Why beer bottle caps have exactly 21 ridges: the science behind the standard

21 ridges: perfect balance for sealing

When engineer William Painter invented the crown cork in 1892, the original design sported 24 ridges. The cap included a cork or paper lining to prevent the beverage from touching metal, and it sealed bottles far better than the cork stoppers or swing-top closures used at the time. As bottled beers spread in popularity, manufacturers started using machines to level up production and that's when problems surfaced. The 24-ridge caps were difficult to fit on bottles efficiently. They often damaged bottle lips or got stuck in capping machinery.

These problems were solved by engineers through trials of fewer ridges-first to 23, then 22-but neither worked well. Eventually, 21 proved ideal, offering the best balance between tight sealing under pressure and ease of capping and removal along with minimum risk of breakage.

Beer and soda bottles have to contain carbon dioxide gas under pressure. A good closure needs to be tight, so as not to allow leaks, but it should release safely at the time of opening. The design of 21 ridges maximizes the contact surface between the edges of the cap and the bottle lip for a secure seal while letting the pressure stay evenly distributed.

The number of 21 ridges provides enough grip for bottle-opener teeth to latch onto, thus making removal easier and safer. Too many ridges on the caps tend to hold too tight and might dent the bottle or make the opening more difficult. That's not ideal when you just want to enjoy your drink.

Over time, the 21-serration design became somewhat of an international standard. With the standardization of bottling and capping machinery, the reliability and efficiency of this design spread to the rest of the world. Today, almost all crown caps on glass beer bottles use 21 ridges irrespective of brand or origin.

More than just a seal, it's smart engineering.

What would appear to be a small, almost insignificant detail-the number of ridges on a beer cap-is actually the result of decades of trial and error, tests of pressure, and industrial evolution. The balance of friction, safety under pressure, manufacturing speed, and user convenience is struck by the 21-ridge crown cap.

So the next time you crack open a cold beer, give a little nod to the hidden engineering behind that familiar pop, and appreciate how a simple choice from the 1890s still shapes our beer bottles today.