10.88 | 0.1088 |
| 250 ft | 27.20 | 0.1088 |
| 500 ft | 54.40 | 0.1088 |
PEX (SDR-9) pipe: volume across sizes
Volume held by a fixed 100 ft run across common PEX (SDR-9) sizes, in US gal. Doubling the diameter roughly quadruples the capacity.
| Nominal size | Bore (in) | Volume in 100 ft (US gal) |
|---|---|---|
| 3/8" | 0.360 | 0.5288 |
| 1/2" | 0.485 | 0.9597 |
| 3/4" | 0.681 | 1.89 |
| 1" | 0.862 | 3.03 |
| 1-1/4" | 1.055 | 4.54 |
| 1-1/2" | 1.245 | 6.32 |
| 2" | 1.633 | 10.88 |
Worked example, step by step
- Bore = 1.633 in, so radius = 0.8165 in.
- Area = π × radius² = 2.0944 in².
- Length = 75 ft = 900 in; volume = 2.0944 × 900 = 1,885.0 in³.
- Convert to the unit you need: 8.16 US gal for a 75 ft run of PEX (SDR-9) 2" pipe.
Frequently asked questions
What is the volume of 1/2 inch PEX per foot?
About 0.0096 US gallons (0.036 litres) per foot, based on the 0.485 in SDR-9 bore. A 100 ft coil holds roughly 0.96 US gallons. Enter your length above for an exact figure.
Why does PEX hold less water than copper of the same size?
PEX has a proportionally thicker wall, so its bore is smaller than same-nominal copper or CPVC. Less bore means less volume per foot.
How do I find total volume of a PEX home-run system?
Calculate one loop and use the number-of-runs field, or run each loop and add the results. The device saves recent calculations to help you tally them.
Does this cover PEX-a, PEX-b and PEX-c?
Yes โ all three are made to ASTM F876 SDR-9 dimensions, so the bore used here applies. Measure and use Custom mode for any non-standard tubing.
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What people say
PEX bore is smaller than people think and this nails it โ my manifold charge finally matched reality.
Loaded the 1/2" ASTM bore automatically. No more guessing off the nominal size.
Covers the sizes I use daily; a PEX-AL-PEX option would be the cherry on top.