The Prop Shop - How to Measure Inboard Propeller Shaft Diameter Without Calipers
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How to Measure Inboard Propeller Shaft Diameter Without Calipers

How to measure shaft diameter without calipers

When measuring propeller shaft diameter, accuracy is very important. For best results, use calipers.  However, when you are on haul out, you don’t always have access to all of the tools that you need. See below for diameter measuring instructions using just a tape measure, pen, and a piece of paper.


  1.  Make sure you are measuring on the fat of the shaft, do not measure the taper.
  2. Wrap the paper tightly around the shaft and mark the point where the paper overlaps.

Wrap paper tightly around shaft

       3. Lay the paper flat and measure from the end to the mark. 

       4. Take this measurement and divide by pi (3.1416). 

In my example, I had a measurement of 3-9/16" which is 3.5625 as a decimal, so: 3.5625 / 3.1416 = 1.134".  1.134 is very close to 1.125 which would be a 1-1/8" shaft.  It isn't the most accurate way to determine the diameter, but will do in a pinch.  If measuring a metric shaft, use same formula as above, substituting MM fo IN.

You should now have a pretty good idea of what your shaft diameter is.  If you are getting any weird numbers, or need some help, please do not hesitate to contact us.

Common SAE Shaft Sizes w/ Decimal Equivalents
Shaft Diameter Shaft Diameter (Decimal)
3/4" .750"
7/8" .875"
1" 1.00"
1-1/8" 1.125"
1-1/4" 1.25"
1-3/8" 1.375"
1-1/2" 1.50"
1-3/4" 1.75"
2" 2.00"
2-1/4" 2.25"
2-1/2" 2.50"
2-3/4" 2.75"
3" 3.00"

Click here for a more detailed chart for determining propeller shaft size (must use calipers).