IBU stands for International Bittering Units and the bitterness of a beer can be quantified and expressed in IBUs. One IBU is the same as one milligram of isomerised alpha acid per litre of beer.

The proper way to work out the bitterness of a beer is by analysing a sample in a laboratory but if, like me, you don't have a spectrophotometer, then you can make do with using a figure calculated from the hop additions.

It's convenient to use brewing software that does this for you but it's just as easy to work it out for yourself. Here's how:

Each hop addition adds bitterness to the beer and the bitterness from all the additions can simply be added together to give the total bitterness. To work out how much bitterness any given hop addition provides, you just do the following:

- Multiply the grams added by the alpha acid percentage of the hop. This gives you a total amount of alpha acid available from that addition. (in grams)
- Multiply this by the utilisation rate, which is the percentage that gets isomerised. I use 26% for 60 minute additions and 5% for 0 minute additions. This gives you a total amount of isomerised alpha acid. (still in grams)
- Multiply this figure by 1000 to get milligrams and divide by the batch size to get a per litre figure. This is now your IBU figure for that particular addition.

For example, if you add 15g of a 12%aa hop at 60 minutes, then you get:

- 12% of 15g is 1.8g
- 26% of 1.8g is 0.468g
- 0.468g x 1000 / 26l = 18 mg/l or 18 IBUs.

Repeat for each hop addition and add them all together to get the total. This could get pretty tedious so instead of doing the sums each time, I've put it all in a spreadsheet which works it out automatically. All I have to do is alter the alpha acid percentages and the weights. The beauty of this is that you can fiddle around with your own utilisation rates.

A few brewers, such as Marble, are starting to cut out the middle hop additions and add the majority of the hops at flameout, making up any remaining bitterness with a small addition at the start of the boil.

According to the Tinseth numbers, which most of the brewing software use as default, flameout additions don't contribute any bitterness. In reality this isn't the case though. I've spoken to one brewer, who does have a spectrophotometer, who says they get 10-14% utilisation from flameout hops. Another pro brewer suggested 4-5% is a good ballpark figure to use. I suspect that for normal homebrew kit, with a 15-20 minute hop stand, a figure somewhere in between is going to be right.

It's important to remember that the utilisation percentages are approximate and will vary a lot depending on the kit you use but the above figures are good enough to get started. If you're adding a big flameout addition and are using a generic brewing calculator to work out the IBUs, then be careful, because the chances are your beer could end up more bitter than expected.

The only way the flameout hops add no significant bitterness is if you cool the wort down to below 80C before adding them, which means little isomerisation takes place. This is what some brewers do, with great success. It's just another way of doing things but as ever, it's probably best to get brewing and see what works for you.

The only way the flameout hops add no significant bitterness is if you cool the wort down to below 80C before adding them, which means little isomerisation takes place. This is what some brewers do, with great success. It's just another way of doing things but as ever, it's probably best to get brewing and see what works for you.