Measuring diameter with tape measure

GailInNM · Mar 23, 2009

Some of us are natural scrounges. Those of us who are often have a small pocket tape measure with us when ever we go out. There are times when I would like to know the diameter of an object that although round is not easy to measure on the end. Think about what the dameter of a 2 liter coke bottle is. One end is rounded and the other end is necked down to the cap.

I have marked the back of my pocket tape measure to read in diameter when wrapped around an object. Start at the end of the tape and mark off increments of Pi times the diameter increment. I mark each half inch and can eyeball to the nearest 1/8 inch, close enough. I use an industrial Sharpie to mark with and then over coat the marking with a bit of clear nail polish overcoat. Just brush a thick coat on with a single stroke. If you try to go back over it, the marker will smear.

To mark, extend the tape and while observing the front side, tick the back side with the marker.
Examples of marking locations are.

1" diameter at 3.14" from end.
1.5" diameter at 4.71" = (Pi X 1.5") = (3.14 x 1.5")
.
.
6 inch diameter at 18.85 inches from end

To use, wrap the tape around the object with the normal marking against the object and see where the end of the tape meets the marks on the back side of the tape.

Photo shows how the back of the tape looks compared to a normal scale. Note the distance from4 to 5 inches on the diameter is about 3.14 inches on the rigid scale.

te_gui · Mar 23, 2009

They make what are called Pi tapes used to measure large diameters, have a vernier scale built in for better resolution. They are very accurate if used correctly

Brian

jmshep · Mar 23, 2009

Something else I didn't know I needed - Thanks for the idea.

John S

hitandmissman · Mar 23, 2009

Now that is a good idea. Will have to make up a couple tapes like that. Sell that idea to a tool company.

rake60 · Mar 23, 2009

Great idea Gail!
I've been using Pi Tapes at work for many years.
In fact I have a .75" - 7" Pi Tape in my home shop.
You need to keep it clean, oiled and free of kinks.
It requires careful handling if you want it to remain
accurate. I wouldn't want to be using it at the scrap
yard to check potential stock sizes.

I'll be marking out the back of my pocket tape for a scale
version. Thanks for the tip!

Rick