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Parallax Error Calculator

This quick calculator shows the amount of parallax error that may occur using a fixed-parallax scope to sight onto an object placed at a known distance. 'Maximum parallax error' is in the context of the viewer setting up with their eye centered on the ocular lens -- using it as is normally intended. Strictly speaking the maximum possible error is double the number you will get below if you sight to your destination by peeking through it from the edge of your scope... which nobody actually does. For a true theoretical maximum double the returned result values.

Objective Lens Diameter

Pick a number from the drop-down. Values are in millimeters (mm)

Fixed Parallax Setting

Enter your scope's fixed parallax range value as a simple number, 1-999, without commas or decimal points.

Starting Range

Enter the starting range value to report, 1-999, without commas or decimal points.

Ending Range

Enter the ending range value to report, 1-999, without commas or decimal points.

Range Chart Increment

Pick the increment for the range chart results


  1. Parallax error in millimeters (Pmm): ( 0.5 x D x ( Abs ( R - P ) ) / P
    • D = Objective diameter in millimeters
    • R = range to target
    • P = fixed parallax setting of scope
  2. Parallax error in inches (Pin): Pmm x 0.0393701
  3. Parallax error in Minutes of Angle: Pin / 1.047 / ( R / 100 )
  4. Parallax error in Milliradians: Pin / 3.6 / ( R / 100 )