About this calculator
How much water a pipe run holds, useful for purging, antifreeze, system fill and working out how long a tap takes to run hot.
How it’s calculated
Volume = π × (inside diameter)² ÷ 4 × length. Purge time = volume ÷ tap flow rate.
Worked example
With these inputs:
- Pipe
- PEX ¾"
- Length of run
- 50 ft
- Tap flow rate (gpm)
- 1.5
the result is:
- Water in the pipe
- 0.92 gal
- Inside diameter
- 0.671"
- Per 100 ft
- 1.84 gal
- Time to purge (wait for hot water)
- 37 seconds
- Water weight
- 8 lb
Tips from the trade
- Long hot-water runs in large pipe waste water and time. Consider a smaller dedicated run or a recirculation loop.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and local building codes before ordering.