Pipe Sizing Calculator
Steel pipe size by water flow rate — closed-loop hydronic systems
Based on the “Allowable Flow Rates for Closed System Piping” chart for standard-weight (Schedule 40) steel pipe. Each size carries a flow range held to a maximum pressure drop of 4 ft/100 ft and a maximum velocity of 10 ft/s. Enter the design flow and the calculator picks the smallest pipe whose capacity covers it; switch the toggles for metric units.
- Flow range
- 78 – 140 US gpm
- Pressure drop
- 1.50 – 4.00 ft/100ft
- Velocity at design flow
- 5.2 ft/s
Allowable flow rates — standard-weight steel pipe
velocity at design flow Q = 120 US gpm
| Pipe size | DN | Flow range (US gpm) | Pressure drop (ft/100ft) | Velocity (ft/s) |
|---|---|---|---|---|
| 1/2" | DN15 | 0 – 2 | 0.00 – 4.00 | 126.7 |
| 3/4" | DN20 | 3 – 4 | 2.50 – 4.00 | 72.2 |
| 1" | DN25 | 5 – 8 | 2.00 – 4.00 | 44.5 |
| 1-1/4" | DN32 | 8 – 16 | 1.25 – 4.00 | 25.7 |
| 1-1/2" | DN40 | 17 – 24 | 2.00 – 4.00 | 18.9 |
| 2" | DN50 | 25 – 48 | 1.25 – 4.00 | 11.5 |
| 2-1/2" | DN65 | 49 – 77 | 2.00 – 4.00 | 8.0 |
| 3"Pick | DN80 | 78 – 140 | 1.50 – 4.00 | 5.2 |
| 4" | DN100 | 141 – 280 | 1.25 – 4.00 | 3.0 |
| 5" | DN125 | 281 – 500 | 1.50 – 4.00 | 1.9 |
| 6" | DN150 | 501 – 800 | 1.75 – 4.00 | 1.3 |
| 8" | DN200 | 801 – 1700 | 1.00 – 4.00 | 0.8 |
| 10" | DN250 | 1701 – 2500 | 1.25 – 2.75 | 0.5 |
| 12" | DN300 | 2501 – 3600 | 1.25 – 2.25 | 0.3 |
| 14" | DN350 | 3601 – 4200 | 1.25 – 2.00 | 0.3 |
| 16" | DN400 | 4201 – 5500 | 1.00 – 1.75 | 0.2 |
| 18" | DN450 | 5501 – 7000 | 0.90 – 1.50 | 0.2 |
| 20" | DN500 | 7001 – 9000 | 0.80 – 1.25 | 0.1 |
| 24" | DN600 | 9001 – 13000 | 0.60 – 1.00 | 0.1 |
Capacities assume a maximum pressure drop of 4 ft/100 ft and a maximum velocity of 10 ft/s. Velocity at the design flow is computed from the Schedule 40 inside diameter; values above 10 ft/s are shown in red. A preliminary selection aid — verify against a full hydraulic calculation before design.
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