Pipe Sizing Calculator
Select a line size by screening candidate NPS against service-specific velocity and pressure-drop criteria, with an API RP 14E erosional velocity check.
When to use this calculator
Use when selecting line size for a new run, or checking whether an existing line is within criteria. Rather than asking you to pick a velocity, it takes the service — pump suction, pump discharge, general process liquid, saturated or superheated vapour — and applies the velocity and ΔP/100 m limits that apply to that service, size range, and material. It screens every candidate NPS at once and returns a table of pass/fail results with a recommended size and the limit that governs it.
Required inputs
- Phase — liquid or vapour — and the design case
- Service and sub-service (e.g. pump suction, non-boiling; main header)
- Pipe material and schedule
- Flow rate, density, and viscosity
- Operating pressure — vapour service, since the criteria band by pressure class
Expected outputs
- A row per candidate NPS: inside diameter, velocity, ΔP/100 m, and pass/fail against each limit
- Recommended NPS with the governing limit (velocity, pressure drop, or erosion)
- Erosional velocity per API RP 14E
- The criterion ID applied, so the basis is traceable
Formula overview
Velocity in m/s, pressure drop in bar per 100 m, NPS in inches, density in kg/m³, operating pressure in barg. The C factor in the erosional velocity equation is dimensional — 100 for continuous service is the common value.
V = Q / A
ΔP/100 m = f · (100/D) · (ρV²/2)
V_e = 1.122 · C / √ρ API RP 14E erosional velocity
Each candidate NPS is then screened against the velocity band and the
ΔP/100 m limit tabulated for that service, size range, and material.Worked example
Water, pump discharge, 50 m³/h, ρ = 998 kg/m³, carbon steel Sch 40:
Candidate 4" Sch 40, ID = 102.3 mm:
A = π/4 × 0.1023² = 0.00822 m²
V = (50/3600) / 0.00822 = 1.69 m/s
criterion for pump discharge: 1.5–3.0 m/s → pass
Erosional check:
V_e = 1.122 × 100 / √998 = 3.55 m/s → 1.69 m/s is well below
3" would give 2.95 m/s — inside the band but close to the top, and its
ΔP/100 m is roughly four times higher. 4" is the recommended size.Common mistakes
- Using the same velocity for suction and discharge — suction should be lower to prevent cavitation.
- Forgetting to round up to the actual nominal size and not re-checking the actual velocity.
- Confusing inside diameter with nominal size (DN is not the true ID).
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