What the pipe volume tells you
Knowing how much water a pipe holds helps with more jobs than you might think: adding antifreeze or inhibitor to a hydronic heating loop, draining a line before winter, sizing an expansion tank, flushing a new well line or understanding why hot water takes so long to reach the kitchen. The calculator works from the inside diameter and the length, then gives the volume, the volume per foot, the weight of the fluid and the wait for hot water at your faucet’s flow rate.
The formulas
- Volume = π × (inside diameter ÷ 2)² × length
- Weight = volume × density (water: 8.34 lb per gallon)
- Wait for hot water = volume ÷ flow rate
1 cubic foot holds 7.48 gallons, and 1 gallon is 231 cubic inches.
Worked example
A bathroom sink is 50 ft of 3/4 in pipe away from the water heater, and the faucet runs at 1.5 gallons per minute.
- Inside radius: 0.375 in; cross-section: π × 0.375² = 0.442 sq in.
- Volume: 0.442 × 600 in = 265 cubic inches, or 1.15 gallons (0.023 gallon per foot).
- That water weighs about 9.6 lb.
- Wait for hot water: 1.15 ÷ 1.5 = 0.77 minute, about 46 seconds, and more in winter when the pipe is cold.
Gallons per foot by pipe size
Based on the nominal size as inside diameter:
| Inside diameter | Gallons per foot | Gallons per 100 ft |
|---|---|---|
| 1/2 in | 0.010 | 1.0 |
| 3/4 in | 0.023 | 2.3 |
| 1 in | 0.041 | 4.1 |
| 1 1/2 in | 0.092 | 9.2 |
| 2 in | 0.163 | 16.3 |
| 4 in | 0.653 | 65.3 |
Use the real inside diameter
Nominal pipe sizes are labels, not measurements. A 3/4 in type L copper tube has an inside diameter of about 0.785 in, type M a little more, and 3/4 in PEX only about 0.68 in. Schedule 40 steel and PVC have their own inside diameters too. Check the manufacturer’s table for accurate numbers, especially on long runs.
Cutting the hot water wait
- Insulate hot water pipes: the water stays warm longer between uses.
- Smaller pipes to distant fixtures hold less water, as long as they still deliver enough flow.
- A recirculation pump keeps hot water near the fixtures, ideally on a timer or a demand button to limit heat loss.
- A point-of-use heater under a far sink removes the wait entirely.
The calculator also handles a 30% glycol mix and heating oil, which are slightly heavier and lighter than water.