PEX to buy = run length × (1 + waste). 120 ft of 1/2 inch PEX with 10% waste is 132 ft, which is one 300 ft coil, and the tubing holds 1.1 gallons of water. This calculator also counts crimp rings, hangers and how far the line grows when it heats up.
How the PEX quantity is calculated
Add up every hot and cold run, from the manifold or main to each fixture, and measure along the pipe rather than in a straight line.
tubing to buy = total length × (1 + waste %), rounded up
coils = tubing to buy ÷ coil length, rounded up
water in the line = length ÷ 100 × gallons per 100 ft for the size
PEX tubing is sold by the foot and in coils, and longer coils exist (up to 1,000 ft, according to PEX supplier Plastic Ranger). Pick a coil length in the calculator to match what your supplier stocks; the calculator does not assume one.
Worked example: 120 ft of 1/2 inch PEX
| Item | Calculation | Result |
|---|---|---|
| Tubing with waste | 120 ft × 1.10 | 132 ft |
| Coils of 300 ft | 132 ÷ 300, rounded up | 1 coil, 168 ft left over |
| Hangers | 1,440 in ÷ 32 in | 45 |
| Water in the line | 120 ÷ 100 × 0.92 gal | 1.10 gal |
| Crimp rings | 8 elbows × 2 + 3 tees × 3 | 25 |
| Growth at +50 °F | 1.2 × 5 × 1 to 2.5 in | 6 to 15 in |
PEX tubing sizes
Inside diameter and volume for common CTS sizes (PexUniverse technical data):
| Nominal size | Inside diameter | Water per 100 ft |
|---|---|---|
| 3/8 in | 0.360 in | 0.50 gal |
| 1/2 in | 0.485 in | 0.92 gal |
| 5/8 in | 0.584 in | 1.34 gal |
| 3/4 in | 0.681 in | 1.83 gal |
| 1 in | 0.875 in | 3.03 gal |
A 1/2 in line usually feeds up to 4 fixtures, 3/4 in up to 14, and 1 in up to 30 fixture units (Plastic Ranger). The water held in a line is the water you waste waiting for hot water to arrive: 100 ft of 3/4 in holds 1.83 gallons.
Hangers and support
Plastic Ranger's installation notes say to fasten the tubing every 32 inches along a joist and at least every 6 ft across beams. The calculator uses 32 in for the whole run, which errs on the side of more clips.
PEX expansion
PEX grows when it heats. PexUniverse gives typical elongation as 1 to 2.5 inches per 100 ft for each 10 °F of temperature rise; Plastic Ranger quotes 1 inch. The calculator shows the range for your run and rise: 120 ft heating from 70 to 120 °F is 6 to 15 in. Leave slack or use expansion loops on long hot runs so the pipe does not bow or pull on its fittings.
The next questions
- Pressure at the fixtures? The water pressure calculator converts head to psi.
- Drains next to the supply lines: the pipe slope calculator.
- Adding up several runs? Use the linear feet calculator.
- New heater at the end of the line? Size a tankless water heater and see the water heater replacement cost.
- See every heating, cooling and plumbing calculator.