Most people sizing a vacuum gripper for the first time start in the wrong place. They ask how many cups they need before they have nailed down the four things that answer the question. Get those four right and the math is almost mechanical. Skip them and you either over build, paying for cups and plenum you do not need, or under build, shipping a tool that drops parts the first time the line runs a heavier box. Here is the actual method we use when sizing vacuum grippers, not a vague rule of thumb.
Start with Four Inputs, Not a Cup Count
Before any calculation makes sense, you need four things.
1. Workpiece mass and pick surface
How heavy is the part, and how much usable flat area do you have to place cups on?
2. Surface material
The material sets your design vacuum level. For cardboard that number is well established at a conservative -250 mbar, which works out to 2.5 N per square cm. For glass, sheet metal, plastic, or shrink wrap, do not assume a value; it varies enough that guessing is how grippers under perform in the field. Why porous surfaces cap the usable vacuum is covered in our vacuum gripper guide.
3. Pick orientation
Is the cup pulling straight up against gravity, a top lift, or holding the part against a vertical face while something else does the lifting, a side or shear pick? This changes the formula, not just the numbers.
4. What is doing the moving
A cobot and an industrial robot pull very different accelerations, roughly 1 m/s2 for a cobot versus 2 m/s2 for an industrial arm. A gripper sized safely on a cobot can fail on an industrial robot running the same part, so this is not a detail to skip. The wider payload math lives in our cobot gripper guide.
How Much Force One Cup Holds
Cup force comes down to F = delta P x A, where A is the sealed area of the cup. And here is the part people get wrong: the sealed area is not the nominal cup size, it is the actual sealing diameter, which is smaller. A 60 mm cup seals on a 38.6 mm line, not 60 mm, and the sealing diameter varies by manufacturer, so the same nominal size from a different supplier will not hold the same force. For cardboard at -250 mbar, using standard 1.5 fold bellows cups, the numbers look like this:
| Nominal Cup Size | Sealing Diameter | Theoretical Force per Cup |
|---|---|---|
| 40 mm | 26.1 mm | About 13.4 N |
| 60 mm | 38.6 mm | About 29.3 N |
| 80 mm | 56.0 mm | About 61.6 N |
Bigger cups hold more, but they also take up more real estate on the box, which matters once you get into layout. Cup and face selection by surface type is its own decision, covered in our gripping face guide.
Doing the Actual Math
For a top lift, cup count follows one formula, with gravity at 9.81 m/s2 and a standard safety factor of 2, which is industry practice and not something to shave down without a specific, justified reason.
TOP LIFT CUP COUNT
N = [ mass x (gravity + acceleration) x safety factor ] / force per cup
Worked example: a 5 kg cardboard box on a cobot at 1 m/s2, using 60 mm cups at 29.3 N each:
N = [ 5 x (9.81 + 1) x 2 ] / 29.3 = 3.7, rounded up to 4 cups
Side and shear picks
Where the cup holds the part against friction rather than pulling straight against gravity, the formula adds a friction coefficient in the denominator, roughly 0.5 for cardboard. Because friction grip is far less efficient than a direct pull, a shear pick generally needs roughly twice the cups of an equivalent top lift. If your application picks from the side, do not skip this; it is the single most common reason a gripper looks under sized once it is running.
Use the theoretical force, once
One rule worth repeating: use the theoretical force per cup in the formula, the full rated number straight from the force table, not a number someone has already reduced to be safe. The safety factor already lives in the formula. Applying it twice, once by shaving the force number and again in the formula, quietly over builds the tool and can mask a genuine sizing problem elsewhere.
Rounding Up Is Not the Last Step
Once you have a raw number, round up, but do not stop there. Offset or unbalanced loads, a part that is long or whose center of mass sits away from the middle of the cup layout, often push the real cup count higher than pure force math suggests. If the load is not centered, the moment it creates has to be resisted by cups positioned to counter it, which is frequently what pushes a design from a simple spider style tool to a larger plated gripper. The same moment logic applies to the whole tool, as covered in our EOAT design guide.
Layout Matters as Much as Count
Once you know how many cups and what size, they still need somewhere to go. As a baseline, keep at least 13.5 mm of edge to edge buffer between cups; tighter spacing down to around 6.8 mm is possible but should only happen with the customer sign off, since it starts trading off serviceability and seal reliability.
If one tool handles multiple box sizes, watch for cup rows that land on box to box seams. Those leak, and typically get valve closed rather than removed, which is fine, but budget for it: a well designed multi box tool keeps the worst case number of simultaneously exposed, unsealed cups low, generally under about 10, using check valves to manage the rest. That zoning discipline is exactly what made the multi pattern tool in our palletizing case study work across every box size on the line.
When a Simple Gripper Is Not Enough
Lighter, spider style tools are the right default for most applications; they are cheaper and simpler. But there is a point where the physics pushes you toward a larger, rigid plate style gripper instead: when the cup count climbs past roughly 8 to 10 branches, when the load is meaningfully offset or unbalanced, since flexible branch arms do not resist that moment as well as a rigid plate does, or when the tool needs to span further than a single piece gripper can reasonably cover. None of this is a judgment call made blind; it is a direct consequence of the same mass, orientation, and layout inputs you started with, and it is where our Area Surface Gripper (ASG) platform takes over from branch style tools.
The Short Version
Before you ask how many cups, answer these: how heavy is the part, what is it sitting on, is this a top lift or a side pick, and what is doing the moving. From there the force table and the formula do the rest, and rounding up plus a look at load balance gets you to a real, defensible cup count instead of a guess.
Suction Cup Sizing Questions
How many suction cups do I need to lift a box?
Run the top lift formula: mass times gravity plus acceleration, times a safety factor of 2, divided by the force per cup. A 5 kg cardboard box on a cobot with 60 mm cups works out to 3.7, so 4 cups, before checking load balance and layout.
What vacuum level should I use for cardboard?
A conservative -250 mbar, about 2.5 N per square cm, is the established design value for cardboard. Higher vacuum levels do not hold on porous board; they just pull air through it, so flow capacity matters more than depth.
Why do side picks need more suction cups?
In a side or shear pick the cups hold the part through friction instead of pulling directly against gravity, and friction grip is far less efficient. With a friction coefficient around 0.5 for cardboard, a shear pick typically needs roughly twice the cups of the same part lifted from the top.
What safety factor is used for vacuum gripper sizing?
Industry practice is a safety factor of 2, applied once, in the formula, against the theoretical force per cup. Reducing the force number and applying the factor again double counts safety and over builds the tool.
Have a Specific Part, Box, or Pick Pattern in Mind?
Send us the weight, surface material, and orientation, and our team will walk through the sizing with you.
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