A lot of the shittier bars will have glasses that look like pints but only hold 14oz, 12oz of beer 2oz of foam.

The German system where there's a line on the glass that needs to be reached by beer under penalty of law puts a smile on my face.

A pint in the USA is 16 fluid ounces. In the UK (in the ‘imperial’ system) it is 20. [Technically the fluid ounce is also different in the two systems, but not enough to matter at this scale.]

Some US bars will serve imperial pints on request and/or offer them for British or Irish beers.

The difference is about 10% which is easily noticeable.

Typically in a British pub you will be served beer in a glass that is calibrated as “pint to brim”, and any head on the beer is taken out of the nominal pint. So usually a pint as served is much closer to half a litre.