Estimate compressive strain from stress, or pad compression, stiffness, load and Young’s modulus using a simplified shape-factor model and converted SI or US inputs.

Small-strain shape-factor approximation for nearly incompressible rubber with bonded or nonslipping loaded faces.

Loaded face area ÷ free-to-bulge lateral area.

Rubber Deflection Formula

The following equation estimates rubber compressive strain using a simplified small-strain shape-factor stiffness approximation. It assumes nearly incompressible rubber with bonded or nonslipping loaded faces and free lateral bulging. It omits finite bulk compressibility, detailed shape effects, creep, temperature and nonlinear behavior; use validated material/load-deflection data for design.

D = L / [Y × (1 + 2 × f²)]
  • Where D is the rubber compressive strain (dimensionless; often reported as % by multiplying by 100). Axial deflection (length) = D × thickness.
  • Y is the underlying material Young’s modulus, in the same stress unit as L (for example, MPa or PSI). Do not apply the shape correction twice by entering an already corrected apparent modulus.
  • f is the shape factor
    • The shape factor is calculated by dividing the compressed (loaded) area by the area that is able to bulge (free area)
  • L is compressive stress, in the same unit as Y. In pad modes, L = total load / (number of identical parallel pads x loaded area per pad).

To calculate rubber deflection (strain), divide the stress by the product of Young’s modulus times (1 plus 2 times the shape factor squared).

What is Rubber Deflection?

Definition:

Rubber deflection is the deformation of a rubber component under an applied load. It may be expressed as a displacement (for example, inches or millimeters of compression) or as strain (dimensionless, often reported as a percent).

When a rubber part is loaded in tension or compression, the applied stress causes strain (a change in length divided by the original length). In simple linear approximations, stress and strain are related by an elastic modulus, but real rubber is often non-linear and its stiffness depends on strain level, temperature, and formulation.

For rubber pads in compression, geometry matters because rubber tends to bulge sideways. The shape factor (loaded area divided by free-to-bulge area) is a common way to account for how constrained the bulging is: higher shape factor generally means higher apparent compressive stiffness and therefore less deflection for the same stress.

Rubber can have relatively high tensile strength and still stretch significantly because tensile strength describes failure resistance, while stiffness/deflection is governed primarily by elastic modulus.

How to Calculate Rubber Deflection

To calculate rubber deflection, take the shape factor raised to the power of 2, multiply by 2, then add 1.

Next, multiply the result from above by the Young’s modulus of the rubber.

Finally, divide compressive stress by the result above, using consistent units. Multiply strain by 100 for percent, or by original thickness for axial compression. The calculator offers direct strain, compression, total-load and material-modulus modes. Identical pads are parallel supports sharing load equally, not stacked layers.