Feet per Minute to l/s Converter & Formula

This calculator converts a flow rate given in feet per minute (ft/min) into litres per second (L/s). You enter the volumetric flow rate in cubic feet per minute (CFM) or a linear velocity in feet per minute along with a cross-sectional area, and the tool returns the equivalent flow in litres per second. The result is shown instantly with the formula used, so you can verify the conversion step by step.

Formula

The conversion relies on the relationship between cubic feet per minute (CFM) and litres per second. One cubic foot equals 28.316846592 litres, and one minute equals 60 seconds. Therefore:

Flow (L/s) = Velocity (ft/min) × Area (ft²) × 0.4719474432

Where:

• Velocity is the linear speed of the fluid in feet per minute (ft/min).
• Area is the cross-sectional area of the duct or pipe in square feet (ft²).
• 0.4719474432 is the conversion factor: (28.316846592 L / 1 ft³) ÷ (60 s / 1 min) = 0.4719474432 L/s per CFM.

If you already have a volumetric flow rate in CFM, simply multiply by 0.4719474432 to get L/s. The calculator above lets you enter velocity and area separately, which is common when you know the air speed in a duct but not the total flow.

Worked Example

Using the default values from the calculator: velocity = 500 ft/min, area = 1 ft².

1. First, compute the volumetric flow in cubic feet per minute: 500 ft/min × 1 ft² = 500 ft³/min (CFM).
2. Convert CFM to L/s: 500 × 0.4719474432 = 235.9737216 L/s.
3. Rounded to two decimals, the result is 235.97 L/s.

This matches the calculator’s headline output. If you had a velocity of 1000 ft/min and an area of 0.5 ft², the flow would be 1000 × 0.5 = 500 CFM, then 500 × 0.4719474432 = 235.97 L/s again—same flow, different combination of speed and area.

Why Convert ft/min to L/s?

Feet per minute is a common unit in HVAC (heating, ventilation, and air conditioning) and industrial ventilation in the United States, where duct velocities are often quoted in ft/min or fpm. Litres per second is the standard unit for volumetric flow in the International System of Units (SI) and is widely used in Europe, Australia, and many other regions for specifying fan performance, air change rates, and water flow. Converting between the two is essential when comparing equipment from different manufacturers, designing systems to meet local codes, or communicating with colleagues who use metric units.

For example, a ventilation engineer in the US might specify a duct velocity of 800 ft/min, but the fan supplier in Germany needs the flow in L/s to select the correct unit. The conversion factor makes this straightforward: multiply the CFM by 0.4719474432. Conversely, to convert L/s to CFM, divide by the same factor.

How to Use the Calculator

Enter the linear velocity in feet per minute and the cross-sectional area in square feet. The calculator assumes the area is perpendicular to the flow direction, which is typical for a duct or pipe. If you know the diameter of a circular duct, you can compute the area as π × (diameter/2)², but that calculation is not built into this tool—you must enter the area directly.

If you only have the volumetric flow in CFM, set the area to 1 and enter the CFM value as the velocity. The result will be the same as multiplying CFM by the conversion factor. For example, 1000 CFM with area 1 ft² gives 1000 × 0.4719474432 = 471.95 L/s.

The output is rounded to two decimal places, which is sufficient for most engineering and practical purposes. For more precise work, you can use the full factor 0.4719474432, but the difference is negligible in real-world applications.

Common Mistakes and Unit Mix-Ups

The most frequent error is confusing linear velocity (ft/min) with volumetric flow (CFM). A velocity of 500 ft/min is not a flow rate until you multiply by an area. Another common mistake is using the wrong conversion factor—some sources quote 0.472, which is fine for rough estimates, but the exact factor is 0.4719474432. Rounding too early can cause small errors, especially when converting large flows.

Also be careful with units: if you measure velocity in metres per second (m/s) and area in square metres (m²), you get cubic metres per second (m³/s), not L/s. To convert m³/s to L/s, multiply by 1000. Similarly, if you have litres per minute (L/min), divide by 60 to get L/s. The calculator only accepts ft/min and ft², so convert all inputs to those units first.

Another trap is using the wrong area—for a rectangular duct, use width × height; for a circular pipe, use π × radius². If you accidentally use the diameter instead of the radius, the area will be four times too large, leading to a flow that is four times the true value.

Limitations of the Conversion

This conversion is purely a unit change; it does not account for fluid properties, pressure, temperature, or friction losses. The result is the volumetric flow at the conditions you specify, but if the fluid is compressible (like air), the actual mass flow depends on pressure and temperature. For most HVAC applications, the volumetric flow at standard conditions is used, but for high-pressure or high-temperature systems, you may need to correct for density.

The calculator also assumes a uniform velocity profile across the area. In real ducts, the velocity is lower near the walls due to friction, so the average velocity is what you should use. If you measure a point velocity with an anemometer, it may not represent the average, especially in turbulent flow. For accurate results, use a traverse method to measure multiple points and average them.

Finally, the conversion factor is exact for the defined units, but rounding in the calculator output (two decimals) introduces a tiny error. For critical applications, use the full factor or a more precise calculator.

Frequently Asked Questions

How do I convert CFM to L/s directly?

Multiply the CFM value by 0.4719474432. For example, 100 CFM × 0.4719474432 = 47.19 L/s. To go the other way, divide L/s by 0.4719474432 to get CFM.

What is the difference between ft/min and CFM?

Feet per minute (ft/min) is a unit of linear velocity—how fast the fluid is moving. Cubic feet per minute (CFM) is a unit of volumetric flow—how much volume passes per minute. They are related by the cross-sectional area: CFM = ft/min × ft². Without an area, you cannot convert between them.

Why is the conversion factor 0.4719474432?

One cubic foot equals 28.316846592 litres, and one minute equals 60 seconds. Dividing 28.316846592 by 60 gives 0.4719474432. This factor converts cubic feet per minute to litres per second.

Can I use this calculator for water flow?

Yes, the conversion is the same for any fluid. The calculator only changes units, not the physical properties. However, for water, you might also need to consider pressure and pipe size, but the volumetric flow conversion remains identical.

What if I have velocity in m/s and area in cm²?

Convert everything to consistent units first. For example, convert m/s to ft/min (1 m/s = 196.850394 ft/min) and cm² to ft² (1 cm² = 0.00107639104 ft²), then use the calculator. Alternatively, convert your flow to m³/s and then to L/s by multiplying by 1000.

Is the result affected by air pressure or temperature?

No, the calculator gives volumetric flow at the conditions you enter. If you need mass flow, you must account for density, which varies with pressure and temperature. For standard air (20°C, 1 atm), density is about 1.204 kg/m³, but that is not part of this conversion.

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in v | Linear velocity | ft/min | 500 | 0
in a | Cross-sectional area | ft² | 1 | 0
out flow | Volumetric flow | L/s | v * a * 0.4719474432 | 2
out cfm | Volumetric flow | ft³/min | v * a | 2