Bond Strength Calculator
Estimate the shear load a lap joint can carry from its bonded area and the adhesive’s shear strength — or find the minimum area needed to hold a target load.
Estimated maximum load
This is the ideal shear capacity (load = area × shear strength). Real joints fail below it because stress concentrates at the overlap ends and peel forces reduce strength — design in a generous safety factor.
Reading a lap-shear rating
When an adhesive is rated at, say, 20 MPa lap-shear, it means each square millimetre of bond can carry about 20 newtons of force acting along the joint before it fails. Multiply by the bonded area and you get the joint’s ideal shear capacity; divide a required load by the rating and you get the smallest bond that could, in theory, hold it. That’s the whole relationship this calculator captures.
The word to keep in mind is ideal. The rating is measured on a standard test coupon under near- perfect conditions. Your surface prep, cure, temperature, and joint geometry all pull the real number down, and stress bunches up at the overlap ends rather than sharing evenly. Treat the result as a ceiling to design comfortably under, not a target to run right up to.
Frequently Asked Questions
How is the maximum load calculated?
For a lap-shear joint, load equals bonded area times shear strength. Because 1 MPa is exactly 1 N/mm², a 500 mm² bond with an adhesive rated at 20 MPa works out to 500 × 20 = 10,000 N, which is about 1,020 kgf. The reverse mode just divides the target load by the shear strength to get the minimum area.
Where do I find the adhesive's shear strength?
From the product's technical data sheet, usually quoted as lap-shear strength in MPa (or psi) on a stated substrate. It varies widely — a few MPa for some flexible adhesives up to 20–30 MPa or more for structural epoxies and acrylics — so use the figure for your actual adhesive and substrate.
Why will a real joint fail below this number?
The formula assumes stress spreads evenly across the whole bond, but in a real single-lap joint it concentrates at the two ends of the overlap, and any bending adds peel forces the adhesive resists poorly. So the calculated figure is an upper bound on ideal shear capacity, not a guaranteed strength — design with a safety factor.
What is kgf and why show it?
Kilogram-force is the pull of gravity on that many kilograms — 1 kgf ≈ 9.81 N. It's an intuitive way to picture a load: 10,000 N is roughly the weight of a one-tonne object. It's shown alongside newtons for a quick sense of scale, not as an engineering unit.
Does this work for peel or cleavage loads?
No. This is a shear model — forces acting along the plane of the bond. Adhesives are far weaker in peel and cleavage, where the load tries to pry the joint apart from an edge. Joints carrying those loads need a different analysis and usually a much larger safety margin.
Estimates for planning — not professional engineering advice; a load-bearing joint should be validated by a qualified engineer and against the adhesive’s technical data sheet.