The right lock length is a measurement, not a guess
Most cyclists buy a lock by feeling: a short U-lock looks neat and secure, a long cable looks flexible, and the choice is made in the shop without measuring anything. Then, at the first awkward railing, the lock will not reach around the frame, the wheel, and the anchor at the same time, and the bike ends up secured by the front wheel alone, or by a cable looped through a quick-release part that a thief can remove in seconds. The calculator on this page turns the problem into a measurement: the thickness of the anchor you lock to, the size of your wheels, and what you want to secure, and from those it gives you the lock length that actually fits the job. A lock that fits is a lock that gets used properly, and a lock used properly is the whole point.
The geometry is simple once it is drawn out. A lock must pass around the anchor, through the frame, and, if you are doing the job properly, through at least one wheel, and then close with a little slack to spare. Slack matters, because a lock pulled drum-tight is hard to close and puts strain on the mechanism, while a lock with generous slack is easier for a thief to attack with leverage tools. The sweet spot is enough length to close comfortably with a few centimetres to spare, and no more, which is why the calculator adds a modest slack allowance rather than a generous one, and why different locking styles need very different lengths.
Locking styles: frame only, frame plus wheel, both wheels
Frame-only locking is the quick stop: the lock secures the frame to the anchor, and the wheels rely on their own security, such as locking skewers or the hope that a thief will not bother. It needs the shortest lock, and it is acceptable for a short stop in a busy, visible place, but it leaves wheels exposed, and quick-release wheels can be removed without tools, which makes frame-only locking a poor choice for an expensive bike left for hours. Frame plus front wheel is the standard good practice for a U-lock: the lock passes through the rear wheel and the frame inside the rear triangle, or through the front wheel and the frame, securing the two most valuable parts with one lock, and it needs a medium length.
Securing both wheels is the gold standard and the longest lock, usually achieved with a U-lock on the frame and rear wheel plus a cable looped through the front wheel and the U-lock, or with a long chain that passes through everything. The calculator's both-wheels figure assumes a single long lock doing the whole job, which is why it is the largest number, and in practice many cyclists split the job between a U-lock and a cable, which is lighter and more flexible. Either way, the principle is the same: every part that can be removed without tools should be secured, and any part left unsecured is a part you may come back without.
Anchors: what to lock to, and what to avoid
The anchor matters as much as the lock, because a superb lock on a weak anchor is no security at all. Good anchors are thick, fixed, and immovable: dedicated bike stands, sturdy railings bolted to the ground, and thick street furniture that cannot be lifted or cut quickly. Bad anchors include thin signposts that can be unbolted, short posts a locked bike can be lifted over, wooden fences that can be cut, and anything a thief can dismantle faster than the lock can be broken. The anchor's thickness also sets the lock length, because a fat post needs a longer lock to get around, which is why the calculator asks for it: locking to a lamp post and locking to a thin stand are different jobs.
Height matters too. Locking low, near the ground, lets a thief use the ground for leverage when attacking the lock, while locking higher, with the lock held off the ground by the bike and the anchor, makes leverage attacks harder. The classic advice is to lock the frame at a height where the lock cannot rest on the ground, and to fill the inside of a U-lock shackle as much as possible, because empty space inside the shackle is space a thief can insert a tool into. A lock that fits snugly around frame and anchor, with little room to spare, is harder to attack than a long, loose lock, which is another reason to measure rather than buy the longest lock available.
U-locks, chains, cables, and folding locks
Lock types trade security against weight and flexibility, and length means something slightly different for each. U-locks are the most secure per kilogram, because their hardened steel shackle resists cutting and leverage, but they are rigid, so their size must match the job exactly, and a U-lock that is too small is useless while one that is too large is heavy and attackable. Chains are flexible and can secure awkward combinations of frame, wheels, and anchor, but a chain long enough for both wheels is heavy, and cheap chains are easily cut, so a chain needs to be a genuinely hardened security chain, not a hardware-store chain. Cables are light and flexible but offer only deterrent-level security, suitable for low-risk stops or as a secondary wheel cable, never as the sole lock for a valuable bike.
Folding locks sit between U-locks and chains: more flexible than a U-lock, lighter than a chain, and reasonably secure in their better versions, but with joints that are potential weak points. Whatever the type, the length rule holds: measure the anchor, the frame, and the wheels, add the slack the calculator allows, and buy the size that fits that job, not the size that looks right in the shop. A lock that is too long is not just heavier; it is less secure, because its slack gives a thief room to work.
Where, how long, and what else to do
Location is a security factor that no lock can replace. A busy, well-lit place with foot traffic and CCTV deters thieves far more than a quiet corner, and the same lock that is adequate for a twenty-minute café stop may be inadequate for an all-day station parking. For long or overnight parking, use two different types of lock, because a thief prepared for one type may not be prepared for the other, and remove everything detachable: lights, bags, computers, and quick-release accessories. Register the bike's frame number, photograph it, and keep the lock's key number safe, because recovery after a theft depends on identification, and insurance claims depend on proof of the lock used.
Electric bikes raise the stakes, because they are more valuable, heavier, and often left at charging points where thieves expect them. The same length maths applies, but the anchor and lock quality matter more, and the battery, if removable, should come with you, because a bike without its battery is a far less attractive target and a battery left on the bike is an expensive, easily removed prize. The same applies to any expensive component: if it comes off without tools, either lock it or take it, and the calculator's both-wheels style is the right mindset even if the implementation uses two locks rather than one long one.
| Locking style | 10 cm anchor, 29 in wheels | 15 cm anchor, 26 in wheels | Typical lock |
|---|---|---|---|
| Frame only | 35 cm | 40 cm | Small U-lock |
| Frame + one wheel | 77 cm | 78 cm | Medium U-lock or chain |
| Frame + both wheels | 124 cm | 121 cm | Long chain or U-lock + cable |
- Measure the anchor thickness before buying a lock.
- Secure the frame and at least one wheel with the main lock.
- Keep the lock off the ground and fill the shackle to resist leverage.
- Lock in busy, well-lit places; use two lock types for long stops.
- Remove batteries, lights, and quick-release accessories.
A lock that fits, on a solid anchor, in a visible place, is the best security most cyclists will ever need. Measure first, buy second, and lock properly every time.