A turbo's trim sizing is calculated by a ratio of measurements from the turbocharger's compressor wheel. The trim spec of a turbo can be a useful measurement in determining the turbo's airflow capability, as generally a higher trim level equates to a higher flow rate. As manufacturers often use the same turbocharger housing on multiple turbos, calculating your turbocharger's trim level is also a way to determine the model and spec of an unknown turbo.
Measure the compressor wheel's inducer diameter with a tape measure. This is the wheel on the intake side of the turbo, also known as the "cold side." The inducer diameter is that of the wheel's smaller blades, by which air is drawn in. Record this measurement in millimeters, as this is the universal measurement for trim levels.
Measure the exducer diameter of the compression wheel. This is the larger portion at the rear of the wheel. Record this measurement in millimeters.
Calculate your turbo's trim spec by squaring the inducer diameter, then dividing it by the square of the exducer diameter. Multiply the result by 100. The product is your turbocharger's trim in millimeters.
