One of the first rules I learnt in do it yourself house renovations was "Measure Once, Cut Twice" and of course the reverse, "Measure Twice, Cut Once".
The latter is definitely the least expensive way to go.
I learnt it the hard way. In spite of being told of this rule before I began my amateur carpentering journey, that was one of my first mistakes. I was fitting a new door in an upstairs bedroom and realized I had to trim the edges of the door to get a good fit. I measured the door opening and was on my way down stairs to trim the door down with my skill saw when the phone rang. I spent about 10 minutes on the phone and after hanging up the call, instead of going back upstairs to re-measure, I relied upon my memory which afterwards I found out was not as good as i thought it was.
The door opening was thirty one and seven eighth inches wide; so I thought. The used door I was fitting was thirty two inches. So I just needed to trim off an inch and an eighth and it would be a good fit. Wrong. The door opening was thirty, not thirty one. So I hacked off an inch too much and ended up having to buy another door.
If you are cutting a length of lumber and you think you have the measurement correct and don't cut enough, it's not a problem. Cut too much and toss away the lumber.
The same rule applies to many other home handyman chores; laying carpet, installing laminate or wood floors, electrical wiring, baseboards and door and window trim and wallpapering. It also applies in painting. Measuring the walls to determine how much paint you need is a great chore where measuring twice pays off, particularly if you are painting with an odd color. A couple of times I have miscalculated in how much paint is required and had go back for an extra bucket only to find a slight, but noticeable change in color in the new paint.
A good measuring tape is every home handyman's best friend. It is best to have at least two. Good tapes come with a clip so you can attach it to your belt. However, I found a lot of the time, I would measure a piece of board or a length of lumber and put the tape down on my workbench or a nearby shelf. I would then traipse upstairs to double check a measurement and find I didn't have the tape with me.