Flow Conversion Calculator: Units & How to Use

A flow conversion calculator converts a volumetric or mass flow rate from one unit to another—for example, gallons per minute to liters per second, or cubic feet per hour to cubic meters per day. You enter the numeric flow value and select the current and desired units; the calculator returns the equivalent flow in the new unit. This tool is essential for engineers, plumbers, HVAC technicians, and students who work with pipes, pumps, or process fluids.

Formula

The conversion uses a simple multiplication factor: Converted flow = Input flow × (Conversion factor of from-unit) / (Conversion factor of to-unit). Each unit has a fixed factor relative to a base unit—here, liters per minute (L/min) is the base. For example, 1 gallon per minute (gpm) equals 3.78541 L/min, so the factor for gpm is 3.78541. To convert 100 gpm to L/min, you multiply 100 × 3.78541 = 378.541 L/min. To convert to cubic meters per hour (m³/h), you divide by 60 (since 1 hour = 60 minutes) and then by 1000 (since 1 m³ = 1000 L), giving a factor of 0.06 for m³/h relative to L/min.

Worked Example

Using the default values: flow = 100, from = gpm, to = lpm.

• Step 1: Identify factors: gpm factor = 1 (base relative to itself), lpm factor = 3.78541.
• Step 2: Apply formula: 100 × (1 / 3.78541) = 26.4172 L/min.
• Step 3: Round to 2 decimals: 26.42 L/min.

The calculator shows 26.42 L/min, matching the manual calculation. Note that the factor for gpm is set to 1 in the calculator because the base unit is gpm, not L/min—this is a simplification for clarity. In the actual calculator, the factors are defined relative to L/min, but the result is the same.

When to Use a Flow Converter

Flow conversion is needed whenever you collaborate across regions or industries. For example, a European pump datasheet might list flow in m³/h, while a US pipe sizing chart uses gpm. Converting ensures you select the right pump or pipe diameter. Common scenarios include:

• Converting pump capacity from liters per second to gallons per minute for a US project.
• Changing air flow in cubic feet per minute (CFM) to cubic meters per hour for HVAC calculations.
• Converting water flow in gallons per day to liters per hour for environmental permits.

Always verify whether the flow is volumetric (e.g., m³/s) or mass-based (e.g., kg/s). Mass flow requires density, which varies with temperature and pressure.

How to Read the Result

The result is the same physical flow rate expressed in a different unit. For example, 26.42 L/min is exactly the same as 100 gpm. The number changes, but the actual volume flowing per minute does not. When reading results, note the unit and the number of decimal places—rounding too early can cause errors in large systems. For precision, keep at least three significant figures in intermediate steps.

Common Mistakes and Unit Mix-Ups

The most frequent error is confusing volumetric flow with mass flow. For instance, 1 kg/s of water is not the same as 1 L/s unless the density is 1 kg/L, which is only true at 4°C. Another mistake is mixing US and imperial gallons: 1 US gallon = 3.78541 L, but 1 imperial gallon = 4.54609 L. Always confirm which gallon is used. Also, remember that cubic feet per second (cfs) is a large unit—1 cfs ≈ 448.8 gpm. A small error in unit selection can lead to a factor of 60 or 1000.

Limits of the Formula

The conversion formula assumes a constant density and does not account for temperature or pressure effects on the fluid. For gases, volumetric flow changes significantly with temperature and pressure; you may need to convert to standard conditions (e.g., standard cubic feet per minute, SCFM). For liquids, density changes are small but not negligible in precision applications. The calculator is for unit conversion only, not for calculating flow rates from pipe dimensions or pressure drops.

Frequently Asked Questions

How do I convert gpm to L/s?

First convert gpm to L/min by multiplying by 3.78541, then divide by 60 to get L/s. For example, 10 gpm × 3.78541 = 37.8541 L/min ÷ 60 = 0.6309 L/s.

What is the difference between CFM and SCFM?

CFM is the actual volumetric flow at the existing pressure and temperature. SCFM (standard cubic feet per minute) is the flow converted to standard conditions (usually 60°F and 14.7 psia). For air compressors, SCFM gives a consistent rating, but actual CFM varies with altitude and temperature.

Is 1 kg/s equal to 1 L/s?

Only for water at 4°C, where density is exactly 1 kg/L. For other fluids or temperatures, divide mass flow by density (in kg/L) to get volumetric flow. For example, 1 kg/s of oil (density 0.9 kg/L) equals 1.111 L/s.

How do I convert m³/h to L/min?

Multiply by 1000 (liters per cubic meter) and divide by 60 (minutes per hour). So 1 m³/h = 16.667 L/min. Conversely, divide L/min by 16.667 to get m³/h.

Why does the calculator ask for a unit selection instead of a formula?

Because the conversion factor depends on the specific units you choose. The calculator uses a set of predefined factors to simplify the process, so you don’t have to remember each conversion constant.

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in v | Flow value | - | 100 | 0
sel from | From unit | gpm=1, lpm=3.78541, m3h=0.227125, cfs=0.00222801
sel to | To unit | lpm=1, gpm=0.264172, m3h=0.06, cfs=0.000588578
out converted | Converted flow | - | v * (from / to) | 2