Vacuum Pump-Down Time Calculator
Estimate how long it takes to evacuate a chamber, tank or system — or size the vacuum pump you need to hit a target pressure within a set time.
The pump-down equation
In the rough vacuum range the gas in a chamber behaves like a well-mixed volume being drawn off at a constant volumetric rate. The pressure then falls exponentially:
Solving for time and pumping speed:
- V — chamber plus piping volume
- S — effective pumping speed at the chamber (pump speed reduced by line conductance)
- p₀, p₁ — start and target absolute pressure
- pᵤ — ultimate pressure of the pump; the chamber can never go below it
The quantity V/S is the system's time constant τ. Each time constant reduces the pressure by a factor of e (≈2.72); going from atmosphere to 1 mbar takes about 6.9 τ.
Getting realistic results
- Account for piping. A long, narrow line can cut the effective speed dramatically. Use the vacuum line sizing calculator to find the effective speed first.
- Check the pump curve. Rotary vane and scroll pumps lose speed below ~1 mbar; Roots blowers only become effective below ~10–50 mbar. See vacuum pump types.
- Moisture dominates below 20 mbar. Water evaporating from wet surfaces can hold the pressure near its vapor pressure for a long time.
References & further reading
- J. F. O’Hanlon & T. A. Gessert. A User’s Guide to Vacuum Technology, 4th ed.. Wiley (2023). Practical design of vacuum systems, pump-down and outgassing.
- K. Jousten (ed.). Handbook of Vacuum Technology, 2nd ed.. Wiley-VCH (2016). Standard reference on gas flow, conductance, pumps and gauges.
- CERN Accelerator School. CAS Vacuum in Accelerators, Platja d’Aro, Spain, 2006 — proceedings (CERN-2007-003). CERN (2007), free to read. Lectures on gas dynamics, conductance, pumps and outgassing.
Frequently asked questions
How do you calculate vacuum pump-down time?
For the rough vacuum range with a constant pumping speed, t = (V / S) × ln((p₀ − pᵤ) / (p₁ − pᵤ)), where V is the chamber volume, S the effective pumping speed, p₀ the start pressure, p₁ the target pressure and pᵤ the pump’s ultimate pressure. Use consistent units, e.g. V in m³ and S in m³/s gives t in seconds.
How long does it take to pump a 100 litre chamber down to 1 mbar?
With a 10 m³/h pump and no leaks or outgassing: t = (0.1 m³ / 10 m³/h) × ln(1013 / 1) ≈ 0.069 h ≈ 4.1 minutes. In practice allow 25–50 % more for pipe losses and falling pump speed near ultimate pressure.
What size vacuum pump do I need?
Rearrange the pump-down equation: S = (V / t) × ln(p₀ / p₁). To evacuate 1 m³ from atmosphere to 1 mbar in 10 minutes you need S = (1 / 600 s) × ln(1013) ≈ 0.0115 m³/s ≈ 41.5 m³/h effective speed, before applying a safety factor. Choose a pump whose speed curve still delivers that at your working pressure.
Why is real pump-down slower than calculated?
Three main reasons: (1) pumping speed falls as you approach the pump’s ultimate pressure, (2) pipes and valves have finite conductance, reducing the effective speed at the chamber, and (3) below about 1 mbar, gas desorbing from surfaces (outgassing, especially water vapour) and small leaks add load. That is why the calculator offers a correction factor.
Does this work for high vacuum?
The formula is reliable in the rough vacuum range (atmosphere down to ~1 mbar). In high vacuum, pump-down is dominated by outgassing rather than volume, and time depends on surface area, materials, cleanliness and bake-out. Use this tool for the roughing stage and allow for outgassing separately.
Related tools & resources
Vacuum Unit Converter
Convert between inHg, psi, kPa, mbar, Torr, microns and % vacuum — absolute and gauge.
Vacuum Line Sizing & Conductance
Pipe conductance, effective pumping speed, gas velocity and minimum line diameter.
Leak Rate & Gas Flow Converter
mbar·L/s, Pa·m³/s, Torr·L/s, atm·cc/s, sccm and slm — plus leak-tightness classes.
Vacuum Pump Types
Rotary vane, scroll, screw, Roots, liquid ring, turbo, diffusion, cryo and ion pumps compared.