Enter the total rod length and the length of the crank stroke into the calculator to determine the rod radio.

Rod Ratio Formula

The following formula is used to calculate the rod ratio of a crankshaft.

RR = PR / CS

  • Where RR is the rod ratio
  • PR is the piston rod length
  • CS is the crank shaft stroke length

Rod Ratio Definition

What is a rod ratio?

A rod ratio is a ratio of the length of a piston rod to the stroke of an engine.

Rod ratios are used to determine the performance or leverage a piston rod has with respect to the crankshaft stroke.

What is a good rod ratio?

Anything above a ratio of 1.57 is considered a good rod ratio, while high-end racing cars can have ratios of 1.75+

How to calculate a rod ratio?

Example Problem:

The following example is a step-by-step guide on how to calculate rod ratio.

First, determine the total length of the piston rod. In this example, the piston rode measures a total of 5 inches.

Next, determine the stroke length of the crankshaft. In this case, the stroke length is measured to be 3 inches.

Finally, calculate the rod ratio using the formula above:


RR = PR / CS

RR = 5/3

RR = 1.6666