What is the difference between stout and porter?

Porter came first. The name appears in London around 1700, for a more heavily hopped and aged brown beer. Stout arose as the name for the stronger version of it.

The Beer Judge Certification Program guidelines call porter the forerunner of all stouts, which were originally called stout porter. Guinness brewed porter from 1799 and brought a heavier version onto the market around 1810. Only at the end of the nineteenth century did Irish brewers begin to lean more heavily on dark malt and roasted barley, and the two names grew apart.

No. There is no law that determines when a brewer may put porter on the label and when stout. Both names are free to use.

What does exist are style guidelines from judging organisations. The Beer Judge Certification Program itself notes that the names of its categories are intended for competitions only and are not always used that way in the brewery. A brewer who calls his 6 per cent beer a porter is therefore doing nothing wrong.

In the guidelines, English porter is listed at 4 to 5.4 per cent alcohol, 18 to 35 IBU and 20 to 30 SRM. Imperial stout goes up to 8 to 12 per cent.

In between, the styles overlap widely. American porter sits at 4.8 to 6.5 per cent and 25 to 50 IBU, Irish dry stout at 3.8 to 5 per cent and 25 to 45 IBU. A robust porter is therefore stronger than an average Irish stout. Only at the top end, with imperial stout at 50 to 90 IBU, do the ranges really diverge.

Four stout variants differ mainly in what goes into the kettle. Dry stout is dry and roasted, sweet stout contains milk sugar, oatmeal stout oats, imperial stout simply much more malt.

Irish dry stout stays light at 3.8 to 5 per cent alcohol. Sweet stout, formerly called milk stout, comes in at 4 to 6 per cent and 20 to 40 IBU; the milk sugar does not ferment and remains in the beer. Oatmeal stout uses 5 to 20 per cent oats in the grain bill and sits at 4.2 to 5.9 per cent. Imperial stout goes back to strong English export porters from the eighteenth century and reaches 8 to 12 per cent.

Look at the alcohol percentage and the style designation on the label, not just the name. The figures for porter and stout overlap, so the word says little about strength.

A rule of thumb that often holds: if it just says porter, expect 4 to 6.5 per cent. If it says stout without anything further, the range is roughly between 3.8 and 6 per cent. If it says imperial, you are in the class of 8 per cent and above.