Pipe sizing calculator
Enter the water flow and a friction rate or velocity to get the required bore, the recommended nominal size and its actual velocity and friction loss. You can also pick a size and check it. The equations with numbers filled in are under Calculation steps; size tables, constants and sources are under Basis further down.
Basis
Friction loss comes from the Darcy-Weisbach equation with the Colebrook friction factor, or from the Hazen-Williams formula. Sizing by friction rate or velocity gives a required bore; the smallest size in the table with at least that bore is chosen and recalculated with its actual bore.
| Item | Equation | Notes · basis |
|---|---|---|
| Friction loss (Darcy-Weisbach) | R = Δp/L = (f / D) · ρV²/2 | R in Pa/m, D the bore in m, ρ in kg/m³, V in m/s. The general equation for any fluid and temperature |
| Friction factor (Colebrook) | 1/√f = −2 log₁₀( ε/(3.7 D) + 2.51/(Re √f) ) | Re ≥ 2300. Same solver as the duct calculator (iterated from the Haaland estimate). The 2300 to 4000 transition range also uses it, which errs on the high side |
| Friction factor (laminar) | f = 64 / Re | Re < 2300 (Hagen-Poiseuille); flagged as laminar in the result |
| Hazen-Williams (SI) | h/L = 10.67 Q^1.852 / (C^1.852 D^4.8704), R = ρ g h/L | Q in m³/s, D in m, h the head loss in m. An empirical fit for turbulent water near room temperature: it ignores viscosity (temperature) and laminar flow. Common for domestic water and fire protection |
| Reynolds number, velocity | Re = V·D / ν, V = Q / (πD²/4) | ν = μ/ρ. Sizing by velocity uses D = √(4Q / (πV)) |
| Choosing the size | bore = outside diameter − 2 × wall ≥ D | The smallest size meeting this is recommended and recalculated with its actual bore. The next size up is shown too |
| Pressure units | 1 kPa/100m = 10 Pa/m, 1 mmAq = 9.80665 Pa | The same convention as the unit converter. 300 Pa/m = 30 kPa/100m = 30.6 mmAq/m |
Pipe size tables
Dimensions in mm; bore = outside diameter − 2 × wall. Only sizes whose dimensions could be checked against a source are listed. Check the standard or the maker's table before ordering.
Carbon steel SGP (JIS G 3452)
Outside diameter and wall of JIS G 3452 SGP carbon steel pipe. Korea's KS D 3507 (SPP) uses the same nominal sizes, but this table was checked against JIS values.
| Size | Outside dia. mm | Wall mm | Bore mm |
|---|---|---|---|
| 15A | 21.7 | 2.8 | 16.1 |
| 20A | 27.2 | 2.8 | 21.6 |
| 25A | 34.0 | 3.2 | 27.6 |
| 32A | 42.7 | 3.5 | 35.7 |
| 40A | 48.6 | 3.5 | 41.6 |
| 50A | 60.5 | 3.8 | 52.9 |
| 65A | 76.3 | 4.2 | 67.9 |
| 80A | 89.1 | 4.2 | 80.7 |
| 90A | 101.6 | 4.2 | 93.2 |
| 100A | 114.3 | 4.5 | 105.3 |
| 125A | 139.8 | 4.5 | 130.8 |
| 150A | 165.2 | 5.0 | 155.2 |
| 175A | 190.7 | 5.3 | 180.1 |
| 200A | 216.3 | 5.8 | 204.7 |
| 225A | 241.8 | 6.2 | 229.4 |
| 250A | 267.4 | 6.6 | 254.2 |
| 300A | 318.5 | 6.9 | 304.7 |
Copper type L (ASTM B88)
ASTM B88 Type L copper tube. Inch dimensions (0.625 in outside diameter and so on) × 25.4, with the Korean nominal name (1/2 = 15A). The KS D 5301 tables themselves were not checked.
| Size | Outside dia. mm | Wall mm | Bore mm |
|---|---|---|---|
| 15A (1/2) | 15.88 | 1.02 | 13.84 |
| 20A (3/4) | 22.23 | 1.14 | 19.94 |
| 25A (1) | 28.58 | 1.27 | 26.04 |
| 32A (1-1/4) | 34.92 | 1.4 | 32.13 |
| 40A (1-1/2) | 41.28 | 1.52 | 38.23 |
| 50A (2) | 53.98 | 1.78 | 50.42 |
| 65A (2-1/2) | 66.68 | 2.03 | 62.61 |
| 80A (3) | 79.38 | 2.29 | 74.8 |
| 100A (4) | 104.78 | 2.79 | 99.19 |
Copper type M (ASTM B88)
ASTM B88 Type M copper tube (same outside diameter as type L, thinner wall).
| Size | Outside dia. mm | Wall mm | Bore mm |
|---|---|---|---|
| 15A (1/2) | 15.88 | 0.71 | 14.45 |
| 20A (3/4) | 22.23 | 0.81 | 20.6 |
| 25A (1) | 28.58 | 0.89 | 26.8 |
| 32A (1-1/4) | 34.92 | 1.07 | 32.79 |
| 40A (1-1/2) | 41.28 | 1.25 | 38.78 |
| 50A (2) | 53.98 | 1.47 | 51.03 |
| 65A (2-1/2) | 66.68 | 1.65 | 63.37 |
| 80A (3) | 79.38 | 1.83 | 75.72 |
| 100A (4) | 104.78 | 2.41 | 99.95 |
Stainless Sch 10S (JIS G 3459)
JIS G 3459 stainless steel pipe, Sch 10S (the same series as KS D 3576). 90A is left out because it could not be checked.
| Size | Outside dia. mm | Wall mm | Bore mm |
|---|---|---|---|
| 15A | 21.7 | 2.1 | 17.5 |
| 20A | 27.2 | 2.1 | 23.0 |
| 25A | 34.0 | 2.8 | 28.4 |
| 32A | 42.7 | 2.8 | 37.1 |
| 40A | 48.6 | 2.8 | 43.0 |
| 50A | 60.5 | 2.8 | 54.9 |
| 65A | 76.3 | 3.0 | 70.3 |
| 80A | 89.1 | 3.0 | 83.1 |
| 100A | 114.3 | 3.0 | 108.3 |
| 125A | 139.8 | 3.4 | 133.0 |
| 150A | 165.2 | 3.4 | 158.4 |
| 200A | 216.3 | 4.0 | 208.3 |
| 250A | 267.4 | 4.0 | 259.4 |
| 300A | 318.5 | 4.5 | 309.5 |
Roughness ε (Darcy-Weisbach)
Material values from Moody's 1944 friction factor chart, converted to mm. Roughness grows with age and water quality, so enter other values with Custom.
| Material | ε (mm) | Moody's value |
|---|---|---|
| Commercial steel | 0.046 | 0.00015 ft |
| Galvanized steel | 0.15 | 0.0005 ft |
| Drawn tubing (copper, brass) | 0.0015 | 0.000005 ft |
Coefficient C (Hazen-Williams)
The ASHRAE Handbook Fundamentals Pipe Sizing chapter gives 150 for plastic and copper, 140 for new steel, and 100 or less for badly corroded or very rough pipe. NFPA 13 (sprinklers) sets 120 for wet-pipe steel and 150 for copper, stainless steel and CPVC. A smaller C gives a larger loss.
| Pipe | C | Source |
|---|---|---|
| Steel 120 (NFPA 13) | 120 | NFPA 13 (wet-pipe steel) |
| New steel 140 (ASHRAE) | 140 | ASHRAE (new steel) |
| Corroded steel 100 (ASHRAE) | 100 | ASHRAE (corroded or rough), NFPA 13 (dry-pipe steel) |
| Copper, stainless, plastic 150 | 150 | ASHRAE (plastic, copper), NFPA 13 (copper, stainless, CPVC) |
Water properties (101.325 kPa)
Density from IAPWS-95 and viscosity from the IAPWS 2008 formulation. The calculator interpolates a 5 °C table (5 to 95 °C) over the four nearest rows; the error between rows is within 0.02 %.
| Temperature °C | Density ρ kg/m³ | Viscosity μ mPa·s | Kinematic viscosity ν m²/s |
|---|---|---|---|
| 5 | 999.967 | 1.5182 | 1.518 × 10⁻⁶ |
| 10 | 999.702 | 1.3059 | 1.306 × 10⁻⁶ |
| 15 | 999.103 | 1.1376 | 1.139 × 10⁻⁶ |
| 20 | 998.207 | 1.0016 | 1.003 × 10⁻⁶ |
| 30 | 995.649 | 0.7972 | 8.007 × 10⁻⁷ |
| 40 | 992.216 | 0.6527 | 6.579 × 10⁻⁷ |
| 50 | 988.035 | 0.5465 | 5.531 × 10⁻⁷ |
| 60 | 983.196 | 0.4660 | 4.740 × 10⁻⁷ |
| 70 | 977.765 | 0.4036 | 4.127 × 10⁻⁷ |
| 80 | 971.790 | 0.3540 | 3.643 × 10⁻⁷ |
| 90 | 965.310 | 0.3142 | 3.255 × 10⁻⁷ |
| 95 | 961.888 | 0.2971 | 3.089 × 10⁻⁷ |
Choosing velocity and friction rate
The ASHRAE Handbook Fundamentals Pipe Sizing chapter describes common HVAC practice as 1.2 m/s or less for pipe of 50 mm and smaller and 400 Pa/m or less for larger pipe. Some references (Carrier 1960) add never more than 4.6 m/s. The calculator checks the recommended size and the next size up against these.
Noise and erosion limits depend on the pipe material, yearly operating hours and air or particles in the water. See that chapter and the maker's data for numbers.
The default 300 Pa/m is an example inside 400 Pa/m. Go lower to cut pump head, higher to cut pipe size.
Worked example
Flow 100 L/min, water at 20 °C, SGP steel (ε 0.046 mm), friction rate 300 Pa/m, Darcy-Weisbach: Q = 1.667 × 10⁻³ m³/s, required bore 44.87 mm → recommended 50A (52.9 mm bore): V = 0.758 m/s, Re = 39,979, f = 0.0245, R = 132.8 Pa/m (13.3 kPa/100m, 13.5 mmAq/m). Next size up, 65A: V = 0.460 m/s, R = 39.0 Pa/m.
With Hazen-Williams and C = 120 the required bore is 47.30 mm and 50A gives R = 173.9 Pa/m, about 30 % more than Darcy for the same pipe. The result depends strongly on the C chosen.
Sources
- ASHRAE Handbook, Fundamentals (SI), Pipe Sizing: Darcy-Weisbach and Colebrook, typical Hazen-Williams C, velocity and friction design practice
- Moody, L. F. (1944), Friction factors for pipe flow, Transactions of the ASME 66: material roughness
- Colebrook, C. F. (1939), Turbulent flow in pipes, Journal of the Institution of Civil Engineers 11
- NFPA 13, Standard for the Installation of Sprinkler Systems: Hazen-Williams C table
- IAPWS R6-95 (water equation of state, density), IAPWS R12-08 (water viscosity)
- JIS G 3452 carbon steel pipes for ordinary piping, JIS G 3459 stainless steel pipes, ASTM B88 seamless copper water tube
Last updated 2026-10-07.