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Flow Measurement Formulas Cheat Sheet
The key flow measurement formulas and rules of thumb on one page: pipe velocity, Reynolds number, the orifice equation, gas standard flow, meter selection rules and conversions.
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A one page A4 sheet to print, pin up in the workshop or keep on your phone. Everything on it is also shown below.
Download PDF A4 · 86 KB Basic relations Volume flow Q = A × v Pipe area A = π × D² ÷ 4 Mass flow qm = ρ × Q Velocity v (m/s) = Q (m³/h) ÷ (2827 × D² (m))
Reynolds number Definition Re = ρ × v × D ÷ μ = v × D ÷ ν Laminar Re < 2300 Transitional 2300 to 4000 Turbulent Re > 4000 (most meters need this)
Orifice plate (ISO 5167-2) Mass flow qm = C ÷ √(1 − β⁴) × ε × (π ÷ 4) × d² × √(2 × ΔP × ρ) Beta ratio β = d ÷ D, usually 0.2 to 0.75 Square law Q ∝ √ΔP, so 50 % flow = 25 % DP Typical C about 0.6 for a sharp edged orifice Permanent pressure loss about (1 − β1.9 ) × ΔP, roughly 40 to 95 % of the DP
Gas: actual to standard flow Ideal gas Qstd = Qact × (P ÷ Pstd ) × (Tstd ÷ T) × (Zstd ÷ Z) Rule Use absolute pressure (bar a) and absolute temperature (K) Normal conditions Nm³: 0 °C, 1.01325 bar a Standard (often) Sm³: 15 °C, 1.01325 bar a (check your contract)
Meter selection rules Meter Key rule Magnetic Conductive liquid, typically ≥ 5 µS/cm; full pipe Coriolis Mass flow and density direct; check pressure drop Vortex Needs Re above about 10,000 to 20,000; no low flow Ultrasonic Clean liquids or gas; long straight runs Turbine Clean, low viscosity fluids; K factor in pulses per litre DP (orifice) Turndown about 3:1 to 4:1 per DP range
Straight pipe runs (guide) Typical: 10 D upstream and 5 D downstream Orifice after two bends out of plane: up to 30 to 40 D upstream (ISO 5167-2 tables) A flow conditioner shortens the requirement Always use the meter data sheet for the final value Velocity guidelines Service Typical velocity Water, pump discharge 1.5 to 3 m/s Water, pump suction 0.6 to 1.5 m/s Steam (saturated) 20 to 35 m/s Compressed air 6 to 10 m/s Magmeter best accuracy 1 to 3 m/s
Conversions From To 1 m³/h 16.667 L/min 1 m³/h 4.4029 US gpm 1 US gallon 3.7854 L 1 kg/s 3.6 t/h 1 ft³/min (CFM) 1.699 m³/h
K factor and totals Flow from pulses Q = f ÷ K Total V = pulse count ÷ K K factor pulses per unit volume (for example per litre)
Related Calculators and Tools Frequently Asked Questions Why is DP flow a square root relationship? Bernoulli's equation gives ΔP proportional to velocity squared, so flow is proportional to the square root of the DP.
What Reynolds number is turbulent? Above about 4000. Most flowmeters are specified for turbulent flow.
What is the difference between Nm³ and Sm³? Both are gas volumes at reference conditions. Nm³ is usually 0 °C and 1.01325 bar a; Sm³ is often 15 °C, but the definition varies, so always check.
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