By Wu Peng, Senior Process Instrumentation Engineer · Last reviewed July 19, 2026
Home › Tools › Mass Flow Rate Calculator
This mass flow rate calculator works in three directions. Give it a volume flow and a density and it returns mass flow in kg/h, kg/s, lb/h, and t/h. Give it a mass flow and a density and it returns volume flow in m³/h, L/min, GPM, and CFM. Or switch to gas mode: enter the actual flow, line pressure, and temperature, and it computes the gas density, the mass flow, and the standard flow in Nm³/h and SCFM. The math is the one relation every conversion runs through, ṁ = ρ × Q.
Use the density at the conditions actually in the pipe. Why that matters, and where to find working density values, is covered in our mass flow vs volumetric flow guide.
Calculator
Mass and volume flow converter
kg/m3
Results update as you type. Gas mode assumes ideal-gas behavior; do not use it for steam. Density presets are typical values at the stated condition.
How the conversion works
Density is the bridge between the two flow rates:
ṁ = ρ × Q and Q = ṁ / ρ
where ṁ is mass flow, Q is volume flow, and ρ is the density of the fluid at the conditions in the pipe. For liquids, density is close to constant, so one measured or tabulated value is usually enough. For gases, density moves with pressure and temperature, which is why the gas mode asks for both and computes the density itself from the ideal gas law.
The classic quick check: 100 GPM of water at 20 °C is 22,672 kg/h, which is 49,982 lb/h. That is where the old rule “100 GPM is about 50,000 pounds per hour” comes from, and this calculator reproduces it exactly.
Worked example
A specification arrives stating 2,800 lbs of anhydrous ammonia per hour through a 1-inch line to a heat exchanger. The piping engineer wants that as a volume flow. Liquid ammonia at 25 °C has a density of about 602 kg/m³, so:
- 2,800 lb/h × 0.4536 = 1,270 kg/h
- 1,270 / 602 = 2.11 m³/h, which is 35.2 L/min or 9.3 GPM
Enter 2,800 lb/h in the mass-to-volume mode with the ammonia preset and the calculator returns the same numbers. Note how light the liquid is: barely 60 percent of the density of water, so the volume flow is two thirds larger than a water line carrying the same mass.
Application example
Fertilizer plant, India. The 2,800 lb/h figure above comes from a real inquiry: a fertilizer producer metering anhydrous ammonia to a heat exchanger on a 1-inch line at up to 15 to 30 bar. Because the process balance runs in mass units and ammonia density moves with temperature, we quoted a Coriolis mass flow meter reading directly in mass, with wetted materials checked against ammonia service.
Gas mode
Gas mode answers the question volumetric readings cannot: how much gas is that, really. It computes the density from the line pressure and temperature using the ideal gas law with the specific gas constant of the selected gas, then reports mass flow plus the two common standard-volume forms: Nm³/h referenced to 0 °C and 1 atm, and SCFM referenced to 60 °F and 14.696 psia. The two references differ, which is exactly why both are shown with their conditions attached.
Try it on a compressed air line: 100 m³/h of actual flow at 6 bar gauge and 35 °C is 793 kg/h, or 613 Nm³/h. The ideal-gas assumption is good to about a percent for air, nitrogen, and methane at these pressures. It is not valid for steam; steam runs on saturation tables instead, which is the business of a steam flow meter with temperature and pressure compensation. And if the line deserves a meter that skips the conversion entirely, a thermal mass flow meter reads gas mass flow directly.
Related guides and tools
The companion mass flow vs volumetric flow guide explains when each flow rate is the right one to specify and carries a density reference table for common fluids. The flow rate units guide handles unit-to-unit conversion, GPM to m³/h and the rest. If you are starting from a pipe size and velocity instead of a flow reading, the pipe velocity calculator gets you to volume flow first.
FAQ
How to get from volumetric flow rate to mass flow rate?
Multiply the volumetric flow rate by the fluid density at the actual line conditions: mass flow = density × volume flow. Keep the units consistent: m³/h times kg/m³ gives kg/h. This calculator handles the unit bookkeeping for you.
How to calculate mass flow rate?
If you know the volume flow, multiply by density. If you know the flow velocity instead, first multiply velocity by the pipe cross-sectional area to get volume flow, then multiply by density: mass flow = density × area × velocity. Density must be taken at the pressure and temperature in the pipe.
What is the mass flow rate and volumetric flow rate?
Volumetric flow rate is the volume of fluid passing a point per unit time, in units like m³/h or GPM. Mass flow rate is the mass passing per unit time, in units like kg/h or lb/h. Density links them, and for gases the two behave very differently when pressure or temperature changes.
How to convert GPM to mass flow rate?
Multiply GPM by 0.2271 to get m³/h, then multiply by the density in kg/m³ to get kg/h. For water at 20 °C, 100 GPM works out to 22,672 kg/h, or very nearly 50,000 lb/h. For any other liquid, swap in its density; the first mode of this calculator does it in one step.
Request a quote
If the conversion keeps landing on a gas or steam line where the volumetric meter is the problem, tell us the fluid, line size, pressure, and temperature. We will recommend a direct mass meter or a compensated build and quote it. Reach our application engineers or use the form below.
Written and technically reviewed by Wu Peng and the Instranova engineering team.