Convert Pa to CFM from velocity pressure and duct or opening area, or reverse CFM to Pa, with kPa, MPa, psi, inH2O and m3/h units included.
| Pa to CFM | CFM to Pa |
|---|---|
| 25 Pa = 1,269 CFM | 500 CFM = 3.88 Pa |
| 50 Pa = 1,794 CFM | 750 CFM = 8.74 Pa |
| 75 Pa = 2,197 CFM | 1,000 CFM = 15.54 Pa |
| 100 Pa = 2,537 CFM | 1,200 CFM = 22.37 Pa |
| 125 Pa = 2,837 CFM | 1,500 CFM = 34.95 Pa |
| 150 Pa = 3,107 CFM | 1,800 CFM = 50.33 Pa |
| 200 Pa = 3,588 CFM | 2,000 CFM = 62.14 Pa |
| 250 Pa = 4,012 CFM | 2,500 CFM = 97.10 Pa |
| 300 Pa = 4,394 CFM | 3,000 CFM = 139.82 Pa |
| 400 Pa = 5,074 CFM | 3,500 CFM = 190.31 Pa |
| 500 Pa = 5,673 CFM | 4,000 CFM = 248.57 Pa |
| 600 Pa = 6,215 CFM | 5,000 CFM = 388.39 Pa |
| 750 Pa = 6,948 CFM | 6,000 CFM = 559.28 Pa |
| 1,000 Pa = 8,023 CFM | 7,000 CFM = 761.24 Pa |
| 1,250 Pa = 8,970 CFM | 8,000 CFM = 994.27 Pa |
| 1,500 Pa = 9,826 CFM | 9,000 CFM = 1,258.37 Pa |
| 2,000 Pa = 11,346 CFM | 10,000 CFM = 1,553.55 Pa |
| 2,500 Pa = 12,686 CFM | 12,000 CFM = 2,237.11 Pa |
| 3,000 Pa = 13,896 CFM | 14,000 CFM = 3,044.95 Pa |
| 4,000 Pa = 16,046 CFM | 16,000 CFM = 3,977.08 Pa |
| Formulas: Q = A * √(2P / rho) and P = (Q / A)2 * rho / 2. Assumes Cd = 1, rho = 1.204 kg/m3, A = 1 sq ft. | |
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Pa to CFM Formula
This ideal, nearly incompressible airflow estimate uses velocity pressure (total minus static) or an appropriate opening pressure difference with an effective coefficient. It is not a direct pressure-to-flow unit conversion:
Where Q is the volumetric flow rate (m3/s, then converted to CFM), Cd is the discharge coefficient (1.0 for an ideal open duct, typically 0.6 to 0.65 for a sharp-edged orifice), A is the cross-sectional area of the opening (m2), delta P is the pressure differential in pascals, and rho is air density in kg/m3. At standard conditions (20 °C, sea level), rho = 1.204 kg/m3.
To convert the result from m3/s to CFM, multiply by 2,118.88. The calculator above handles all unit conversions automatically.
What is Pa to CFM?
Pa to CFM converts a pressure reading (pascals) into an airflow rate (Cubic Feet per Minute) across a known opening area. Pascals measure the force per unit area driving air through a system. CFM measures the volume of air passing a point each minute. The two are related through the area of the duct or opening and the density of the air.
Use suitable average velocity-pressure measurements or a known opening with an appropriate coefficient, area and actual air density. Arbitrary fan or duct static pressure, or a pressure drop across an unknown filter or system, is insufficient. The simple model omits fan/system curves, approach/beta corrections, gas expansion and compressibility; it is not a calibrated meter or a compressed-gas/choked-flow model.
Pressure Types in Airflow Systems
HVAC and ventilation systems reference three distinct pressure measurements. Static pressure (SP) is the outward force air exerts on duct walls, independent of airflow direction. Velocity pressure (VP) is the kinetic energy component, proportional to the square of air speed. Total pressure (TP) equals static plus velocity pressure. When using this calculator, clarify which pressure type your measurement represents. A pitot-static measurement gives the difference between total and static pressure; a traverse may be needed to determine average velocity. A manometer tap on the side of a duct gives static pressure, which cannot by itself establish airflow.
For reference: 1 inch of water column (in. wc) equals 249.09 Pa. Most residential HVAC systems operate between 25 and 100 Pa (0.1 to 0.4 in. wc) of static pressure. Commercial systems often run at 250 to 750 Pa (1 to 3 in. wc).
Air Density by Temperature and Altitude
The formula above assumes standard air density of 1.204 kg/m3 (0.075 lb/ft3) at 20 °C and sea level. Real conditions rarely match this standard exactly. As temperature rises or altitude increases, air becomes less dense, meaning a given pressure differential produces more volumetric flow (higher CFM) but carries fewer pounds of air per minute. The tables below show how air density shifts.
| Temperature | Density (kg/m3) | Density (lb/ft3) | Change vs 20 °C |
|---|---|---|---|
| -10 °C (14 °F) | 1.341 | 0.0837 | +11.4% |
| 0 °C (32 °F) | 1.292 | 0.0807 | +7.3% |
| 10 °C (50 °F) | 1.247 | 0.0778 | +3.5% |
| 20 °C (68 °F) | 1.204 | 0.0752 | Reference |
| 30 °C (86 °F) | 1.164 | 0.0727 | -3.3% |
| 40 °C (104 °F) | 1.127 | 0.0704 | -6.4% |
| 50 °C (122 °F) | 1.092 | 0.0682 | -9.3% |
| Altitude | Density (kg/m3) | Correction Factor |
|---|---|---|
| Sea Level (0 ft) | 1.204 | 1.000 |
| 1,000 ft (305 m) | 1.161 | 0.964 |
| 2,000 ft (610 m) | 1.120 | 0.930 |
| 3,000 ft (914 m) | 1.079 | 0.896 |
| 4,000 ft (1,219 m) | 1.040 | 0.864 |
| 5,000 ft (1,524 m) | 1.002 | 0.832 |
| 6,000 ft (1,829 m) | 0.965 | 0.801 |
| 7,000 ft (2,134 m) | 0.929 | 0.772 |
| To adjust, replace rho in the formula with the density for your conditions. | ||
SCFM vs ACFM
Standard CFM (SCFM) is referenced to standard conditions: 68 F (20 C), 14.696 psia, and 0% relative humidity. Actual CFM (ACFM) is the volume at the real temperature, pressure, and humidity of the system. At sea level and 68 F, SCFM equals ACFM. At higher altitudes or temperatures, ACFM is larger than SCFM for the same mass flow rate because the air is less dense and occupies more volume. The conversion is: ACFM = SCFM * (Pstandard / Pactual) * (Tactual / Tstandard), where temperatures are absolute (Kelvin, or Rankine = °F + 459.67) and pressures are absolute. The reference conditions must be declared because SCFM conventions vary.
The modeled output is actual CFM at the supplied air density. For the same dry-air mass flow, reference CFM = ACFM × (actual density / reference density), so multiply by the density ratio rather than dividing. The calculator’s disclosed reference density is 1.204 kg/m³; other SCFM reference conditions differ. Temperature/altitude mode estimates dry air from a standard-atmosphere pressure profile and does not measure weather, humidity or duct absolute pressure.
Estimate Compressed Air Flow Through an Opening
Choose Compressed air to open air for a known round opening venting to the atmosphere, such as a leak with a known equivalent diameter. Enter the supply gauge pressure and opening diameter. Optional settings let you replace the default discharge coefficient of 0.61, air temperature of 20 °C and atmospheric pressure of 101.325 kPa. A pressure reading alone does not determine compressor capacity. For rated flow at a given pressure, use the compressor manufacturer’s data.
The estimate uses the ideal-gas flow relation for dry air with an effective discharge coefficient. Set P₁ equal to gauge pressure plus atmospheric pressure and P₂ equal to atmospheric pressure, both in pascals absolute. Let r = P₂/P₁, A = πd²/4 in square metres, T₁ be the upstream temperature in kelvins, R = 287.05 J/(kg·K), γ = 1.4, and Cd be the opening’s discharge coefficient.
Mass flow in kg/s is ṁ = Cd × A × P₁ × F / √(R × T₁). Above the critical ratio r ≈ 0.5283, flow is subsonic and F = √[(2γ/(γ − 1)) × (r2/γ − r(γ + 1)/γ)]. At or below the critical ratio, flow is choked in the model and F = √γ × [2/(γ + 1)](γ + 1)/(2(γ − 1)). Divide mass flow by dry-air density at 20 °C and 101.325 kPa, then multiply by 2,118.88 to report standard cubic feet per minute (SCFM). Standard conditions vary between references, so match the reference before comparing results.
Example: At 100 psi gauge and 20 °C, a 1/8 in opening with Cd = 0.61 vents to 101.325 kPa atmospheric pressure. Upstream absolute pressure is about 791 kPa, so r ≈ 0.128 and the model predicts choked flow. The estimate is about 15.9 SCFM. The result depends on the opening shape and coefficient. Use measured data for installed equipment, and a different model for restrictions that discharge into a pressurized line.
References: NASA Glenn mass-flow equations and the U.S. Department of Energy compressed air leak guide for the sharp-edged opening estimate.
Common Duct Sizes and Typical Airflow
The table below gives the cross-sectional area for standard round duct sizes and the resulting CFM at a velocity of 900 fpm (a common residential design velocity) and 1,500 fpm (typical commercial design velocity).
| Duct Diameter | Area (sq ft) | CFM at 900 fpm | CFM at 1,500 fpm |
|---|---|---|---|
| 4 in | 0.087 | 79 | 131 |
| 6 in | 0.196 | 177 | 295 |
| 8 in | 0.349 | 314 | 524 |
| 10 in | 0.545 | 491 | 818 |
| 12 in | 0.785 | 707 | 1,178 |
| 14 in | 1.069 | 962 | 1,604 |
| 16 in | 1.396 | 1,257 | 2,094 |
| 18 in | 1.767 | 1,590 | 2,651 |
| 20 in | 2.182 | 1,963 | 3,272 |
| 24 in | 3.142 | 2,827 | 4,712 |
| Illustrative area × velocity examples only. Actual duct design requires the applicable noise, loss, balancing and system-performance criteria. | |||
How to Calculate Pa to CFM
Ideal example: A 12-inch round duct has a representative average velocity pressure (total minus static) of 250 Pa. With dry-air density 1.204 kg/m³ and Cd = 1, what is the modeled actual airflow?
- Calculate the duct area: A = π × (0.5 ft)² = 0.785398 ft² = 0.072965877 m².
- Apply the ideal formula: Q = 1 × 0.072965877 × √(2 × 250 / 1.204) ≈ 1.486934 m³/s; modeled mean velocity ≈ 20.378479 m/s.
- Convert to actual CFM: 1.486933566 × 2,118.880003 ≈ 3,150.63 ACFM.
- Cross-check: 3,150.6338 / 0.785398 ≈ 4,011.51 fpm. This arithmetic does not establish suitability, acceptable noise, pressure losses or fan performance for any application.
At velocity pressures of 25 to 75 Pa, the same ideal 12-inch round section gives approximately 996.32 to 1,725.67 ACFM under the stated density and coefficient. The 1 ft² conversion table uses a larger area. These are model examples, not residential duct-design recommendations.
