ToolnozaQuick answer: the air tank fill time formula
For a constant-temperature ideal-gas estimate, fill time t = V × (P₂ − P₁) ÷ (P₀ × Q). V is receiver volume, P₁ and P₂ are starting and target pressures, P₀ is the pressure used to reference free-air flow, and Q is delivered airflow referenced to that same condition. Use the same pressure unit throughout and matching volume units for V and Q. If V is in liters and Q in liters per minute, t is in minutes.
With no air consumption during filling, set demand to zero. With consumption, use net flow Q = Qcompressor − Qdemand, with both flows referenced to the same conditions. Toolnoza’s dedicated calculator lets you enter the ambient absolute pressure instead of fixing it at 1 bar. It estimates filling time under constant-temperature, steady-flow assumptions, not vessel design.
Use the air tank fill time calculator
Open the Compressor Tank Fill Time Calculator using the button above. Enter receiver volume in liters, starting and target gauge pressure in bar, compressor FAD in L/min, air consumption during filling in L/min FAD and ambient absolute pressure in bar. For the worked example, enter 50, 6, 8, 150, 0 and 1 respectively.
Read the estimated time in minutes and seconds. The tool uses net flow after subtracting consumption; compressor delivery must exceed demand. Convert gallon volumes, psi pressures or CFM flows before entering them in the fields labeled liters, bar and L/min.
Worked example: a 50-liter tank from 6 to 8 bar
Assume a 50 L receiver, starting pressure of 6 bar gauge, target pressure of 8 bar gauge and constant delivered flow of 150 L/min. The pressure increase is 8 − 6 = 2 bar. Approximate additional free-air volume is 50 × 2 ÷ 1 = 100 L.
Fill time is 100 ÷ 150 = 0.6667 minutes, or about 40 seconds. This is the time to raise pressure from 6 to 8 bar, not to fill from atmospheric pressure. From 0 to 8 bar gauge, the same idealized calculation gives 400 ÷ 150 = 2.67 minutes, or 160 seconds.
Gauge pressure, absolute pressure and reference pressure
Gauge pressure measures pressure above local atmospheric pressure. Absolute pressure includes atmospheric pressure. When the same atmospheric reference applies to both endpoints, subtracting two gauge readings gives the same pressure increase as subtracting the corresponding absolute pressures. Do not mix a gauge starting value with an absolute target value.
The denominator P₀ is an absolute reference pressure. It does not disappear in the general formula. The worked example assumes 1 bar; entering 1.01325 bar would give a slightly different result. High-altitude or temperature-sensitive applications need reference conditions that match the compressor data.
FAD, CFM and unit conversions
Use delivered airflow at the relevant operating pressure, rather than piston displacement. FAD is free air delivery; SCFM refers to flow stated at defined standard conditions. Check the manufacturer’s pressure rating and reference temperature and pressure before comparing flow values. A rating at a lower pressure may overstate flow available during the final part of filling.
1 cubic foot = 28.316846592 liters, so 5 CFM converts to about 141.58 L/min when the flow reference is consistent. One US gallon is 3.785411784 L; one imperial gallon is 4.54609 L. Convert psi to bar using approximately 0.0689476 bar per psi. Do not enter gallon or psi numbers directly in fields labeled liters and bar.
Why the real fill time can be longer
Delivered flow can decrease as receiver pressure rises. Air demand, leaks, compressor unloading and duty-cycle limits also reduce net air entering the receiver. Hot compressed air may reach the target pressure sooner and then lose pressure as it cools. The simple calculation assumes constant temperature, no air draw and continuous steady delivery.
For an approximate demand adjustment, replace Q with net flow Qcompressor − Qdemand, using flow values referenced to the same conditions. If demand equals or exceeds delivery, the target cannot be reached under those conditions. For accurate commissioning, compare a measured pressure-rise interval with the estimate and use the manufacturer’s performance data.
Fill time and receiver sizing are different questions
Fill time describes how long an existing tank takes to reach a target pressure. Receiver sizing considers storage, demand peaks, usable pressure range and compressor cycling. A shorter fill-time estimate does not establish that a tank or compressor is suitable for the installation.
Stay within the manufacturer’s pressure and duty-cycle limits. This guide and calculator do not authorize changing pressure-switch or relief-valve settings. The technical reference below explains compressed-air receiver behavior and flow conventions; the numerical example here is an ideal-gas calculation.