Divide Any Length In Half, Without Using Maths

You don't need a calculator to quickly find the center of something that's an odd length, such as 37 5/8 inches (95cm). The smart folks at This Old House show us two ways to quickly divide a length in half using just your tape measure.

In the video above, Tom Silva demonstrates the two methods. The first is easier because it involves only one tape measure:

  1. Measure your item. In this case, a board.
  2. Round up to a larger number that's easily divisible by two. In this example, 40in (102cm).

  3. Pull a tape measure diagonally across the board until the 40in (102cm) graduation aligns with one edge.
  4. Make a pencil line at the 20in (51cm) graduation line (half of 40). That will be the center of the 37 5/8 inch board.

The second method is to use two tape measures, pulling each across from opposite edges, and find where the identical measurement is. That's the center.

Maths is fun, but sometimes maths shortcuts are even more fun.

Ask Tom: How to Find Measurements Without Maths [This Old House]


    Finding the centre of 95cm isn't complex maths, because we use a base ten counting system, not the stupid imperial system.

      Yeah, the Yanks are backward in so many ways, they're one of the few countries that still use imperial. Plus they still carry friggin guns for Christs sake, it's like the wild west over there.

        Basically "third world" outside the large cities. Despite being base 10 many can't even "read" a metric tape measure. The school system needs a massive reform to get students engaged in science and mathematics if they are not going to be left behind by the BRIC (Brazil, Russia, India, China) countries. Their current solution is the green card to import the best people, and a significant percentage come from BRIC.

        One of the few countries? Three countries: US, Myanmar and Liberia. There was a study done in 1968 which determined that metrification was feasible and would be good for America. It was voted down. NASA lost the Mars orbiter in 1998 due to Lockheed Martin using Imperial.

        This, gun control and healthcare makes me wonder how America can so constantly do the opposite of everyone else and still think they're doing everything the best way.

        Also, when the guy said something like "19 and 13/18ths" near the end I nearly lost it. What a stupid system.

        Last edited 29/03/16 12:03 am

      If you can't measure a board and work out where halfway is, you shouldn't have access to cutting tools. Even if you use Inches it's not that frickin' hard.

      And the first method is close enough is good enough.

    Yes those solutions are all good. But how do I do it without maths?

      In fairness, their second method does not in fact require maths; maths is only required when using the method described (in an article which claims to describe how to solve the problem WITHOUT maths).

      Here's a different method. This works a bit better with a cloth tape measure as used in sewing, but variations work with anything foldable, even a piece of string. (Actually, I usually use a piece of string.) A length of cotton may be better.
      1. Take your tape measure and stretch it between your endpoints, with one end of the tape measure at one endpoint.
      2. Secure the tape measure at the other endpoint.
      3. Fold the tape measure back along its length until the end of it is back at your second endpoint (ensuring that it is taut.).
      4. The place where the tape folds is the centre.

      If you're finding the middle you're seeking is not along an edge, you can mark the approximate centre when the tape is at full length and use that to make sure that the orientation of your tape remains correct. Or else fold the string/tape along its length first then fold the end back.

      The string method also lets you measure along a curved path, which is rather harder when using a tape measure.

      Alternatively (on a flat surface), make up a simple (large) compass and use the methods found in high school geometry. (Or was it primary school? This is not rocket science.) It's not as if making a simple compass is hard. Two lengths of wood, two washers, a nut and bolt, a nail for the sharp part at one end and a clamp to hold the pencil at the other. Although I suppose geometry counts as maths.

      Do people seriously have trouble figuring this stuff out? Even with maths it's not very hard; dividing by two is not a complex operation.

      Last edited 28/03/16 1:31 pm

        Personally, dividing by 2 in a Base 12 number system is very complex operation.

    For christ's sake. It's easier just doing it in your head. If you can't figure out how to divide a number by two then something's wrong.

    Step 1, use the metric system.

    Step 2, use extremely basic math.

    Here's how woodworkers do it,

    1) Using any measuring implement (even a stick will do) eye ball what you think is the half way point, referencing your stick/ruler/whatever against one side and marking the surface you are measuring.

    2) Make a mark at the same distance, this time referencing from the opposite edge.

    3) You now have two marks on your surface, likely to be not much more than 1cm apart. You can now easily mark the centre point by eye.

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