Tape measure and square

Fractions are the native format, not an inconvenience

A tape measure is marked in sixteenths and a carpenter thinks in them, so every input on this site takes a measurement written the way it appears on the board — 23 1/2, 23-1/2 or plain 23.5, whichever you typed. Converting to decimals in your head before you can do arithmetic is the fastest way to introduce an error that nothing downstream will catch.

Adding two of them is the operation people reach for a calculator to do and then get wrong, because a phone calculator will not take the input. The answer comes back snapped to the nearest sixteenth, which is the finest mark on the tape — a readout of 31 127/256 would help nobody.

A tape proves a corner square, and a square does not

A framing square is about a foot long, and a foot of reference does not tell you much about a wall sixteen feet long — a corner that reads square across twelve inches can be an inch out across the room. The tool for that is the tape, and the method is the 3-4-5 triangle: measure three units along one wall, four along the other, and if the diagonal between those marks is exactly five units, the corner is square. It is Pythagoras, and it works at any scale.

Bigger is better here, because the error you are hunting grows with the distance you measure over. Scale the triangle up to the largest multiple the space allows.

Two diagonals, not one

For a rectangle — a cabinet carcase, a door, a deck frame — there is a faster check than a triangle: measure both diagonals. If they are equal, the thing is square, and if they are not, it is racked toward the longer one. That single comparison is worth doing on every glue-up before the clamps come off, because a carcase that dries out of square stays that way.

The tolerance is a judgement rather than a constant. The square check calculator treats an eighth of an inch as the line, which is right for framing and generous for a drawer.

The hook on the end is meant to move

The metal tab at the end of a tape slides by about the thickness of itself. That is not wear and it is not a fault: it moves out by its own thickness when you pull against it and in by the same when you push, so an inside measurement and an outside measurement both read true.

Which means a bent hook is a tape that lies, and two different tapes can disagree by a sixteenth. Use one tape for a whole job wherever the parts have to meet.

What this page does not tell you

• —The 3-4-5 check proves a corner, not a room. A rectangle needs its diagonals compared as well, because four square corners is not the same claim as four equal sides.
• —An eighth of an inch is the tolerance this site uses for a square check and it is not a standard. Casework wants far tighter, and a deck frame will forgive more.
• —Measure twice is advice about attention, not about tools, and no calculator here replaces it.