Grip size comes from the length of your hand
Not the thickness of your fingers, and not the measurement you would use for a glove. Grip makers size from hand length: the distance from the crease where your palm meets your wrist to the tip of your middle finger, with the hand open flat. A glove is sized from the circumference around your knuckles, which is a different number describing a different thing. People swap the two constantly and end up a full size out.
Golf Pride, who make more grips than anyone, publish four bands from that measurement: under 7 inches is undersize, 7 to 8 is standard, 8 to 9 is midsize, and 9 or more is jumbo. We use theirs rather than averaging the field, because the published charts genuinely disagree. One large golf site puts the standard band at 6.6 to 7.5 inches, which is most of an inch away from the maker's own answer, and there is no standards body to settle it.
The formula
midsize = standard + 1/16 in | jumbo = standard + 1/8 in
build up tape ≈ 1/64 in of diameter per wrap
Hand length is measured wrist crease to middle fingertip. Undersize sits about 1/64 inch under standard, midsize about 1/16 inch over, and jumbo about 1/8 inch over. Those two facts settle the tape arithmetic: 1/16 of an inch is four 64ths, so four wraps of build up tape under a standard grip lands you on midsize.
Worked example
Tape only goes one way
This is the part that catches people out. Build up tape adds diameter, and there is no version of it that removes any. If you measure into the undersize band you have to buy undersize grips, or pay a club builder to ream out the grip core, which is a workshop job rather than something you do on the kitchen table. Going up a size is cheap and reversible. Going down is neither.
There is also a practical ceiling on the trick. Four wraps to reach midsize is routine and nobody will notice. Eight wraps to reach jumbo is a different proposition: the tape starts building unevenly, it adds real weight up at the butt end of the club, and it changes the taper of the grip so the lower hand no longer sits where the maker intended. If you need jumbo, buy jumbo.
Why grip size shows up in your ball flight
Grip size controls how freely your hands can release the club, and the release is what squares the face. A grip that is too small lets the hands work too actively, so the face closes early and the ball turns over to the left for a right handed golfer. A grip that is too large quiets the hands and slows the release, and the same golfer starts leaking shots out to the right.
That is worth knowing before you rebuild a swing. If your miss is a persistent hook or a persistent block, checking the grip costs you five minutes and possibly a roll of tape. It is not always the answer, but it is the cheapest thing on the list of possible answers, and an awful lot of golfers are playing a grip size they were never fitted for because it is simply what the clubs came with.
The five second check that beats any chart
Put a glove on and take your normal grip. Look at your top hand. The fingertips of your middle and ring fingers should just barely brush the pad at the base of your thumb. If they are digging into your palm, the grip is too small for you. If they do not reach the pad at all, it is too large. That is Golf Pride's own fit test and it is better than this page, because it measures your actual hold on an actual club rather than a hand lying flat on a table.
Two things sit outside all of this. Your putter grip is a separate question, fitted by feel and by how much you want the hands taken out of the stroke, which is why oversized putter grips exist in shapes no iron grip comes in. And if you are sizing clubs for a child, junior grips are a real product rather than just a small standard, so start there instead of at the bottom of this chart. For the clubs themselves, the golf club size calculator handles length and flex.