What is the difference between a stout and an imperial stout?

According to the 2021 BJCP style guide, an Irish stout is at 3.8 to 5 percent alcohol, an imperial stout at 8 to 12 percent.

In between lie two more styles from the same guide: sweet stout at 4 to 6 percent and American stout at 5 to 7 percent. The word stout therefore covers a whole family, from just under 4 to 7 percent, and imperial stout only begins where that family ends.

The actual alcoholic strength by volume is always stated on a label, because it is mandatory for every drink above 1.2 percent. That number is the quickest way to see which category a bottle falls into.

More grain goes into the brew, so the original gravity comes out higher and the yeast has more sugar to ferment.

The style guide expresses this in specific gravity. An Irish stout starts at 1.036 to 1.044 and ends at 1.007 to 1.011. An imperial stout starts at 1.075 to 1.115 and ends at 1.018 to 1.030. More alcohol and more unfermented sugar therefore remain, which keeps the beer fuller in the mouth.

To keep that weight in balance, more hops go in as well: 50 to 90 IBU for an imperial stout, against 25 to 45 IBU for an Irish stout.

Imperial goes back to strong English porters that were brewed for export.

The BJCP style guide describes those beers as strong English porters that were brewed for export and reportedly were popular at the Russian imperial court. After the Napoleonic wars trade was restored and these beers were increasingly also sold in England itself.

After that the style almost disappeared, until it was picked up again in modern speciality beer. The older name Russian imperial stout and the current style name imperial stout refer to the same category.

No, no beer style is legally defined in the Netherlands.

The Warenwetbesluit Gereserveerde aanduidingen does define the designation beer, as a drink obtained after alcoholic fermentation of wort, mainly prepared from starch- and sugar-containing raw materials, hops and brewing water. It also regulates a handful of Dutch categories such as alcohol-free beer, low-alcohol beer, oud bruin, pils and bokbier. Stout, porter and imperial stout do not appear in it.

The BJCP style guide is a judging document for beer competitions, not a law. A brewer may therefore also put the word imperial on a beer of 7 percent. So look at the alcohol percentage on the label and not at the word on the label.

Besides Irish stout, the 2021 style guide also recognises sweet stout, American stout and imperial stout as separate styles.

Sweet stout is at 4 to 6 percent alcohol with 20 to 40 IBU and an original gravity of 1.044 to 1.060. American stout is at 5 to 7 percent with 35 to 75 IBU and an original gravity of 1.050 to 1.075. Imperial stout closes the line-up at 8 to 12 percent with 50 to 90 IBU.

The bitterness therefore rises with the alcohol content: from 20 IBU at the bottom to 90 IBU at the top. The colour does not. Sweet stout, American stout and imperial stout are all three at 30 to 40 SRM, so dark says nothing about strong.