Liters per Minute to kg/s: Flow Conversion

This calculator converts a volumetric flow rate in liters per minute (L/min) to a mass flow rate in kilograms per second (kg/s). You enter the flow rate in L/min and the fluid density in kg/m³, and it returns the equivalent mass flow in kg/s. The conversion depends entirely on the fluid’s density, so the same volumetric flow of air and water will give very different mass flows.

Formula

The relationship is straightforward:

ṁ = Q × ρ / 60000

• ṁ = mass flow rate (kg/s)
• Q = volumetric flow rate (L/min)
• ρ = fluid density (kg/m³)
• 60000 = conversion factor (60 seconds per minute × 1000 liters per cubic meter)

Why 60000? Because 1 L/min = 0.001 m³ / 60 s = 1/60000 m³/s. Multiplying by density (kg/m³) gives kg/s.

Worked Example

Using the calculator defaults: Q = 100 L/min and ρ = 1000 kg/m³ (water at about 4°C).

1. Convert L/min to m³/s: 100 L/min ÷ 1000 = 0.1 m³/min; 0.1 ÷ 60 = 0.0016667 m³/s.
2. Multiply by density: 0.0016667 m³/s × 1000 kg/m³ = 1.6667 kg/s.
3. Rounded to 4 decimals: 1.6667 kg/s.

The calculator’s first output shows exactly 1.6667 kg/s for these inputs.

Why Density Matters

Mass flow is a measure of how much actual material (in kilograms) passes a point per second. Volumetric flow only tells you the volume. Since density varies with temperature, pressure, and fluid composition, the same L/min reading can correspond to very different kg/s values. For example:

• Water at 4°C: density ≈ 1000 kg/m³ → 100 L/min = 1.667 kg/s
• Air at 20°C and 1 atm: density ≈ 1.204 kg/m³ → 100 L/min = 0.002008 kg/s
• Gasoline at 15°C: density ≈ 750 kg/m³ → 100 L/min = 1.25 kg/s

Always use the density of the fluid at its actual operating temperature and pressure. For gases, pressure has a large effect; for liquids, temperature is the main factor.

Common Mistakes and How to Avoid Them

• Using water density for other fluids: Only use 1000 kg/m³ if the fluid is essentially water. Check the fluid’s density from a reliable source.
• Forgetting temperature effects: Density changes with temperature. Water at 80°C has a density of about 972 kg/m³, not 1000. This introduces about a 3% error.
• Mixing units: If your flow is in m³/h or gallons per minute, convert to L/min first. 1 m³/h = 16.667 L/min; 1 US gpm ≈ 3.785 L/min.
• Rounding too early: Round only the final answer, not intermediate steps. In the example, rounding 0.0016667 to 0.0017 before multiplying gives 1.7 kg/s instead of 1.6667 kg/s.

When to Use This Conversion

This conversion is common in engineering, plumbing, HVAC, chemical processing, and fluid dynamics. You need it whenever you are sizing pumps, calculating heat transfer, dosing chemicals, or comparing fluid flows in different units. For instance, a pump rated at 500 L/min moving a liquid with density 1200 kg/m³ delivers 10 kg/s, which matters for structural loads and motor power.

It is also useful for environmental measurements—like estimating the mass of pollutants discharged per second from a pipe—or for converting between specifications in different unit systems.

Limits of the Formula

The formula assumes a single, uniform fluid with constant density across the flow. It does not account for:

• Mixtures: If the fluid is a mixture (e.g., air with water droplets), you need the average density, which is hard to determine.
• Compressibility: For gases at high pressure, density changes along the flow path. Use the local density or an average.
• Non-Newtonian fluids: Some fluids have density that varies with shear rate, but this is rare; most liquids are nearly incompressible.

For most practical purposes with liquids, the formula is accurate to well under 1% if you use the correct density. For gases, it is only valid if the pressure and temperature are constant.

Frequently Asked Questions

How do I convert kg/s back to L/min?

Reverse the formula: Q (L/min) = ṁ (kg/s) × 60000 / ρ (kg/m³). For example, 2 kg/s of water (ρ=1000) gives 2 × 60000 / 1000 = 120 L/min.

What is the density of water at room temperature?

At 20°C, pure water has a density of about 998 kg/m³. At 25°C, it is about 997 kg/m³. For most rough calculations, 1000 kg/m³ is fine, but for precise work use the temperature-specific value.

Can I use this for gases like air?

Yes, but you must use the density of the gas at its actual pressure and temperature. Air at 20°C and 1 atm is about 1.204 kg/m³. At higher pressures, density increases proportionally (ideal gas law).

Why is the conversion factor 60000 and not 1000?

Because you are converting both liters to cubic meters (÷1000) and minutes to seconds (÷60). Combined, that is ÷60000. If you had flow in m³/s, the factor would just be the density.

Does viscosity affect the conversion?

No. Viscosity affects pressure drop and pump performance, but it does not change the relationship between volume and mass. The conversion only needs density.

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in v | Flow rate | L/min | 100 | 0
in rho | Density | kg/m³ | 1000 | 0
out m | Mass flow | kg/s | v * rho / 60000 | 4