Some bike lock makers provide their own internal security ratings for guidance. Those can be helpful, but the most secure locks we’ve tested use standard ratings from objective sources, including Sold Secure, which offers ratings of multiple vehicle types between Bronze and Diamond, and ART, which uses a number scale from 1 to 5.

So what do these ratings actually mean? Here’s a guide to two of the top lock-security firms and their rating systems:

Sold Secure was established in 1992 by Northumbria and Essex police in the UK. The brand is now owned and operated by the Master Locksmiths Association, a not-for-profit trade group. It uses a four-level rating system, as listed below.

Sold Secure Rating Levels: Bronze (lowest, secure against basic tools); Silver (midrange, secure against enhanced tools/determined attacks); Gold (second best, resistant against dedicated tools/attacks); Diamond (highest, resistant against destructive attacks and “special tools”).

The firm also uses separate ratings for different vehicle types, like bikes versus motorcycles (which it now calls pedal cycles and powered cycles, respectively). For example, the Hiplok DX has a Sold Secure Pedal Cycle Diamond rating and a Sold Secure Powered Cycle Gold rating. It’s a good score, but it isn’t as secure as top grinder-resistant options like the Litelok X1 or tougher locks like the Litelock X3 and Hiplock D1000, all three of which get Diamond ratings in both categories. Grinder-resistant locks will usually say so directly on the brand’s site as well. When in doubt, you can always go to the Sold Secure website to check whether your lock is rated.

ART is a Dutch independent security firm that uses a five-star security system centered on vehicle type, durability, weather, environmental resistance, and resistance to attacks.

ART Rating Levels: 1/5 (minimum security rating, fit for a second bike lock only); 2/5 (higher security rating that represents good theft protection for bicycles, the minimum for most insurance companies); 3/5 (bicycle security for high risk of theft, also suitable for mopeds and scooters for insurance purposes); 4/5 (very high security rating for bicycles, good theft protection for motorcycles while traveling); 5/5 (highest security rating, sometimes required to insure motorcycles stored at home).