Flexural strength is a measurement that indicates a material's resistance to deforming when it is placed under a load. The values needed to calculate flexural strength are measured by experimentation, with rectangular samples of the material placed under load in a 3- or 4-point testing setup. (A 3-point setup is shown in the accompanying photo.)

Gather the following data from the load experiment:

Maximum applied load, which we will call "P";

Material span length between points in the test setup, which we will call "L";

Width of the material specimen, which we will call "b"; and

Average depth of the specimen, which we will call "d."

Convert all measurements to the following units:

Maximum applied load in pounds;

Material span length in inches;

Width of the material specimen in inches; and

Average depth of the specimen in inches.

Substitute the numerical values for this data in the following equation for calculating flexural strength:

R = P_L/b_d^2

where R = Flexural strength, in units of lbs. per square inch, and d^2 indicates the quantity d squared.

To further clarify, multiply P by L and then divide that quantity by the quantity of b times d squared. The result is the calculated flexural strength.

#### Tip

During experimentation, the flexural yield strength is usually reported instead of flexural strength for materials that do not crack under the loads placed upon them in the flexure test.