Are all TCG cards the same size?

No, they're not. Trading card games split into two sizes. Most Western-designed games, Magic: The Gathering, Pokemon, One Piece, Disney Lorcana, and Flesh and Blood, use the "standard" size at 63x88mm. Games with Japanese design roots, Yu-Gi-Oh!, Cardfight!! Vanguard, and Weiss Schwarz, use a smaller card at 59x86mm. The two sizes aren't interchangeable with the same sleeves, and that's the root of most "why don't my sleeves fit" questions.

Standard size: 63x88mm

This is the size most players run into first, since Magic and Pokemon are the two biggest TCGs in the West and both use it. Standard cards are 63mm wide, 88mm tall, and 0.305mm thick. In inches, that's about 2.48 x 3.46. One Piece Card Game, Disney Lorcana, and Flesh and Blood all launched on this same template, which means a Magic player picking up One Piece doesn't need new sleeves, new toploaders, or new binder pages. It's all the same footprint.

Japanese small size: 59x86mm

Yu-Gi-Oh! is the game most people are surprised by. Its cards measure 59x86mm and 0.28mm thick, noticeably narrower than a standard card even though the height is close. Cardfight!! Vanguard and Weiss Schwarz share this same smaller size. Sleeves built for standard cards will physically fit over a Japanese-size card (there's extra width to spare), but the card will slide around inside, shuffle poorly, and look loose in a binder pocket built for the bigger size. Buying sleeves labeled "Japanese size" fixes this. Several major brands make a Japanese-size line specifically for this reason: Dragon Shield, KMC, Ultimate Guard, and Ultra Pro all sell a small-size version alongside their standard line.

A third category: sports and playing cards

Worth a mention since it trips people up: sports cards (baseball, basketball, football) and standard poker playing cards run 63.5x88.9mm, which is a fraction bigger than a standard TCG card and converts to an exact 2.5 x 3.5 in. It's close enough to standard TCG size that a lot of sleeve packaging just says "standard" for both, but if you're precision-fitting a graded sports card into a toploader built around 63x88mm gaming cards, expect it to sit slightly tighter than a Pokemon card would.

Why this trips up new players

Nobody reads the fine print on a pack of sleeves before buying, and most sleeve boxes look nearly identical on a store shelf unless you check the label. Someone who plays Pokemon and picks up Yu-Gi-Oh! for the first time will grab whatever sleeves they already have at home, and those sleeves will technically close around the Yu-Gi-Oh! card while leaving a visible gap on both long edges. It's not a defect in the sleeve. It's a mismatch that only shows up once you compare a Japanese-size card side by side with a standard one.

Quick answers

Why are Yu-Gi-Oh cards smaller than Pokemon cards? Yu-Gi-Oh! uses the 59x86mm Japanese card size, while Pokemon uses the 63x88mm standard size. The difference is about 4mm on the width, which is enough to make standard sleeves fit loosely.

Can I use standard sleeves for Yu-Gi-Oh cards? You can fit them in, but the card will shift around inside the sleeve and the fit will look sloppy in a binder. Japanese-size sleeves are built for the smaller card and hold it snugly.

Is One Piece the same size as Pokemon? Yes. Both use the 63x88mm standard size, so the same sleeves, toploaders, and binder pages work for either.

Are Magic cards the same size as Pokemon cards? Yes, both are 63x88mm. It's one of the more useful facts in this hobby if you play more than one game, since it means your sleeve collection doubles as gear for both.

Two sizes, five standard-size games, three Japanese-size games. Once you know which bucket your game falls into, sleeve shopping stops being a guessing game.