Compressed Air Pipe Size Calculator: Shop Line Pressure Drop

Plumbing a garage or shop for compressed air? The pipe size decides how much of your compressor's pressure actually reaches the drops. Enter the airflow, pressure and length of the run, add the elbows and tees, and we'll show the pressure drop and air velocity for every common steel and copper size, then pick the smallest one that hits your target. Pressure drop falls with roughly the fifth power of the bore, so going up one pipe size usually cuts the drop by more than half.

How the calculator works

It uses the Harris formula, a long-standing empirical equation for pressure loss in compressed air lines:

Pressure drop (PSI) = 0.1025 × L × Q² ÷ (3600 × r × d^5.31)

• L is the equivalent length in feet: the straight run plus an allowance for fittings. Each 90° elbow counts as 30 pipe diameters and each tee taken through the branch as 60 pipe diameters, which are standard textbook allowances.
• Q is the flow in CFM of free air, the figure on your compressor's spec sheet. Dividing Q² by 3600 converts it from cubic feet per minute to cubic feet per second, the unit the formula was built on.
• r is the compression ratio, (line PSI + 14.7) ÷ 14.7. Air at higher pressure is denser and slower, so it loses less pressure in the same pipe.
• d is the actual inside diameter in inches, which is not the same as the nominal size. A 3/4" Schedule 40 steel pipe measures 0.824" inside; 3/4" Type L copper is 0.785".

The 0.1025 coefficient was fitted to steel pipe. Copper has a smoother bore, so the copper figures here run slightly on the safe side.

Velocity is the compressed volume flow divided by the bore area. Fast air drags water past your drop legs and into tools, which is why many shop layouts aim to keep main-line velocity at or below roughly 20 to 30 feet per second, and the table flags anything above 30.

Practical tips for plumbing a shop air system

• Size the main for the compressor, not today's tool. The main line should carry the full output of the compressor, and of any bigger compressor you might buy later. Pipe is cheap to oversize now and expensive to replace.
• A loop beats a dead-end run. When the main runs in a ring, air reaches each drop from two directions, so the real drop is lower than this calculator shows for a single run. Treat the result as conservative for a loop.
• Take drops off the top of the main with a gooseneck so water running along the bottom of the pipe doesn't fall into the drop, and slope the main slightly away from the compressor toward a drain.
• Do not use PVC. Ordinary PVC water pipe is not made for compressed air and can shatter into sharp fragments if it fails. Use steel, copper or a purpose-made aluminum or plastic air piping system rated for the pressure.
• The hose is part of the system. A well-sized pipe can still be wasted by a long, narrow hose at the drop. Check it with our air hose pressure drop calculator, and confirm your total demand with the air compressor size calculator.

Results are estimates for clean, straight pipe at room temperature. Old, rusty steel pipe, quick couplers, filters, regulators and dryers all add their own losses on top, so leave some margin.