Reading centering like a grader
Corners you can protect. Warp you can press out. Surface you can keep clean. Centering was decided at the factory and nothing you do changes it, which makes it the first thing worth measuring and the cheapest reason to not spend a grading fee.
What the numbers actually describe
Centering is expressed as a ratio of the border widths on opposite sides. A card measured 55/45 has one border slightly wider than the other, and one measured 50/50 is perfectly centered. You measure it twice, left to right and top to bottom, and the worse of the two is the one that will be quoted. Every card also has a back, and the back has its own, usually looser, allowance.
How to measure it in under a minute
Lay the card on a flat mat in even light and put a centering overlay on top. Read the border widths at the midpoint of each side, not at the corners, where a slightly off cut will lie to you. Measure to the edge of the printed border, not the edge of the cardstock. Write down both ratios before you form an opinion, because it is very easy to talk yourself into a number you want.
The thresholds that matter
PSA publishes 55/45 to 60/40 on the front and 75/25 on the back as the requirement for Gem Mint 10, and 60/40 front with 90/10 back for Mint 9. Other graders set their own tolerances and some run tighter on modern cards, so check the current standard for the company you are actually submitting to. Treat these as gates rather than targets: a card that fails the centering gate cannot reach that grade no matter how perfect everything else is.
Both directions, and the back
Plenty of cards look centered because the eye checks left to right and stops. Top to bottom is where modern cards commonly fail, and the back is where vintage commonly fails. Measure all three before deciding anything.
Do the arithmetic before the envelope
Once you have the ratio, the decision is arithmetic, not hope. Look up recent sales for the card at the grade your centering allows and at the grade you were hoping for. Subtract the grading fee, shipping both ways, and insurance. If the realistic grade does not clear that number by a comfortable margin, the card is worth more raw and you have just saved yourself a two month wait.
When the measurement is borderline
Borderline cards are where submission strategy lives. If a card sits right on a threshold, the rest of its condition decides whether to send it, because a grader looking at a marginal card will not give you the benefit of the doubt anywhere else. Perfect corners and a clean surface can carry a borderline measurement. A borderline measurement plus a print line cannot.
Our centering kit includes the overlay, a 30X loupe, and cloths, which is everything this routine needs. It is in the Grading prep collection, and the full pre-submission checklist is in our guide on prepping a raw card for grading.