Mass flow rate is the product of density and volumetric flow rate: ṁ = ρ × Q
- ṁ = mass flow rate (kg/s)
- ρ = density (kg/m³)
- Q = volumetric flow rate (m³/s)
For gases, density can be estimated using ρ = P / (R × T), where P is absolute pressure (Pa), T is temperature (K), and R is the specific gas constant (J/(kg·K)).
- Enter the volumetric flow rate and select its unit.
- Choose an output mass-flow unit for your target application.
- Select a density method: direct density, specific gravity, or ideal gas.
- Fill the required fields for the selected density method.
- Press Calculate to view results above the form.
- Use Download CSV or Download PDF to save outputs.
| Scenario | Volumetric flow (Q) | Density (ρ) | Mass flow (ṁ) |
|---|---|---|---|
| Water line | 0.050 m³/s | 998 kg/m³ | 49.9 kg/s |
| Oil transfer | 1200 L/min | 0.88 g/cm³ | 17.6 kg/s |
| Ventilation air | 800 ft³/min | 1.20 kg/m³ | 0.453 kg/s |
| Natural gas estimate | 0.020 m³/s | 0.80 kg/m³ | 0.016 kg/s |
- Use absolute pressure for the ideal-gas density method.
- For liquids, density can change with temperature; verify conditions.
- If your flow meter reports standard conditions, use the same basis.
- For slurries or wet gases, lab density data is recommended.
1) Why this conversion matters in real systems
Volumetric flow (Q) is convenient for meters and pumps, but many design limits depend on mass flow (ṁ). Combustion, drying, mixing, and heat transfer often scale with kg/s, not m³/s. Converting correctly improves energy balances, material accounting, and safety documentation.
2) Core relationship and unit consistency
The calculator uses the fundamental relation ṁ = ρ × Q. The key requirement is consistent base units: Q in m³/s and ρ in kg/m³. For reference, 1 L/s equals 0.001 m³/s, and 1 ft³/min equals 0.000471947 m³/s.
3) Typical density data you can start from
Density is the main driver of uncertainty. Water is about 998 kg/m³ near 20°C, light hydraulic oils often range 850–920 kg/m³, and seawater is commonly near 1025 kg/m³. For gases at room conditions, air is near 1.2 kg/m³. Always confirm the datasheet basis and temperature.
4) Specific gravity workflow for liquids
If you have specific gravity (SG), the calculator multiplies SG by a chosen reference density. This is common for fuels and solvents where SG is reported without a full density table. Example: SG = 0.88 with a 1000 kg/m³ reference gives ρ ≈ 880 kg/m³ for quick estimates.
5) Ideal gas estimate for compressible flow
For gases, density depends strongly on absolute pressure and temperature. The ideal-gas option uses ρ = P / (R × T), with P in Pa and T in Kelvin. At 101.325 kPa and 25°C, air density computes close to 1.18 kg/m³, aligning with standard engineering ranges.
6) Worked example with numbers
Suppose Q = 1200 L/min of oil and density is 0.88 g/cm³. First, Q becomes 0.02 m³/s. Density becomes 880 kg/m³. Then ṁ = 880 × 0.02 = 17.6 kg/s. This matches the example table and demonstrates how unit conversion affects the final result.
7) Measurement considerations and accuracy
Flow meters may report actual or standardized volumetric flow. Standardized flow already assumes reference conditions, so you must use matching density conditions to avoid double correction. For liquids, temperature shifts can change density by a few percent, which propagates linearly to mass flow.
8) Engineering uses and quick validation checks
Mass flow supports pump power estimates, heat duty calculations, dosing rates, and emissions reporting. A quick validation is dimensional: kg/m³ × m³/s yields kg/s. Another check is plausibility: high CFM air values should still produce relatively small kg/s compared with liquids.
1) What is the difference between volumetric and mass flow?
Volumetric flow measures volume per time, such as m³/s or L/min. Mass flow measures mass per time, such as kg/s. Mass flow depends on density, so the same volumetric flow can imply different mass flow values.
2) Which density method should I choose?
Use direct density when you have a reliable datasheet or lab value. Use specific gravity when SG is provided. Use the ideal-gas method for gases when you know absolute pressure and temperature at the measurement point.
3) Why does temperature matter so much?
Density changes with temperature. Liquids often change modestly, but gases can change significantly. Because mass flow equals density times volumetric flow, any density shift produces the same percentage change in mass flow.
4) Should I use gauge pressure or absolute pressure for gases?
Use absolute pressure. Gauge pressure must be converted by adding local atmospheric pressure. Using gauge pressure directly can underpredict density and mass flow, especially at low operating pressures.
5) What if my meter reports standard cubic feet per minute?
Standard flow is referenced to specified conditions. Convert using the density at the same standard conditions, or convert the standard flow back to actual conditions first. Mixing bases can cause noticeable mass flow errors.
6) How can I sanity-check the result quickly?
Confirm units reduce to kg/s. Compare against typical densities: water near 1000 kg/m³, air near 1.2 kg/m³. If a gas result seems as large as a liquid result at the same Q, recheck density settings.
7) Can this be used for slurries or wet gases?
Yes, but accuracy depends on using a representative density. Slurries and wet gases can vary with composition and loading. If possible, measure density or use process data rather than relying on ideal-gas assumptions.