How do you calculate cross sectional area in rebar?

Most rebar (steel reinforcement bar) is approximately round.

Measure the diameter of the bar, that is, the thickness of the bar.

Divide the measurement by 2 to get the radius.

Use the formula pi x r2 (3.14 x radius x radius) to get the cross sectional area. The formula works for all measurement units whether they are inches, millimeters, centimeters or others.

An alternative is to use (pi x d^2)/4. (3.14x diameter x diameter divided by 4) You do not then need to divide the original diameter.