Updated 19 October 2026

Snap-fit cantilever calculator: strain and force

A cantilever snap fit bends once on the way in and must neither break nor take a permanent set doing it. The number that decides that is the strain at the root of the arm. This calculator gives it from the geometry alone, and the forces once you add the modulus and friction from your material datasheet.

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How it is calculated

strain e = 1.5 x t x y / L^2 force P = b x t^2 x E x e / (6 x L) mating W = P x (μ + tan α) / (1 - μ x tan α)

• The arm is a straight cantilever of constant rectangular section, fixed at the root, deflected by the undercut depth y at its tip. These are the standard beam formulas used in the snap-fit design guides of the plastics suppliers.
• The separation force uses the same formula with the retention angle in place of α. When μ x tan of the angle reaches 1, the joint does not move at all: at the insertion angle it cannot be assembled, at the retention angle it never releases.
• E is the modulus at the strain reached. Above the proportional limit use the secant modulus from the stress-strain curve, or the force comes out too high.

Printed snap fits: design guidance

• Print the arm lying flat, so its length runs along the layers. An arm standing upright bends across the layer lines, where a printed part is weakest.
• A datasheet strain is measured on a test bar, often moulded. A printed arm in the same material can fail earlier, so test a printed sample before committing a design.
• Round the inside corner at the root. A sharp corner concentrates the strain exactly where it is already highest.
• An arm that tapers to half its root thickness at the tip takes a larger deflection for the same root strain than the straight arm calculated here, so this result is on the safe side for a tapered arm.
• Tough, ductile materials such as PETG, nylon (PA12) and PC suit snap fits better than brittle ones. See the materials pages.

From the numbers to a part

Once the strain sits inside the allowable figure, the next number is the price of the part. Upload the model and the instant quote returns a price and a delivery date in under a minute, with no account. 3Descu is an ISO 9001:2015 certified factory with 200+ printers, FDM and SLA only. A part that does not match the agreed specification is reprinted or refunded.

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