Centering is the one grading criterion you can measure before you pay. Here is what the ratio means, three ways to measure it, and where the lines are at each grading company.
Look at one axis at a time. Add the left and right borders together and express the larger one as a share of the total: a 3.3 mm left border and a 2.7 mm right border is 3.3 ÷ 6.0 = 55% on the left, so the card is 55/45 left to right. Do the same for top and bottom. The grading services quote the worse axis: a card that is 52/48 one way and 60/40 the other is a 60/40 card. Front and back are judged separately, and where a company publishes a back figure it is looser than the front (PSA: ~55/45 front and 75/25 back for a GEM-MT 10).
Measure each border in millimetres at the middle of the side, from the cut edge to the printed frame. Good enough when the card is obviously in or out; not good enough near a line, because the 55/45 line on a standard 63 mm card is a difference of about 0.3 mm between the two borders.
Phone apps that show a grid you line the card up with, or physical jigs with printed lines. Fast, and fine for a sanity check. They depend on your eye placing the lines, and a phone held at a slight angle adds a percent or two of error on its own.
Photograph the card straight on, out of the sleeve, on a plain surface that contrasts with the border, and let software find the outline, correct the perspective and locate the border edges. This is what SlabbyPrep does: it fits the card’s outline, maps it to an upright 63 × 88 mm card, finds where the border colour ends on each side, and reports both axes to a hundredth of a millimetre, with a magnifier to correct any line by hand. On synthetic test photographs with known answers it lands within 1.5 percentage points every time, usually within half a point.
Each company publishes its own line per grade, and they differ more than people think: PSA GEM-MT 10 ~55/45, BGS Pristine 10 / Black Label 50/50, CGC Pristine 10 50/50, SGC 10 Pristine 50/50, TAG Pristine 10 ~51/49, ACE Gem Mint 10 60/40 for the top grade on the front. The full table with quotes and dates is at centering standards by grade, and each ratio has its own page: 52/48 · 53/47 · 55/45 · 57/43 · 58/42 · 60/40 · 62/38.
On one axis, the larger border is 60% of the two borders added together and the smaller is 40%. A card with a 3 mm left border and a 2 mm right border is 60/40 left to right. Services quote the worse of the two axes.
The border: from the card’s cut edge to the edge of the printed frame or artwork, on each side. Measuring the artwork itself gives the same ratio only if the card was cut perfectly square, which is exactly what you are checking.
Both. PSA and BGS publish a separate, looser back tolerance (PSA: 75/25 for a 10, 90/10 for a 9); the others judge the front. Measure the back too if you are aiming for a 10.
Yes. The catch is the photo: a tilted phone turns a centred card into an off-centre one, and a glossy sleeve hides the border edge. Shoot straight on, out of the sleeve, on a plain contrasting surface, and use a tool that corrects perspective before it measures.
For a card that is clearly in or clearly out, yes. Near a line it is not: the PSA 10 line at 55/45 on a 63 mm card is a difference of about 0.3 mm between the two borders, which is under the resolution of a ruler and inside the wobble of a hand-held caliper.
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