Pipe Sizing Calculator

Find the optimal pipe size for your flow conditions. Enter a flow rate and fluid, then set velocity or pressure drop limits to see which pipe sizes are suitable.

Fluid

Temperature

Density998.2 kg/m³

Viscosity1.002 cP

Pipe & Flow

Flow Rate

Material

Schedule

Pipe Length

Constraints

Sizing Criteria

Velocity Guideline

Max Velocity

Max Pressure Drop

ResultsRecommended: NPS 5"

Display:

Vel.

dP/L

NPS	DN	ID (mm)	Velocity (m/s)	dP (kPa)	dP/L (kPa/100m)	Re	Regime
1/8"	6	6.8	75.76	1391759.9	1391759.9	515,802	Turbulent
1/4"	8	9.2	41.37	280529.2	280529.2	381,183	Turbulent
3/8"	10	12.5	22.56	56680.9	56680.9	281,442	Turbulent
1/2"	15	15.8	14.17	16745.2	16745.2	223,072	Turbulent
3/4"	20	20.9	8.07	3864.3	3864.3	168,387	Turbulent
1"	25	26.6	4.98	1108.5	1108.5	132,270	Turbulent
1-1/4"	32	35.1	2.88	272.2	272.2	100,544	Turbulent
1-1/2"	40	40.9	2.11	124.5	124.5	86,181	Turbulent
2"	50	52.5	1.28	35.51	35.51	67,127	Turbulent
2-1/2"	65	62.7	0.899	14.68	14.68	56,197	Turbulent
3"	80	77.9	0.582	5.04	5.04	45,225	Turbulent
3-1/2"	90	90.1	0.435	2.47	2.47	39,107	Turbulent
4"	100	102.3	0.338	1.34	1.34	34,464	Turbulent
5" Best fit	125	128.2	0.215	0.448	0.448	27,492	Turbulent
6"	150	154.1	0.149	0.185	0.185	22,877	Turbulent
8"	200	202.7	0.086	0.050	0.050	17,385	Turbulent
10"	250	254.5	0.055	0.017	0.017	13,847	Turbulent
12"	300	303.2	0.038	0.00729	0.00729	11,623	Turbulent
14"	350	333.3	0.032	0.00465	0.00465	10,572	Turbulent
16"	400	381.0	0.024	0.00247	0.00247	9,250	Turbulent
18"	450	428.7	0.019	0.00141	0.00141	8,222	Turbulent
20"	500	477.8	0.015	0.000843	0.000843	7,376	Turbulent
24"	600	574.6	0.011	0.000353	0.000353	6,133	Turbulent

How it works: For each pipe size in the selected schedule, the calculator computes velocity and pressure drop using Darcy-Weisbach with Colebrook-White friction factor. The smallest pipe that satisfies your constraints is recommended. Pipe roughness is based on the selected material (0.0450 mm).

\frac{\Delta P}{L} = f \cdot \frac{1}{D} \cdot \frac{\rho V^2}{2}, \qquad f = \frac{64}{Re}\ (Re<2000),\quad \frac{1}{\sqrt f} = -2\log_{10}\!\left(\frac{\varepsilon/D}{3.71} + \frac{2.51}{Re\sqrt f}\right)\ (Re>4000)

$\Delta P / L$ — pressure drop per unit length (Pa/m; shown as kPa per 100 m or psi per 100 ft)
$f$ — Darcy friction factor — 64/Re in laminar flow, Colebrook-White in turbulent flow, interpolated between Re 2,000 and 4,000
$D$ — pipe internal diameter (m) — the ID column, from the selected schedule
$\rho$ — fluid density (kg/m³) at the operating temperature (and pressure, for gases)
$V$ — mean velocity (m/s) = Q / (π·D²/4)
$Re$ — Reynolds number = ρ·V·D/μ, with μ the dynamic viscosity
$\varepsilon$ — absolute roughness of the pipe wall (m), set by the material

Inputs and outputs explained

NPS, DN and ID. NPS is the nominal pipe size in inches and DN its metric label (NPS 4 = DN 100); neither is a real dimension. The hydraulics use the internal diameter (ID) column, which depends on the schedule — the table shows all three.
Schedule. Wall thickness class for the material (ASME B36.10M for carbon steel, B36.19M for stainless, plus the EN and copper families). A heavier schedule has a smaller bore for the same NPS, so the same flow runs faster and loses more pressure.
Material roughness. Absolute roughness ε set by the material (carbon steel 0.045 mm, stainless 0.015 mm, PVC 0.0015 mm…). It enters the Colebrook-White friction factor as ε/D; the value in use is shown under the table.
Sizing criteria. Max velocity, max pressure drop, or both. A size passes when it satisfies every selected criterion; the smallest passing size is recommended.
Velocity guideline. Presets for the max velocity: water general 2.5 m/s, pump suction 1.5, pump discharge 3.0, saturated steam 25, superheated steam 40, air/gas low pressure 15, high pressure 25, viscous oil 1.0 — practice bands from Crane TP-410 and piping handbooks, not code limits. Pick Custom to enter your own.
Max pressure drop (dP/L). The allowable friction loss per unit length, e.g. kPa per 100 m or psi per 100 ft, so the criterion is independent of the pipe length. dP in the table is that gradient times the entered length.
Best fit. The smallest size in the schedule that passes; larger passing sizes cost more but run slower and lose less. Sizes marked ✗ fail at least one criterion (the failing column is red).
Gas presets. Density is the ideal-gas value at the stated pressure and temperature, and flow can be entered at normal conditions (Nm³/h, 0 °C and 101.325 kPa); the sizing treats the gas as incompressible along the pipe, which is fine while the drop stays under ~10% of the absolute inlet pressure.

Worked example

Water at 20 °C (ρ = 998.2 kg/m³, μ = 1.002 mPa·s, i.e. cP — from the page's water correlation), 10 m³/h through 100 m of carbon steel Schedule 40 (ε = 0.045 mm), sizing on both criteria: ≤ 2.5 m/s and ≤ 1 kPa per 100 m — the calculator's defaults. Every size in the schedule is checked; the figures below are the three around the answer.

For each candidate bore: V = Q/(π·D²/4), Re = ρ·V·D/μ, f from Colebrook-White (or 64/Re if laminar), then dP/L = f·(1/D)·ρ·V²/2.
NPS 4 (DN 100, ID 102.3 mm): V = 0.338 m/s, dP/L = 1.338 kPa/100 m — fails at least one limit, so it is rejected.
NPS 5 (DN 125, ID 128.2 mm): V = 0.215 m/s, Re = 27,492, f = 0.0249, dP/L = 0.448 kPa/100 m — passes both, and it is the smallest size that does, so it is the recommendation (dP over 100 m: 0.45 kPa).
NPS 6 (DN 150) also passes — V = 0.149 m/s, dP/L = 0.185 kPa/100 m — with more margin, at the cost of a larger pipe.

These figures are computed by the same functions that fill the table above (and that the SimuPipe solver uses for every pipe), so entering the defaults reproduces them exactly.

Assumptions and limits

Straight pipe only: no fittings, valves, elevation change or equipment. Add fittings on the friction loss calculator, or build the line in the simulator, before trusting the total drop.
A hydraulic answer, not a mechanical one. The schedule is chosen for bore, not pressure rating — wall thickness for design pressure and temperature is a separate check (ASME B31.1 / B31.3 or EN 13480).
Velocity guidelines are practice bands for noise, erosion and water hammer, not code limits; pump suction lines are usually held lower to protect NPSH, and slurries or erosive fluids need a minimum velocity too.
Constant properties at the stated temperature; liquids incompressible, gases at the stated pressure with the ~10%-drop rule above. Two-phase and non-Newtonian flow are outside the method.
The friction factor uses f = 64/Re below Re 2,000, Colebrook-White above 4,000 and an interpolation between — sizes landing in that band are marked Transitional and carry the largest uncertainty.
The smallest passing size is the cheapest that meets the limits, not necessarily the most economical over the system's life; pumping energy usually favours the next size up on long runs.

The table and the SimuPipe solver share the same Reynolds, Colebrook-White and Darcy-Weisbach code. The solver's accuracy is documented case by case on the validation page (53 published cases, including Janna and Crane TP-410 pipe-friction problems).

References

None of these methods are ours — check them at the source.

Crane Co. Flow of Fluids Through Valves, Fittings, and Pipe, Technical Paper No. 410 — velocity practice bands, roughness values and the friction basis.
ASME B36.10M, Welded and Seamless Wrought Steel Pipe; ASME B36.19M, Stainless Steel Pipe — the schedule dimensions (OD, wall, bore) behind the size table.
Colebrook, C. F. (1939). "Turbulent flow in pipes…" Journal of the Institution of Civil Engineers, 11(4), 133–156. doi:10.1680/ijoti.1939.13150
Moody, L. F. (1944). "Friction factors for pipe flow." Transactions of the ASME, 66, 671–684.
Nayyar, M. L. (ed.). Piping Handbook, 7th ed., McGraw-Hill — velocity and pressure-drop sizing practice by service.
White, F. M. Fluid Mechanics, McGraw-Hill — Darcy-Weisbach and the friction-factor regimes.

How to Size a Pipe

Pipe sizing is the process of selecting the correct pipe diameter for a given flow rate, fluid, and set of constraints. The two most common criteria are:

Maximum velocity — keeping fluid velocity below recommended limits prevents erosion, noise, and water hammer. Typical limits are 1.5-3 m/s for water and 15-25 m/s for gases.
Maximum pressure drop — a pressure drop constraint (usually expressed as kPa per 100m) ensures the system can deliver the required pressure at the endpoint. Common limits range from 0.5-2 kPa/100m for gravity systems to 5+ kPa/100m for pumped systems.

This calculator uses the Darcy-Weisbach equation with the Colebrook-White implicit friction factor to compute pressure loss for each available pipe size in the selected schedule. The Reynolds number determines whether the flow is laminar or turbulent.

For full pipe network analysis with multiple branches, pumps, valves, and control devices, try SimuPipe — our browser-based pipe network simulation tool.

Pump suction lines are a special case — there the limit is not velocity or pressure drop but the available NPSH, and the term that usually decides it is the liquid's temperature rather than the pipe. See: NPSHa vs NPSHr and why pumps cavitate.

Frequently Asked Questions

How do I choose the right pipe size?

Pipe sizing is a balance between velocity, pressure drop, and cost. Higher velocities mean smaller (cheaper) pipes but more pressure drop and noise. A common approach is to set a maximum velocity limit (e.g. 2-3 m/s for water, 20-30 m/s for gas) or a maximum pressure drop per unit length, then find the smallest standard pipe size that meets the criteria.

What is a good flow velocity for water pipes?

For general industrial water service, 1.5-3 m/s is typical. Suction lines to pumps should be slower (0.5-1.5 m/s) to avoid cavitation. Fire water mains can run up to 4-5 m/s during emergencies. Higher velocities increase erosion risk and water hammer. For chilled water and HVAC systems, 1-2.5 m/s is common to limit noise and pump energy.

What velocity should I use for compressed air?

For compressed air headers, 6-10 m/s is recommended to limit pressure drop. Branch lines can run up to 15 m/s. Distribution mains in large plants should stay below 8 m/s. Higher velocities cause excessive pressure drop and can carry condensate moisture further into the system. Always check that the total pressure drop is within your system's allowance.

What is the maximum allowable pressure drop?

There is no single answer — it depends on the system. For process piping, a rule of thumb is 0.1-0.5 bar per 100 m of pipe. For compressed air, total system losses should stay under 10% of compressor discharge pressure. For pump suction lines, the available NPSH sets the limit. This calculator lets you evaluate pressure drop for each candidate pipe size to find the best fit.

Why does pipe schedule matter for sizing?

Pipe schedule determines wall thickness and therefore internal diameter. A thicker wall (higher schedule) means a smaller bore for the same nominal size, which increases velocity and pressure drop. For example, NPS 4 Schedule 40 has an ID of 102.3 mm, while Schedule 80 has 97.2 mm. This calculator uses pipe schedule data to compute the actual internal diameter for accurate sizing.

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