How the flow rate converter works
Type a flow in any unit and every other unit updates: m³/h, L/min, L/s, US and UK gallons per minute, cubic feet per minute, oil barrels per day, and mass flow in kg/h, kg/s, t/h or lb/h. The volume units are exact by definition; the mass units need the density of the fluid as it flows.
The second part relates flow to differential pressure on an orifice plate, venturi, nozzle or averaging pitot tube: enter the flow and Δp at full scale, then either a flow or a measured Δp.
Volume flow and mass flow
Mass flow is volume flow times density. Water at 20 °C has 998.21 kg/m³, so 10 m³/h of water is 9982 kg/h. A light oil at 850 kg/m³ gives 8500 kg/h for the same volume. Use the density at the flowing temperature: water at 80 °C is about 2 % lighter than at 20 °C.
Gases are different: their density depends on pressure and temperature, so a gas flow is usually stated as a normal or standard volume (Nm³/h, SCFM) at defined reference conditions. Those references differ between industries and standards, so they are not converted here.
Square-root flow on a DP flowmeter
A differential-pressure flowmeter produces a Δp that grows with the square of the flow: Q = Qmax × √(Δp / Δpmax). Half the flow gives a quarter of the Δp. A transmitter with a linear Δp output therefore needs square-root extraction, either inside the transmitter or in the control system, but not in both. The 4–20 mA converter scales such a signal with its square-root option.
Low flow and turndown
Because of the square law, the low end of a DP flowmeter is coarse: at 10 % of flow the Δp is only 1 % of span, so a Δp error of 0.1 % of span becomes a flow error of 0.5 % of full scale. Most transmitters and controllers cut off low flow below about 5 to 10 %. If the process runs at low flow for long periods, consider a second transmitter for the low range, or a meter with a pulse output; the pulse output calculator checks its frequency against your counter.
Frequently asked questions
Is a US gallon the same as a UK gallon?
No. A US gallon is 3.785411784 L, an imperial (UK) gallon 4.54609 L, about 20 % more. Check which one a datasheet means; “gpm” alone is usually US.
Why does my flow reading differ from the design?
The square-root relation holds only with the density and discharge coefficient of the design point. A different temperature, pressure or fluid changes the density; wear on the orifice edge changes the coefficient.
Can I size an orifice plate here?
No. The bore and the Δp at full scale come from a sizing calculation to ISO 5167, which accounts for the pipe, the fluid and the Reynolds number. This calculator works from that result.