Flow calculators

Flow unit conversion, DP-to-flow, orifice sizing and gas flow correction.

Flow Unit Converter

m³/h, L/min, GPM, BBL/d, kg/h, Nm³/h, SCFM in one place.

Pipe Velocity Calculator

Velocity from flow rate and pipe ID — with recommended velocity limits.

Mass ↔ Volumetric Flow Converter

kg/h ↔ m³/h with density — edit any of the three.

Actual ↔ Standard Gas Flow Converter (Am³/h ↔ Nm³/h, ACFM ↔ SCFM)

Gas-law correction between operating and reference conditions.

Reynolds Number Calculator — Laminar or Turbulent Flow

Laminar or turbulent? Re from velocity, diameter, density and viscosity.

Pipe Schedule & Dimensions Lookup (ASME B36.10M)

NPS + schedule → OD, wall, ID, weight and internal area.

Orifice Plate Sizing Calculator — Bore & Beta Ratio (ISO 5167)

Bore and beta ratio from flow and differential pressure.

Pipe Pressure Drop Calculator (Darcy-Weisbach & Colebrook)

Friction loss per 100 m with the real friction factor, not a chart.

Flow Meter K-Factor Calculator (Pulses to Flow Rate)

K-factor, pulse frequency, flow rate and meter factor in one place.

About flow calculations

Flow measurement is the most technology-diverse category in instrumentation, and the one where the underlying physics most often surprises people. Differential-pressure elements produce a signal proportional to the square of flow. Gases carry no meaningful volumetric number without stated reference conditions. Meters that work beautifully in turbulent flow can fail entirely in laminar flow.

These tools cover the calculations that decide whether a flow measurement is trustworthy: the true pipe bore from the schedule, velocity and Reynolds number from that bore, orifice plate sizing to ISO 5167, correction between actual and standard gas flow, and conversion between mass and volumetric bases.

A recurring theme runs through all of them: nominal is not actual. Nominal pipe size is a label rather than a dimension, and volumetric gas flow is meaningless without conditions attached. Sizing calculations that start from nominal values inherit errors that no amount of instrument accuracy can recover.

How these tools fit together

  1. Get the real internal diameter. Look up the actual bore from nominal size and schedule. Using nominal diameter shifts velocity and Reynolds number by several percent before you start.
  2. Check the flow regime. Calculate velocity and Reynolds number at both minimum and maximum flow. Most inferential meters require turbulent flow to obey their calibration.
  3. Size the element. For DP flow, size the orifice plate to a beta ratio between 0.4 and 0.6 where accuracy and permanent pressure loss balance well.
  4. Fix the reference conditions. For gas, state whether flows are actual or standard and at which reference — a difference of several percent hides in this question.

Frequently asked questions

What is a reasonable liquid velocity in a pipe?

Commonly 1 to 3 m/s for pumped liquid lines. Below that, solids may settle; above it, pressure drop and erosion rise sharply. Gas lines run much faster, typically 15 to 30 m/s.

Why does DP flow measurement need square root extraction?

Differential pressure across a restriction rises with the square of flow rate, so recovering flow requires taking the square root. It must be applied exactly once in the signal chain — either in the transmitter or in the control system, never in both.

All calculators are provided for reference and education. Verify independently before use in safety-critical work — see our disclaimer.