How Slab Measures Centering

slab-grader.com/how-we-measure

What centering means

Centering is the position of the printed artwork within the card's border. Graders report it as a ratio: left-to-right (LR) and top-to-bottom (TB). A perfectly centered card is 50/50 on both axes. A reading of 55/45 LR means the card is shifted 5 percentage points toward the left border.

Each grading service publishes its own centering thresholds. Slab displays the raw LR and TB ratios so you can check them against PSA's, BGS's, or CGC's published charts. We report the numbers; the chart tells you what they mean for your grade.

How the measurement works

For each scan, Slab aligns your photo to a reference render of that exact card using feature matching. The pixel-level alignment reveals how far the card sits from its intended design center. From that shift, Slab computes the LR and TB border ratios directly — no manual annotation, no lookup tables.

The measurement method varies by card type. Standard bordered cards use a border-gradient detection approach. Full-art cards, foil rares, and card backs use reference-image registration, where Slab locates the printed artwork in the scan and measures how far it shifted. Both approaches produce the same LR/TB ratio output.

Accuracy figures on this page come from a two-step validation protocol: a calibration exam against published machine-measured truth values, followed by an independent on-device check on a real phone scan of a physically measured card. Both steps must pass before a reading goes live for that axis. The table below shows exam outcomes for each validated card type.

Calibration exams

Before any centering number shows for a card type, it completes a two-step calibration:

Calibration exam. Slab measures a set of real physical cards whose centering values are sourced from TAG's published machine measurements, then compares its readings against those published values. TAG provides per-card machine-measured LR and TB percentages as a public record; Slab uses that record as the truth source for its calibration exams. TAG is not affiliated with Slab and does not endorse or certify Slab's measurements. The result is a mean absolute error (MAE) in percentage points — the typical gap between Slab's reading and the actual value. Lower is better; anything under 0.5 pt is well inside a single grading-tier boundary.
Device-proof step. Calibration alone is not enough. Each axis also requires a cross-check using a real scan taken on a phone, compared against an independently measured physical card. Only when both legs pass does the number go live for that axis.

The table below lists every card type Slab has calibrated. Accuracy figures are the measured outcomes of the calibration exams — they are read directly from the calibration register and never typed by hand, so the table and the register cannot drift.

Status guide

Live showing in app — calibrated and device-verified Calibrated exam passed; device proof pending In progress calibration exam not yet complete

Calibrated means the measurement passed its exam on reference cards but has not yet been cross-checked on a live phone scan against an independently measured physical card. Numbers are held until that second step completes. An absent number is a deliberate signal — not a gap in the interface.

Accuracy by card type

MAE = mean absolute error in percentage points. Figures come from the calibration exam held-out set for each card type. Each axis is certified independently.

Pokémon

Card type	LR accuracy	TB accuracy
Standard front (SM / SV era)	0.341 pt Calibrated	0.075 pt Live
Classic era (Base Set – Neo Destiny)	0.217 pt Calibrated	0.044 pt Calibrated
e-Reader era (Expedition – Skyridge, EN)	0.387 pt Calibrated	0.224 pt Calibrated
VMAX	0.044 pt Calibrated	0.005 pt Calibrated
Full Art / Illustration Rare (SIR)	0.191 pt Calibrated	0.176 pt Calibrated
Alt-Art V (SWSH era)	0.257 pt Calibrated	0.146 pt Calibrated
Rainbow Rare	0.235 pt Calibrated	0.117 pt Calibrated
Gold Secret Rare	0.424 pt Calibrated	0.214 pt Calibrated
Trainer Gallery	0.455 pt Calibrated	0.169 pt Calibrated
Hyper Rare / Crown Rare (SV era)	0.346 pt Calibrated	0.182 pt Calibrated
Card back (English)	0.351 pt Calibrated	0.040 pt Calibrated
Card back (Japanese)	0.469 pt Calibrated	0.175 pt Calibrated

Magic: The Gathering

Card type	LR accuracy	TB accuracy
Standard (modern border)	0.463 pt Calibrated	0.230 pt Calibrated
Mystical Archive / Bonus Sheet	0.279 pt Calibrated	0.143 pt Calibrated
Borderless / Extended Art	0.058 pt Calibrated	0.039 pt Calibrated
Showcase — retro, dungeon & moonlit	0.357 pt Calibrated	0.184 pt Calibrated
Showcase — narrow dark border	0.278 pt Calibrated	0.142 pt Calibrated
Showcase — thin dark border	0.465 pt Calibrated	0.218 pt Calibrated
Card back	0.384 pt Calibrated	0.195 pt Calibrated

One Piece

Card type	LR accuracy	TB accuracy
Character / Event / Stage / DON!! cards	0.416 pt Calibrated	0.206 pt Calibrated
Leader cards	0.400 pt Calibrated	0.193 pt Calibrated
Alt Art	0.400 pt Calibrated	0.104 pt Calibrated
Secret Rare (SEC)	0.324 pt Calibrated	0.172 pt Calibrated
Card back	0.192 pt Calibrated	0.003 pt Calibrated

Lorcana

Card type	LR accuracy	TB accuracy
Standard	0.223 pt Live	0.213 pt Calibrated
Enchanted / Epic	0.275 pt Calibrated	0.161 pt Calibrated
Card back	0.361 pt Calibrated	0.209 pt Calibrated

Conservative verdict philosophy

Where Slab cannot make a confident measurement — poor scan quality, card type still in calibration, or any axis pending device proof — it shows nothing. A blank is a deliberate signal that the measurement is not yet reliable enough to display, not a gap in the interface.

Where a measurement is available and the centering lands exactly on a tier boundary, Slab rounds toward the higher grade. The thresholds used are taken directly from each grading service's published criteria.

Card types still In progress are never estimated or approximated. Slab waits until the calibration exam passes before showing any number for that type.