Mass Flow Rate vs Volumetric Flow Rate: Formulas, Units & Converter (2026)

Last Updated: 2026-11-16

Quick Answer: Mass Flow vs Volumetric Flow in One Table

Volumetric flow rate (Q) is the volume of fluid passing a point per unit time, calculated as Q = A × v and reported in m³/h, L/min, US GPM or CFM. Mass flow rate (ṁ) is the mass passing per unit time, calculated as ṁ = ρ × Q and reported in kg/h, t/h or lb/h. The two are linked only through density, so for an incompressible liquid at a stable temperature they are interchangeable, while for gases, steam and any liquid whose density drifts they are not. If you are paying for product by weight, dosing a recipe, or billing gas, you need mass flow; if you are sizing a pump, checking a pipe velocity or balancing a cooling loop, volumetric flow is what the engineering is built on.

Jump to the live converter if you only need a number, or keep reading for the formulas, unit tables, worked examples and the meter-selection logic behind each choice.

The Volumetric Flow Rate Equation

The volumetric flow rate equation is the first thing every piping calculation uses, and it only needs two inputs: the pipe’s internal area and the average velocity of the fluid across that area.

Q = A × v

where
Q = volumetric flow rate (m³/s)
A = internal cross-sectional area of the pipe (m²)
v = average velocity of the fluid (m/s)

A = π × D² / 4      (D = internal diameter in metres, not nominal size)

Two practical cautions. First, D is the actual bore, not the nominal pipe size. A “2-inch” schedule 40 steel pipe has an internal diameter of 52.5 mm, a DN50 PVC pipe may be 46 to 51 mm, and the difference alone shifts Q by 10 to 20%. Second, v is the average velocity. Fluid moves faster in the centre of a pipe than at the wall, so a single-point measurement such as a pitot tube or an insertion probe must be corrected to the mean. Full-bore meters such as electromagnetic flowmeters average the velocity across the whole cross-section by design, which is one reason they are preferred for billing.

Rearranged, the same equation gives you the velocity from a known flow, which is what you need to check erosion limits and noise:

v = Q / A

As a rule of thumb for water, 1 to 3 m/s is the normal design band for pipework, below 0.5 m/s most velocity-based meters lose accuracy, and above 5 m/s you start to see erosion in elbows and cavitation at valves.

The Mass Flow Rate Formula

Mass flow rate is volumetric flow multiplied by the density of the fluid at the conditions where the flow is measured:

ṁ = ρ × Q = ρ × A × v

where
ṁ = mass flow rate (kg/s)
ρ = fluid density at operating conditions (kg/m³)
Q = volumetric flow rate (m³/s)

And the reverse:

Q = ṁ / ρ

The formula is trivial. The difficulty is always the value of ρ. Water changes density by about 3% between 4 °C and 80 °C. Diesel changes by roughly 0.08% per °C, so a tank that warms from 10 °C to 35 °C holds 2% less mass per litre. Steam at 10 bar(a) is almost twice as dense as steam at 5 bar(a). Any time you convert a volumetric reading into tonnes or kilograms, the density you use must be the density at the meter, at the moment of measurement, not the value printed in a handbook at 20 °C.

Gas Density, Standard Conditions and the Actual-vs-Standard Correction

For gases, density is calculated from pressure and temperature using the ideal gas law:

ρ = P × M / (R × T)

where
P = absolute pressure (Pa)
M = molar mass (kg/mol); air = 0.02897, methane = 0.01604
R = 8.314 J/(mol·K)
T = absolute temperature (K)

Because gas volume depends so strongly on P and T, the industry reports gas flow at a reference condition instead of the actual condition. The reference is not universal:

Mixing these up produces errors of 5 to 8% before anyone has made a measurement. Always write the reference condition on the purchase order and on the meter tag.

To move between standard and actual volume:

Q_actual = Q_std × (P_std / P_actual) × (T_actual / T_std)

P in absolute units, T in kelvin

Only Coriolis and thermal mass meters escape this correction, because they read mass, and mass does not care what the pressure is. A volumetric gas meter such as a gas turbine flowmeter or a vortex meter must be paired with a pressure transmitter, a temperature sensor and a flow computer to deliver a standard or mass figure.

Interactive Converter: Volumetric ↔ Mass Flow

Enter a flow rate in any unit and a density. The converter outputs the equivalent value in every volumetric and mass unit used on this page. The density presets are typical values at 20 °C and 1 atm (water, diesel, gasoline, air, natural gas) and at 10 bar absolute for saturated steam; replace them with your own measured density whenever you have one.

Volumetric Flow Rate Units: Conversion Table

Each row shows what one unit of the row header equals in every column. The factors are exact or rounded to six significant figures; US gallons and cubic feet are used throughout (the UK imperial gallon is 20% larger and is a common source of error on export projects).

Mass Flow Rate Units: Conversion Table

Note that “t” here is the metric tonne (1,000 kg). US short tons (907.185 kg) and UK long tons (1,016.05 kg) still appear in some specifications, so confirm which one a datasheet means before you size a Coriolis meter.

Density of Common Fluids for Converting Between Mass and Volume

Steam values are from the IAPWS-IF97 saturated-steam tables; hydrocarbon ranges follow ASTM D1298 and EN ISO 3675 density classes. Where a range is given, measure your own product or ask the supplier for the certificate of analysis.

Four Worked Examples

Example 1: Water in a DN50 pipe at 2 m/s

A DN50 steel pipe with a 50 mm internal bore carries water at an average velocity of 2 m/s and 20 °C.

A = π × 0.050² / 4 = 0.0019635 m²
Q = A × v = 0.0019635 × 2 = 0.003927 m³/s = 14.14 m³/h  (= 62.2 US GPM)
ṁ = ρ × Q = 998.2 × 0.003927 = 3.920 kg/s = 14,110 kg/h ≈ 14.1 t/h

For water, the number of cubic metres per hour and the number of tonnes per hour are nearly identical, which is why water engineers rarely think about mass flow at all. A clamp-on transit-time ultrasonic flowmeter would read this directly as velocity, multiply by the programmed pipe area and report 14.1 m³/h at ±1%.

Example 2: Diesel transfer, 100 m³/h to tonnes per hour

A depot pump moves 100 m³/h of diesel at 840 kg/m³.

ṁ = 840 × 100 = 84,000 kg/h = 84 t/h

If the diesel warms from 15 °C to 30 °C during the day, its density drops about 1.2%, to roughly 830 kg/m³, and the same 100 m³/h now delivers 83 t/h. Over a 10,000-tonne annual throughput that is a 120-tonne discrepancy between the volumetric meter and the weighbridge. This is the classic reason fuel custody transfer is done either with a Coriolis meter or with a volumetric meter plus temperature correction to 15 °C under API MPMS Chapter 11.

Example 3: Compressed air, 500 SCFM at 7 bar gauge

A compressor is rated at 500 SCFM (14.696 psia, 60 °F). What actual volume flows in the 7 bar(g) distribution header at 30 °C?

P_actual = 7 + 1.01325 = 8.013 bar(a)
T_actual = 30 + 273.15 = 303.15 K ; T_std = 288.7 K
Q_actual = 500 × (1.01325 / 8.013) × (303.15 / 288.7)
         = 500 × 0.12645 × 1.0500
         = 66.4 ACFM

The header sees only 66 ft³/min of gas, even though the compressor “makes” 500. Size the pipe and the flow meter for 66 ACFM, but buy and bill in SCFM. A vortex flowmeter in this line would measure the 66 ACFM; the flow computer, with a pressure and a temperature input, converts it back to 500 SCFM. Without those two inputs the meter’s reading is only correct at one operating point.

Example 4: Boiler steam, 2 t/h through a vortex meter

A boiler exports 2 t/h of saturated steam at a design pressure of 10 bar(a), where the density is 5.15 kg/m³.

Q = ṁ / ρ = 2,000 / 5.15 = 388 m³/h

The vortex meter is calibrated in volume and reports 388 m³/h. A flow computer with the fixed density of 5.15 kg/m³ displays 2.0 t/h. Now the plant turns down and the header pressure drops to 8 bar(a), where saturated steam density is 4.16 kg/m³. The meter still sees 388 m³/h of vapour, the flow computer still multiplies by 5.15, and it still reports 2.0 t/h. The true mass flow is 388 × 4.16 = 1,614 kg/h. The uncompensated reading is 24% high, and the boiler-house efficiency report is wrong by the same margin. A pressure transmitter on the line and a density table in the flow computer remove the error; this is called pressure-temperature compensation and should be specified on every steam meter, whatever the technology.

Which Flow Meters Measure Mass, and Which Measure Volume

Coriolis meters vibrate a tube and measure the phase shift caused by the mass of fluid passing through it. Because the phase shift depends only on mass, not on what the fluid is, a single meter can run diesel, biodiesel, ethanol and water and remain accurate on all of them. The same tube’s resonant frequency gives density, so the meter also outputs volume flow and, for a two-component mixture, concentration. The U-tube YSK series covers DN3 to DN150 with ranges from 0–144 kg/h at DN3 to 0–360,000 kg/h at DN150, works from −200 °C to +300 °C, and outputs 4–20 mA, pulse, RS485, HART and Modbus. Where a straight-run or self-draining design matters, for example in food, pharmaceutical or paint lines, the straight-tube YSK-Z is the alternative, and the micro Coriolis YSK-Z02 handles the small flows found in additive dosing, catalyst injection and laboratory pilot plants.

Thermal mass meters heat an element and measure how much heat the gas carries away, which is proportional to mass flow for a known gas composition. They are the usual choice for compressed air, nitrogen and biogas where the gas is clean and the composition is stable.

Every other technology measures volume or velocity. That is not a weakness; it simply means the mass conversion has to happen somewhere else. For liquids at a stable temperature, a fixed density in the transmitter is fine. For steam and gas, the flow computer needs live pressure and temperature inputs. The vortex meter is the workhorse here because it is cheap, has no moving parts and tolerates 350 °C steam, and because a compensated vortex meter reports mass flow at roughly ±1.5%, which is adequate for internal energy accounting though not for custody transfer.

When You Must Buy a Mass Flow Meter

A mass flow meter costs several times what a volumetric meter costs in the same line size, so the decision should be driven by the economics of the measurement, not by the technology being newer. Buy mass flow when any of the following applies:

1. Custody transfer by weight. Fuel oil, LPG, chemicals and edible oils are traded in tonnes. A Coriolis meter at ±0.1% of reading removes the density argument from every invoice. Compare that with Example 2, where temperature alone moved the figure by 1.2%.
2. Batching and recipe dosing. Reactions, coatings and food recipes are formulated by mass. A volumetric meter dosing a resin whose density varies by batch will produce off-spec product that no later measurement can fix.
3. Gas billing. Natural gas, hydrogen and industrial gases are sold by energy content, which follows mass, not actual volume. Either a Coriolis or thermal meter, or a volumetric meter with full P/T/compressibility correction (AGA 8), is required.
4. Fluids whose density changes during operation. Multi-product pipelines, cryogenic liquids, polymer melts, slurries and anything measured across a wide temperature range.
5. Process control on mass balance. Reactor feeds, combustion air-to-fuel ratio and evaporator duty are all mass-based. A volume signal adds a density error into the control loop.

Stay with a volumetric meter when the fluid is clean water or a stable liquid, the temperature is steady, and the result is used for pump control, leak detection, utility sub-metering or irrigation. For those applications an electromagnetic or ultrasonic meter at a fraction of the price is the engineering-correct choice, and our comparison of electromagnetic vs turbine vs ultrasonic flow meters walks through the selection in detail. The wider landscape of all nine measurement technologies is covered in types of flow meters compared.

Specifying the Meter: What to Put on the Enquiry

Getting the mass-versus-volume question right on paper saves most of the back-and-forth we see on flow meter orders. SIBXSUN supplies all of the technologies above, sourced from partner factories in China, and a complete enquiry lets us quote the correct meter the first time:

• State the quantity you actually need: m³/h, Nm³/h, kg/h or t/h. If it is a standard volume, write the reference condition (0 °C or 15 °C or 60 °F).
• Give minimum, normal and maximum flow, not just the maximum. Turndown decides whether a vortex meter will read at low load or whether you need Coriolis.
• Give operating pressure and temperature ranges, both of them, because density correction depends on the spread, not the design point.
• Name the fluid and its density or composition. “Hydrocarbon” is not enough; “diesel, 840 kg/m³ at 15 °C, up to 7% biodiesel” is.
• Say what the output feeds: a 4–20 mA loop into a PLC, a pulse into a totaliser, RS485 Modbus into a SCADA, or HART for configuration. Our guide to flow rate vs pressure in pipes covers the pressure-drop side of the same sizing exercise, and if the specification arrives in psi or feet of head, pressure units explained has the conversions.

For electromagnetic meters specifically, the electromagnetic flowmeter supplier page lists sizes, liners and lead times, and if you are comparing sources, see our review of flow meter brands and suppliers in China. The full range of Coriolis, vortex, turbine, ultrasonic and radar meters is in the flow meters category.

Frequently Asked Questions

What is the formula for volumetric flow rate?

Volumetric flow rate is Q = A × v, where A is the internal cross-sectional area of the pipe (A = πD²/4) and v is the average fluid velocity. In SI units Q comes out in m³/s; multiply by 3,600 for m³/h. For a DN50 pipe (0.05 m bore) at 2 m/s the result is 0.003927 m³/s, or about 14.1 m³/h.

How do you convert volumetric flow rate to mass flow rate?

Multiply the volumetric flow rate by the fluid density at the actual operating temperature and pressure: ṁ = ρ × Q. For 100 m³/h of diesel at 840 kg/m³ the mass flow is 84,000 kg/h, or 84 t/h. For gases, density must be calculated from the actual pressure and temperature, not from a datasheet value at standard conditions.

What is the difference between SCFM and ACFM?

SCFM (standard cubic feet per minute) is gas flow corrected to a reference condition, usually 14.696 psia and 60 °F in the US. ACFM (actual cubic feet per minute) is the real volume moving through the pipe at operating pressure and temperature. Compressed air at 500 SCFM delivered at 7 bar gauge and 30 °C occupies only about 66 ACFM because the gas is compressed roughly eightfold.

Which flow meters measure mass flow directly?

Coriolis mass flow meters and thermal mass flow meters measure mass flow without needing a separate density value. Coriolis meters such as the SIBXSUN YSK series also output volume flow and density simultaneously. Electromagnetic, vortex, turbine, ultrasonic and orifice-plate meters measure volume or velocity and need a density input or pressure-temperature compensation to report mass.

When do I need a mass flow meter instead of a volumetric flow meter?

Buy a mass flow meter when the fluid density changes during operation or when the quantity being bought, sold or dosed is defined by weight. Typical cases are custody transfer of fuels and chemicals, batching and recipe dosing, natural gas billing, steam accounting where pressure varies, and cryogenic or multi-product lines. For clean water, cooling loops and most utility measurement, a volumetric meter with a fixed density is accurate enough and costs far less.

Not sure whether your line needs Coriolis or a compensated vortex meter? Send us the fluid, the flow range and the operating pressure and temperature and we will size both options with prices and lead times within 24 hours.