mmHg to PSI: Vacuum Cup Holding Force by Cup Size

Sizing Vacuum Cups from a Gauge Reading in mmHg

A vacuum gripper never pulls on anything. It removes air from a small volume and lets the atmosphere outside press the part against the cup. That means the whole design question — how many cups, how big, arranged how — comes down to one differential pressure and one sealed area. European cup and generator data sheets often state that differential in millimetres of mercury or in percent vacuum, while the force arithmetic on a North American drawing is done in pounds per square inch, so the two units meet on every gripper sizing sheet.

Conversion factor: 1 mmHg = 0.019336775 psi. A common design set point of 456 mmHg below atmosphere — 60 % of full vacuum — is therefore 8.82 psi, and a Ø40 mm cup with 1.95 in² of effective area holds about 17.2 lbf (76 N) before any safety factor.

What Sets the Ceiling

Atmosphere does the gripping

Even a perfect vacuum only gives you the 760 mmHg the air outside is worth — 14.70 psi at sea level, and less on a plant floor a thousand metres up. There is no such thing as a stronger vacuum.

The generator decides how close you get

A compressed-air venturi ejector typically tops out near 85 % of full vacuum, roughly 650 mmHg; an electric rotary-vane pump can reach past 90 %. The last few percent cost the most air or the most watts.

Porous parts leak by design

Corrugated board, untreated timber and sintered parts bleed air continuously, so the achievable level is set by how much flow the generator can move, not by the depth it could reach on a sealed test plate.

Only a fraction is usable

Component suppliers commonly halve the calculated force for a flat horizontal lift and quarter it when the part hangs vertically or the axis accelerates, which is where most sizing errors are caught.

Taking a Set Point Through to Cup Force

The route from a vacuum switch setting to a force figure is short, and only the first step needs this page.

Enter the vacuum level you can hold

Use the level the system actually maintains on the part, not the generator's catalogue maximum — type it into the mmHg field and the psi differential builds as you type. A comma instead of a dot is accepted, and stray spaces in the figure are dropped.

Multiply by the sealed area

Force in pounds is the psi figure times the effective area in square inches — use the sealing lip diameter from the cup drawing, not the outside diameter, because the bellows convolutions do not carry load.

Move the figure into the sizing sheet

The copy button on either field yields the number by itself, with no unit text attached, so it can be pasted straight into a spreadsheet cell that already multiplies by area and divides by the safety factor. Ctrl + C in the field works the same way.

Reverse it for an imperial data sheet

When the cup catalogue is written in psi and the vacuum switch on the machine is graduated in mercury, the ↔ button runs psi → mmHg instead. By hand that direction multiplies by 51.7149.

Remember what the gauge is telling you: a vacuum reading is how far below the surrounding air you have pulled, which is exactly the differential that presses the part on. It is not an absolute pressure, so never subtract it from atmosphere a second time when calculating force.

Vacuum Level Against Holding Force per Cup

Theoretical holding force for two common cup sizes, calculated as differential pressure times the sealed circle area — 1.95 in² for a Ø40 mm lip, 12.17 in² for a Ø100 mm lip. These are the figures before any safety factor is applied, and they assume a smooth, non-porous surface and sea-level ambient.

Vacuum below ambient	Share of full vacuum	Differential (psi)	Ø40 mm cup	Ø100 mm cup
152 mmHg	20 %	2.94 psi	25.5 N (5.7 lbf)	159 N (35.8 lbf)
304 mmHg	40 %	5.88 psi	50.9 N (11.4 lbf)	318 N (71.6 lbf)
456 mmHg	60 %	8.82 psi	76.4 N (17.2 lbf)	477 N (107.3 lbf)
608 mmHg	80 %	11.76 psi	101.9 N (22.9 lbf)	637 N (143.1 lbf)
650 mmHg	86 %	12.57 psi	108.9 N (24.5 lbf)	681 N (153.0 lbf)
760 mmHg	100 %	14.70 psi	127.3 N (28.6 lbf)	796 N (178.9 lbf)

The relationship is strictly linear in pressure but quadratic in diameter, which is why the Ø100 mm cup at 40 % out-lifts the Ø40 mm cup at full vacuum by a factor of two and a half. Chasing a deeper vacuum is nearly always the expensive way to gain force; adding cup area, or one more cup, is the cheap one.

Handy Details During Gripper Selection

Sweep a whole set-point range

Both boxes stay editable, so stepping 300, 400, 500, 600 mmHg to see where the force curve meets your part weight takes seconds.

Match the catalogue you are holding

One press of ↔ makes psi the input, the direction you need when the cup is specified imperially and the machine's vacuum switch is not.

Every unit a vacuum spec might use

The searchable lists on both sides cover kPa, mbar, inHg and the rest, which is where most vacuum component sheets live once you leave mercury behind.

Numbers your calculation can consume

Results run to eight decimals and the copy button strips the unit away, so nothing has to be retyped or cleaned up in the force spreadsheet.

Vacuum Handling Questions

How much can a 100 mm vacuum cup actually lift?

At a realistic 608 mmHg (11.76 psi) the theoretical figure is about 637 N. Halve it for a horizontal lift of a flat sheet and you have roughly 318 N of working capacity, or a quarter — about 159 N — if the part is carried on edge or the axis accelerates hard. The full-vacuum figure of 796 N is a number to design away from, not towards.

Why does a gripper never reach the full 760 mmHg?

Three things stop it. The generator has a limit of its own — a venturi ejector runs out of pumping capability around 85 %. Every joint, hose and cup lip leaks a little, and the generator has to keep replacing that air. And altitude quietly lowers the ceiling: at 1000 m the surrounding air is only about 675 mmHg, so 100 % of what is available there is already 11 % less force than at the coast.

What safety factor should be applied to a calculated cup force?

Component makers generally publish a factor of 2 for a horizontal lift where the cup face is level and the load hangs straight down, and 4 or more where the part is vertical, tilted, or moved by an axis with real acceleration — in those cases friction at the cup lip, not the vacuum, is resisting the load. Local machinery safety rules for lifting over people are stricter again and take precedence over any catalogue figure.

Why does a porous part need suction flow rather than a deeper vacuum?

Because air keeps coming through the material. The level that settles under the cup is wherever the generator's flow curve crosses the leakage rate, so a pump specified only by its ultimate vacuum can sit at 200 mmHg on cardboard while a high-flow ejector holds 450 mmHg on the same box. For porous handling, read the flow figure in the data sheet before the depth figure.

Is a vacuum chuck for machining sized the same way as a lifting cup?

The pressure arithmetic is identical, but the load case is not. A chuck clamps the workpiece down and then resists cutting forces sideways through friction, so what counts is the clamping force multiplied by the friction coefficient of the seal and surface. A 200 × 300 mm sealed pocket at 608 mmHg presses down with roughly 4.9 kN, yet only a fraction of that is available against side load — which is why deep cuts on a vacuum table are taken as light passes.

mmHg to PSI