Halve Any Tape-Measure Fraction in Your Head by Doubling the Denominator

The Problem

Finding the center of a board that measures 6-11/16 stops most people cold. The common workaround — angling the tape until it reads a round number and marking the middle of that — works, but it puts the tape and the mark out in the field of the board when you often need the number itself, down at the edge, or written on a cut list.

The Technique

Even whole number: halve the whole number, keep the fraction's top, double its bottom.

1. Take 6-11/16. Halve the whole number: 3.
2. Keep the numerator (11) and double the denominator (16 becomes 32).
3. Half of 6-11/16 is 3-11/32. Done — no decimals, no calculator.

Odd whole number: the leftover half folds into the fraction.

1. Take 7-5/8. Round the whole number down to 6 and halve it: 3.
2. Double the denominator as before: 8 becomes 16.
3. For the new numerator, add the original numerator and denominator together: 5 + 8 = 13.
4. Half of 7-5/8 is 3-13/16.

Why the odd-number version works: the half you dropped when rounding 7 down to 6 is worth 8/16 — exactly the original denominator over the new one. Adding the denominator to the numerator puts that missing half back.