How to measure a recessed space accurately

[DTR]

The other option - which is much simpler - is to make an adjustable stick which is fitted into the space then immediately transferred to the wood that needs to be the same size. This has the advantages that the tool needs no graduation and that you can make the fit as tight or loose as it needs to be. Also, there is no risk of transcription or calculation errors.

I much prefer that way of working whenever possible.

+1

I tend to use two plain sticks of whatever is handy, pinch them inside the opening then put a pencil mark across both. Then it doesn't matter if the sticks come apart, get knocked etc, I just line up the pencil marks.

I do like Pete's home-made gauge though!