If you buy one lock, buy a U-lock. Its shape is its security: a short, thick hardened shackle is hard to cut, hard to lever, and leaves little open space for a bottle jack or crowbar — which is exactly the gap a big padlock-and-chain or a long shackle gives a thief. Pound for pound, nothing else protects a bike as well. The trade-off is reach: a U can only lock to something it fits around, so you match the opening to the racks you actually use.
The single spec that matters most is shackle diameter. Thicker defeats bolt cutters and buys time against a saw; 18mm is the practical ceiling for something you can carry, 16mm is a strong all-rounder, and 12–13mm is fine for lower-risk parking. We rank on that published diameter, the maker’s own rating and independent Sold Securegrades, and honest fit — we haven’t cut these open ourselves. A smaller “mini” body is more secure but locks to less, so the right pick is a balance between the security you need and the racks you park at. Whatever you choose, using it correctly matters as much as the rating.
Our security pick is the 18mm end of that scale, and it is the one most worth reading about in detail before you commit to carrying it — the Fahgettaboudit Mini review goes through the published specs, the two costs nobody puts on the box, and who should buy the step-down instead.
How to choose a U-lock size
The tension in every U-lock buy is security versus reach. A compact “mini” leaves almost no room for a tool, which is why it’s the most secure shape — but it also means you can only lock the frame and one wheel to a slim rack or post, not a fat one. A full-size body fits more but gives a thief more room to work. The fix for a too-roomy U-lock is free: fill the empty space with the bike and as much solid object as you can, so there’s no gap for a jack. Our locking guide shows the frame-and-wheel technique that gets the most security out of whatever size you own.
Match the rating to the risk. A 12mm Silver-rated U-lock is a genuine lock and a fine choice for low-risk parking or backup duty, but it’s a step below the Gold you want for a bike left in a busy city. For a heavy, valuable e-bike, jump straight to the top tier or a hardened chain — see the best e-bike locks. If the choice has narrowed to the 13mm mini against the 16mm full-size, that specific decision gets its own page: Kryptonite Evolution vs New York. And if a U simply won’t reach the racks on your route, a chain trades some strength-to-weight for the reach to wrap around almost anything.
Two situations change this ranking. On a crowded campus, where you lock up five times a day between buildings, the lock you carry beats the stronger lock you left in your room — best locks for a college campus. And when two bikes have to share one lock, a U-lock usually cannot reach around both frames plus an anchor: how to lock two bikes together covers the length you need instead.
