What Size U Lock For Your Bike?

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A good U lock is an excellent way to protect your bike.

Well-made U locks are very resistant to common tools and techniques used by bike thieves, and you can use them in a variety of ways to secure a bike.

When we talk about the size of the U lock, there are three different factors at play.

So let's take them one by one.

Size of a U lock: Lock Area Size

When people talk about the size of a U lock, this is the measure they are generally referring to.

The overall measurements of a U lock determine how big the lock area or internal dimensions are.

Because U locks are rigid, you can't just be wrap them around light posts, traffic signs, or another convenient anchoring spot. Therefore the size of the lock area matters.

The internal area needs to be wide enough to fit around these kinds of objects if you are using them instead of a bike rack.

And if the bars of a U lock are too short you may not have enough length to close the lock unless you can get your bike frame resting right against it.

Length and width also need to be bigger if you are using your U lock with a scooter or moped.

However, big U locks with a lot of interior space also pose a security risk.

The most common method of defeating a U lock is to insert a crowbar or bottle jack inside and pry it open in that way.

For optimal versatility look for a U lock that is wide enough so that it will fit around light posts and street signs but is also as small as possible.

This will reduce available space for a jack or lever to pry it open.

What Size U Lock Should I Use for My Bike?

Key measure

If you need a wider U lock the widest locks measure about 4 – 4.5 inches.

Look for a length of 8 - 9 inches and remember that you want the smallest U lock that will work for you.

Size of a U Lock: Crossbar Size

The diameter of the crossbar is a critical factor in U lock security. While U locks are usually made from hardened steel, you can cut the bars with bolt cutters if they're too thin.

The bars on a U lock should be at least 13 mm for medium security.

At 16 mm or more they you can't cut them with bolt cutters and are considered high security.

Of course, all that thick steel adds weight so most people opt for the thinnest crossbars that work for their security needs.

Key measure

13-15 mm crossbars for medium security, 16+ mm for high security

Size of a U Lock: Mini U Locks

Today, you can get very small U locks that aren't designed to attach a bike frame to a bike rack.

Instead, these compact U locks can be used in place of padlocks to secure cables or chains.

You can also used them as a simple device to lock the rotation of the rear wheel to immovable objects.

This will render the bike non-rideable without attaching the frame (also called the Sheldon method of bike locking).

These mini U locks are loved by many bike riders.

They meet the requirement of being small enough that tools can't fit and get leverage inside. 

They are also lighter and more portable than their full-sized counterparts.

The smallest U locks are barely bigger than a robust padlock and you can used them in a wide range of ways to secure a bike.

Choosing the Right Size U Lock for U

Remember, if a U lock is to be your primary bike security method, then it needs to be as small as possible but still do the job.

Here are some things to consider.

The size of your bike or vehicle frame and wheels

The smallest U locks may not be able to fit around the fatter tires of a mountain bike.

The thicker frames of scooters or electric bikes, or other non-standard bike frames and wheels could be prohibitive.

The size of your bike rack or attachment point

If you frequently find yourself locking your bike to light posts, fence railings, or other convenient places, you may need a U lock that is wide enough to fit around these items.

Congestion at the bike rack

Small U locks really only work when you can get your bike frame flat against a rack.

If you often park in places where there are a lot of bikes, and it isn't possible to park flush against a rack, you may need a longer U lock.

Other security measures

If you are combining a U lock with a chain, cable, second U lock, etc., then a U lock becomes part of your security ecosystem.

Maybe you use a big, heavy bike chain to lock your bike overnight, and a big U lock for parking a bike at work or school all day, but a mini U lock for quick errands in safer areas.

You may end up with two or three different locks, depending on your need for speed, convenience, and portability vs. your need for security.

If you aren't sure how big a U lock you need, a good way to find out is to make a mock-up U lock out of cardboard.

Take it from place to place and see if it fits in the different situations where you lock your bike, and if it's too big or small.

By doing this hands-on practice, you will get a good sense for how wide and how long you need your U lock to be.


U locks come in more sizes, thicknesses, and weights than ever before, and can be used in a wide range of ways to lock a bike.

Because we have so many options, it is easy to find a U lock that will fit your needs perfectly, at any size.

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