FPS to CFM Calculator: Convert Airflow and Velocity

An FPS to CFM calculator converts air velocity in feet per second (ft/s) and duct or opening area in square feet (ft²) into airflow in cubic feet per minute (CFM). Enter the velocity and cross-sectional area to find the volume of air moving each minute; the calculator also gives the equivalent flow in cubic metres per hour.

Formula

Airflow in cubic feet per minute equals air velocity in feet per second multiplied by the area perpendicular to the airflow in square feet, then multiplied by 60 seconds per minute:

Q = v × A × 60

• Q is airflow in cubic feet per minute (CFM).
• v is average air velocity in feet per second (ft/s, also written FPS).
• A is the flow area in square feet (ft²).
• 60 converts seconds to minutes.

The metric output uses 1 CFM = 0.0283168466 cubic metres per minute, or approximately 1.6990108 m³/h. FPS is a speed, while CFM is a volume flow rate: area is required to convert one into the other.

Worked Example

Using the calculator’s default values, velocity is 10 ft/s and area is 1 ft².

1. Multiply velocity by area: 10 ft/s × 1 ft² = 10 ft³/s.
2. Convert seconds to minutes: 10 ft³/s × 60 s/min = 600 ft³/min.
3. Convert to metric flow: 600 × 1.6990108 = 1,019.40648 m³/h.

Rounded to the calculator’s two decimal places, the result is 600.00 CFM, or 1,019.41 m³/h.

What the Conversion Measures

Velocity describes how fast air travels past a point. Volumetric airflow describes how much space that moving air occupies over time. For a uniform flow through a fixed cross-section, multiplying average velocity by area gives volume per second. Multiplying by 60 expresses that volume per minute.

This is why there is no single conversion factor from FPS to CFM. A velocity of 10 ft/s through 1 ft² gives 600 CFM, while the same velocity through 2 ft² gives 1,200 CFM. Conversely, a stated CFM value does not determine velocity unless the flow area is known.

How to Measure the Area

Use the area open to the airflow, measured perpendicular to its direction. For a rectangular duct, multiply its inside width by its inside height. For a circular duct, use the inside diameter and the circle-area formula:

A = π × (D ÷ 2)²

Convert the dimensions to feet before calculating square feet. For example, a 12-inch by 12-inch opening is 1 ft² because each dimension is 1 ft. A circular duct with an inside diameter of 12 inches has an area of about 0.785 ft², not 1 ft². That difference affects the airflow result.

For grilles, filters, or other obstructions, distinguish the full face area from the actual free area through which air passes. Use the area that matches the velocity measurement: a velocity measured across the face should be paired with the corresponding face area and interpreted as an average, while a measurement of velocity through open passages needs the open area.

Taking and Interpreting Velocity Readings

Air speed is rarely identical at every point in a duct or opening. A single reading can therefore misrepresent the average velocity used in the formula. To estimate airflow more reliably, take readings at several representative points across the cross-section and calculate an appropriate average, following the measurement instrument’s instructions. Then multiply that average by the matching area.

Check that the instrument reports ft/s. Some meters display metres per second (m/s), feet per minute (FPM), or another unit. Convert before entering a value: 1 m/s is approximately 3.28084 ft/s, and 1 FPM is 1/60 ft/s. Entering FPM as if it were FPS makes the calculated airflow 60 times too high.

The calculator returns a mathematical estimate, not a measurement of every real-world airflow condition. Turbulence, leaks, bends, obstructions, changing fan operation, and nonuniform velocity can make actual flow differ. A professional airflow measurement may be needed when a precise result matters.

Common Mistakes and Limits

• Leaving out area: Velocity alone cannot determine CFM. Include the duct or opening’s cross-sectional area.
• Using the wrong area units: If dimensions are in inches, convert them to feet before multiplying for ft². Do not enter a value in square inches as though it were square feet; 1 ft² = 144 in².
• Mixing measurement locations: Pair velocity with the cross-sectional area at the same location. A duct’s dimensions may differ from a grille’s dimensions.
• Assuming a point reading is an average: Uneven flow can make one reading too high or too low to represent the whole section.
• Rounding too early: Keep extra digits during intermediate calculations, then round the final result. The displayed decimals do not imply that the underlying measurement is equally precise.

The formula assumes a meaningful average velocity across a defined area and consistent units. It does not correct for density, temperature, pressure, or humidity, which may matter when comparing mass flow or conditions at different states. CFM here means volumetric flow at the conditions of the measurement, not a mass-flow quantity.

Frequently Asked Questions

How do I convert FPS to CFM?
Multiply average velocity in ft/s by cross-sectional area in ft² and by 60. The result is cubic feet per minute: CFM = FPS × ft² × 60.

Can I convert FPS to CFM without knowing the area?
No. FPS measures speed, while CFM measures volume per time. You need the area through which the air moves, as well as a suitable average velocity.

What is the difference between FPS and FPM?
FPS means feet per second; FPM means feet per minute. There are 60 seconds in a minute, so 1 FPS equals 60 FPM. Confusing them causes a factor-of-60 error.

How do I convert CFM to FPS?
Rearrange the formula: FPS = CFM ÷ (area in ft² × 60). The result depends on the area used and should be compared with a representative velocity measurement.

What is the metric equivalent of CFM?
One CFM is approximately 1.699 m³/h, or 0.0004719 m³/s. The calculator displays m³/h as a convenient metric flow-rate unit.

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in v | Air velocity | ft/s | 10 | 0
in a | Cross-sectional area | ft² | 1 | 0
out cfm | Airflow | CFM | v * a * 60 | 2
out m3h | Airflow | m³/h | cfm * 1.6990108 | 2