Chamber balance
The equation V·dp/dt = Q − S·p is solved, so outgassing, leakage and process gas affect both time and attainable pressure.
Pumping system sizing
Vacuum pump sizing calculator
Preliminary sizing of actual pump speed from chamber data, process time, gas load, connection conductance and catalogue data.
Engineering model
Choose how to size the pump
Simple mode estimates the load from chamber material and seals. Advanced mode accepts measured system parameters and catalogue data.
Orange indicates a seal:—
Suggested system type—
Estimated assumptions
Estimated internal surface area— m²
Estimated seal length— m
Total gas load— mbar·L/s
Backing / roughing pump
Minimum nominal speed— m³/h
Connection conductance at stage endpoint— L/s
High-vacuum pump
Minimum nominal speed— L/s
Connection conductance at stage endpoint— L/s
Commercially available models
Example pumps meeting the calculated minimum
The list is filtered by nominal speed, ultimate pressure and cleanliness requirements.
Backing pump
High-vacuum pump
These are preliminary sizing examples, not a commercial offer. Before purchase, verify the exact variant, S(p) curve, connections, permissible foreline pressure and current availability.
- The connection limits flow below the required chamber speed. A larger pump alone cannot meet the requirement; increase diameter, shorten the path or add another port.
- Connection length is less than ten diameters. The calculator applied a short-port inlet-conductance correction; use catalogue conductance for valves, elbows and complex components.
- This is preliminary sizing based on typical outgassing values after about one hour of pumping. Before purchase, compare the result with the selected pump’s S(p) curve; for high vacuum, replace the estimate with a rate-of-rise measurement where possible.
Recommended architecture—
Gas balance
Outgassing load— mbar·L/s
Total gas load— mbar·L/s
Equilibrium pressure at sized speed— mbar
Recommended pump ultimate pressure— mbar
Backing / roughing pump
Required effective speed at chamber— L/s
Connection conductance at stage endpoint— L/s
Minimum nominal speed— m³/h
Flow regime at stage endpoint—
High-vacuum pump
Required effective speed at chamber— L/s
Connection conductance at stage endpoint— L/s
Minimum nominal speed— L/s
Flow regime at stage endpoint—
- Connection length is less than ten diameters. The calculator applied a short-port inlet-conductance correction; use catalogue conductance for valves, elbows and complex components.
- The connection limits flow below the required chamber speed. A larger pump alone cannot meet the requirement; increase diameter, shorten the path or add another port.
- No maximum gas throughput was entered for the high-vacuum pump. Backing-pump speed was checked only against pump-down time; Qmax/pfore from the main-pump manual must also be verified.
- In UHV, the result is governed mainly by actual outgassing, permeation, seals and bakeout procedure. Use measured gas loads for the design.
- The result is the minimum speed on the actual S(p) curve, not merely the pump nameplate value. Also verify ultimate pressure, maximum throughput, gas compatibility and permissible foreline pressure.
Calculation method
From process requirements to pump specifications
The model separates transient pump-down demand from the steady gas load that limits final pressure.
Conductance
The full Knudsen equation varies conductance with pressure and distinguishes laminar, transitional and molecular flow.
Two pumping stages
Below 10⁻³ mbar, roughing and high-vacuum stages are sized separately, and the backing pump can also be checked against main-pump maximum throughput.
Manufacturer curve
The nominal value is corrected by the actual speed fraction at the operating point, allowing comparison with a specific S(p) curve.
Technical basis
Sources and methods from vacuum-equipment manufacturers
The model is based on relationships published by Leybold, Edwards and Kurt J. Lesker. Data for a specific pump must always be confirmed in its current manufacturer manual.
Pump sizing in practice
Vacuum pump sizing: common questions
The calculator result is a starting point for selecting an architecture and catalogue models. Final sizing should account for the actual installation and manufacturer curves.
What data is needed to size a vacuum pump?
The essentials are chamber volume, initial and target pressure, required pump-down time, and connection diameter and length. A more accurate result also needs surface area, outgassing rate, leak rate and process load.
Why is nominal pump speed higher than the required chamber speed?
Pipework, valves, elbows and filters limit conductance, so only part of the pump’s nominal speed is available at the chamber. The narrower or longer the connection, the larger the difference.
Is the result sufficient to purchase a specific pump?
No. This is preliminary sizing. Before purchase, verify the S(p) curve, ultimate pressure, maximum gas throughput, permissible foreline pressure, connections, process medium and material compatibility.
When is a two-stage pumping system needed?
For high and ultra-high vacuum, the main pump, such as a turbomolecular pump, requires a backing pump. The calculator then separates evacuation from atmospheric pressure from the high-vacuum stage.