Pipe size calculator
Supply sizing is a pressure budget, not a diameter guess. What you have at the meter has to cover elevation, the meter and fitting losses, and still leave working pressure at the worst fixture. Enter the demand in water supply fixture units (WSFU), the developed length and the pressure you measured, and the calculator reads the trunk size off that budget.
Pre-filled with a two-bathroom house on a 55 psi municipal supply. Change any field.
Minimum trunk size
- Peak demand
- 20.0gpm
- Usable pressure
- 27.7psi
- Equivalent length
- 135ft
- Allowable friction
- 20.5psi / 100 ft
| Size | Capacity | Derated |
|---|---|---|
| 1/2" | 6.5 | 6.5 |
| 3/4" | 13.3 | 13.3 |
| 1" | 22.9 | 22.9 |
| 1-1/4" | 34.9 | 34.9 |
| 1-1/2" | 49.2 | 49.2 |
| 2" | 84.6 | 84.6 |
How it sizes
The IPC sizing method is a chain of subtractions. Start with the static pressure measured at a hose bibb. Take off 1 psi for every 2.31 ft the highest fixture sits above the meter. Take off the meter and backflow losses. Reserve the working pressure the worst fixture needs — 15 psi is the usual floor for a tank water closet, more for a pressure-balancing shower valve. What is left is the pressure you may spend on friction over the whole run.
Divide that by the equivalent length — the measured run plus the loss the fittings add, which on an ordinary house branch runs about half the straight length again — and you get an allowable friction rate in psi per 100 ft. Then read the demand across the table at that rate. The calculator derates the capacity column when the friction budget is thin, which is why a long run on low pressure jumps a size.
Measure, do not assume
Worked example
Two bathrooms, a kitchen, a laundry and two hose bibbs come to roughly 20 WSFU. The meter sits 90 ft of pipe from the far bathroom and the highest fixture is 10 ft up. At 55 psi:
| Step | Value |
|---|---|
| Static pressure at the bibb | 55.0 psi |
| Less elevation, 10 ft ÷ 2.31 | −4.3 psi |
| Less meter and fittings at the entry | −8.0 psi |
| Less working pressure kept at the fixture | −15.0 psi |
| Available for friction | 27.7 psi |
| Equivalent length, 90 ft × 1.5 | 135 ft |
| Allowable friction rate | 20.5 psi / 100 ft |
Twenty WSFU is about 20 gpm on the flush-tank curve. At that friction rate a 3/4" copper type L trunk carries it; PEX of the same nominal size does not, because its inside diameter is smaller. That is the single most common sizing mistake on a repipe — swapping material without re-reading the table.
Fixture unit values used
| Fixture | Cold | Hot | Total |
|---|---|---|---|
| Water closet, flush tank | 2.2 | — | 2.2 |
| Lavatory | 0.5 | 0.5 | 0.7 |
| Bathtub or tub/shower | 1.0 | 1.0 | 1.4 |
| Shower, separate | 1.0 | 1.0 | 1.4 |
| Kitchen sink | 1.0 | 1.0 | 1.4 |
| Dishwasher | — | 1.4 | 1.4 |
| Clothes washer | 1.0 | 1.0 | 1.4 |
| Hose bibb | 2.5 | — | 2.5 |
Hot and cold do not add to the total: a fixture cannot draw both at full rate at once, so the code assigns a combined value lower than the sum.
Code notes
IPC UPC Both codes publish sizing tables; they differ in the fixture unit values and in how the water service is treated. Your authority having jurisdiction (AHJ) decides which code edition applies and may have amended the table. Size to their adopted edition, then have the plan checked.
Code violation