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Sure, you can use a calculator or even your head to find the circumference of a round duct by measuring its diameter, but with the Cooper Tool Tinner’s Circumference Rule all you need to measure is the diameter — the ruler will read the circumference.

The tempered medium-weight steel rule measures in inches by 1/16″ on the top edge and circumference inches by 1/8″ on the bottom edge. The black markings are easy to read, and on the reverse side Cooper has printed relevant formulas and tables for easy reference.

The rulers come in 3′ and 4′ lengths — both rulers measure 1-1/4″ wide and 1/16″ thick. At $60 on the bottom end these circumference rulers aren’t cheap, so practicing your arithmetic might be a little more cost effective.

Tinner’s Circumference Rule [Cooper Tools]
Street Pricing [Google Products]
Via Amazon [What’s This?]

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14 Responses to Measure Round With A Straight Ruler

  1. Swedub says:

    Or you can just get a seamstress tape measure. You can find them for under $10. Luckily my mom has been sowing before I was every born so she had quite a few I could ‘barrow’.


  2. aaron says:

    yeah, this is silly.

  3. aaron says:

    or use a thread or shoelace, even a strip of paper. yeesh.

  4. jeffrey immer says:

    2pi*R it’s not that hard i always have a calculator around

  5. Dazrin says:

    Can’t we just use (3 * Dia) and add a little? It is not like you can accurately get down to the 1/8″ when you have a straight ruler measuring a round surface.

    (What’s the difference between an engineer and a scientist? To a scientist, pi is 3.14159….. To an engineer, pi is 3.)

  6. jeffrey immer says:

    ha “(What’s the difference between an engineer and a scientist? To a scientist, pi is 3.14159….. To an engineer, pi is 3.)”
    i just use the pi button on my calculator

  7. fred says:

    On a prior ost – I noted:

    Wheeler Rex makes a pipe diameter tape:


    My pipe supplier use to give these away as gifts

  8. shopmonger says:

    WOW is Pi really that hard

  9. aaron says:

    i know, it’s completely irrational! lol

  10. Coach James says:

    “i know, it’s completely irrational! lol”


  11. T says:

    Fred is apparently the only one of you folks who’s used a pi tape. They’re everywhere in the OCTG world.


  12. David Bryan says:

    When I was a lad we just used 22/7, and it’s a pretty useful back-of-the-envelope estimator. I don’t see this kind of ruler as very useful for measuring a duct to determine its circumference– you’d have to have a duct that’s nearly perfectly round, and I see a lot of ductwork that isn’t like that.
    If the relationship between the two sets of numbers were reversed, with the circumference marks at full length and the diameter marks at C/pi, you could use the ruler directly to mark a piece of metal for making a cylinder or duct of a given diameter. You’d really only need the C/pi marks. When I used to use a 45 degree ruler a lot, everybody else I knew would pinch the ruler and turn it over to see what the conversion was but I’d just double the offset length I wanted and read the measurement directly off of the offset length side. I always thought they’d be easier for people to use if the relationship between those numbers was reversed, too.
    I don’t think I know any engineers that would agree with that “pi is 3” comment.

  13. Zathrus says:

    Actually, every engineer I know would beat you about the head for that “pi is 3” comment.

  14. Derrick says:

    You’re all missing the point. This ruler is/was designed for pattern development and layout in the sheet metal trade. You can take a square, or a table, make a mark on each axis, at 18″. You then bend the rule so that 0 on the circumference side is at one mark and 90 is at the other. Each increment is now 1 degree. You can also use the ruler as a flexible/irregular curve for connecting points on square to rounds, elbows, etc. There are also a ton of useful formulas and calculations relating to the trade on the backside of the rule. This is a very heavy duty straight edge as well. Well, I think that is it and for the record, in the trade we use Pi=3.14, always. I have never looked at this ruler to find circumference even though it is always on the bench.

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