--- title: "Compressed air CFM calculator: tubing flow and pressure drop" canonical: "https://yuannaitube.com/compressed-air-flow-calculator/" language: "en-US" --- TOOLS # Compressed air CFM calculator: tubing flow and pressure drop Enter your tubing size, length, supply pressure and air consumption. The calculator returns the air velocity, the pressure drop and the pressure left at the end of the line, and it works backward to the largest flow your tubing can carry within a pressure drop you set. Flow is entered as free air (the volume at atmospheric pressure), the way compressors and air tools are rated. ## Maximum free-air flow for a 0.5 bar pressure drop Supply pressure 7 bar gauge. Calculated with the same method as the tool above. | Tubing size | ID (mm) | L/min at 3 m | L/min at 10 m | cfm at 10 m | | --- | --- | --- | --- | --- | | 4 × 2.5 mm | 2.5 | 30 | 16 | 0.6 | | 6 × 4 mm | 4 | 106 | 55 | 2.0 | | 8 × 5 mm | 5 | 194 | 101 | 3.6 | | 8 × 5.5 mm | 5.5 | 251 | 131 | 4.6 | | 8 × 6 mm | 6 | 318 | 166 | 5.9 | | 10 × 6.5 mm | 6.5 | 395 | 206 | 7.3 | | 10 × 8 mm | 8 | 692 | 361 | 12.8 | | 12 × 8 mm | 8 | 692 | 361 | 12.8 | | 14 × 10 mm | 10 | 1,265 | 660 | 23.3 | | 16 × 12 mm | 12 | 2,071 | 1,080 | 38.2 | | 1/8 in | 1.58 | 9 | 5 | 0.2 | | 1/4 in | 4.35 | 133 | 70 | 2.5 | | 3/8 in | 6.35 | 371 | 193 | 6.8 | | 1/2 in | 9.6 | 1,133 | 591 | 20.9 | ## How the calculator works Two checks are applied to the same line. ### Air velocity Free-air flow is converted to the volume at line pressure with Boyle’s law, using the ratio of atmospheric pressure to absolute line pressure, and divided by the inner cross-section of the tube. This is the method used in the [Oak Ridge National Laboratory MEASUR air velocity calculator]() . Atmospheric pressure is taken as 1.01325 bar absolute. ### Pressure drop Pressure drop uses the empirical compressed air formula published by Engineering ToolBox: **dp = 7.57 · q1.85 · L · 104 / (d5 · p)** where dp is the pressure drop (kg/cm²), q is the air flow at atmospheric conditions (m³/min), L is the pipe length (m), d is the inside diameter (mm) and p is the initial gauge pressure (kg/cm²). The source notes that the single formula is intended for drops below about 1 kg/cm² (14–15 psi) and recommends dividing longer or higher-drop lines into parts, using each part’s end pressure as the start pressure of the next. The calculator does this automatically with 100 equal sections, so large drops are handled the way the source describes. See the [Engineering ToolBox compressed air pressure drop page]() . The formula is a general empirical relation for compressed air lines, not a measurement of a particular tube. Fittings, valves and tight bends add loss on top of the tube itself, and temperature is not an input. Use the result to size a line and compare options; for a critical circuit, send us the layout and we will check it against the data sheet values. ## Reading the results ### Velocity The Oak Ridge guidance for compressed air piping is about 20 ft/s (6 m/s) for main headers, no more than 30 ft/s (9 m/s) in branches, and an upper limit of 50 ft/s (15 m/s) for line drops and feed lines. These figures are written for plant piping. Short tubing to a single cylinder or tool can run above them; for a continuous high-flow run, choose a larger bore. ### Pressure drop A smaller bore costs far more pressure than a longer line does: in the formula, drop rises with the 1.85th power of flow, falls with the fifth power of the inner diameter, and grows only in proportion to length. Going from a 4 mm to a 6 mm inner diameter cuts the drop for the same flow by about 87 percent, because (4/6)5 is about 0.13. Compare sizes with the same pressure and flow before you choose. ## Choosing the tubing Inner diameters in the size list are the values on our PA6, PA12 and PU data sheets. For other grades enter the inner diameter from the data sheet. Working pressure, temperature and bend radius are separate checks: see the [tubing size chart]() for every size we list with its rated pressure. - [PA6 nylon tubing]() : the highest pressure ratings in the range on most sizes. - [PA12 nylon tubing]() : stable dimensions in wet service. - [Polyether tubing]() : flexible polyurethane for humid and washdown lines. - [PU tubing]() : the tightest bend radii, for moving machinery. ## Can you convert psi to cfm? No. Pressure (psi or bar) and flow (cfm or L/min) are different quantities, so there is no direct conversion between them. The flow a line carries depends on the pressure difference across it, the tube bore and length, and the restrictions in the circuit. That is why this calculator asks for all four inputs. To convert units of the same kind, use the [tubing unit converter]() : 1 cfm = 28.3168 L/min, and 1 bar = 14.5038 psi. ## FAQ What is free air flow (FAD, SCFM)? It is the air volume measured at atmospheric conditions before compression. Compressor output and air-tool consumption are normally quoted this way, so enter consumption as free air. Inside the line the same mass of air occupies a smaller volume, which the calculator accounts for when it computes velocity. What pressure drop is acceptable in a tubing line? The Engineering ToolBox page treats drops above about 1 kg/cm² (14–15 psi) as outside the range of the simple formula. The right limit for your machine is the pressure the tool or cylinder needs at the end of the line, so set the supply pressure and the allowed drop from that requirement. Does the result include fittings and valves? No. The calculation covers the straight tube. Push-in fittings, valves, filters and bends each add resistance, so leave margin when a line has several of them. Can I use this for tubing other than PA6, PA12 and PU? Yes. Select Custom or type any inner diameter in millimeters. The calculation depends on the inner diameter, not on the tube material. ## Related pages - [Tubing size chart]() : OD, ID, wall and working pressure for every size. - [Tubing unit converter]() : bar, psi, millimeters and inches. - [Pneumatic tubing guide]() : choosing material and size for air lines. - [Data sheets]() : rated values as printed. Need a line sized or checked? [Send your layout and requirements]() .