Brew your own beer
- Alcohol by volume, percent (rounded down, as duty is)
- 4.8
- Degrees the beer attenuated
- 37.0
- Original gravity, degrees
- 48.0
The factor table and the 20°C measuring temperature come from HMRC's alcoholic products technical guide; the molecular weights, the molar volume and the Henry's law constants behind the priming figures come from the NIST Chemistry WebBook and CODATA.
Brewing your own beer is four decisions and three measurements, and the measurements are the part that decides whether the next batch is the same as this one. You mash a grain bill at a temperature you have to hit, you boil, you take a gravity reading before the yeast goes in and another when it has stopped, and you package with enough sugar to carry the fizz you want. Everything a beginner reads concentrates on the first part and waves at the second, which is why the second batch is a surprise. This page runs the day in the order it happens and hands each measurement to the free calculator that works it: strike water and sparge volume before you start, alcohol by volume when fermentation stops, and priming sugar at packaging. HMRC's own guide, which is where the strength arithmetic here comes from, measures alcoholic strength at 20°C and rounds it down to one decimal place for duty: a level of care worth borrowing even when nobody is charging you duty.
Open the ABV calculator Free to use. No account, no card, no trial clock.
Work the water before you fill the kettle
Weigh the grain bill, pick a mash temperature and a thickness, and the mash water calculator gives you the strike temperature and the sparge volume. The strike temperature is not the mash temperature: the grain arrives cold and takes roughly four tenths of the heat water does per kilogram, so a 66°C mash at three litres per kilogram wants water at about 72°C.
Take the first gravity reading before the yeast goes in
Fill the trial jar from the fermenter, read the hydrometer, and correct it to 20°C from your hydrometer's own card if the sample is warmer or colder. That reading is the original gravity, and HMRC's guide is emphatic that it be taken as soon as possible after filling and before fermentation starts.
Leave it alone until the gravity stops moving
Two readings the same, two or three days apart, is the only reliable sign that fermentation has finished. A single reading that looks right can be a stall, and priming a beer that has not finished is how bottles come apart.
Work the strength the way the guide does
Put both readings into the ABV calculator. It takes the degrees the beer attenuated and multiplies by the factor the Laboratory of the Government Chemist's table gives for that band, then rounds down. On a drop from 1.048 to 1.011 that is 37 degrees at 0.130, giving 4.8%, the 131.25 shortcut most calculators use says 4.86%.
Prime from what the beer is already carrying
The priming sugar calculator starts from the carbon dioxide the beer already holds at the warmest temperature it saw after fermentation finished, worked from the Henry's law constants NIST publishes, and adds only the difference. A beer that finished at 12°C needs noticeably less sugar than the same beer at 24°C.
Write down what your kit actually did
Boil-off, grain absorption and brewhouse efficiency are properties of your kettle, your bag and your crush, not of the recipe. Measure them once, put them in the boxes, and every recipe after that lands where it says it will. This is the part Mashzo Pro keeps for you.
What brewers ask about the Brew your own beer
Do I need all-grain kit to start?
No. Extract brewing skips the mash entirely and every measurement after it is identical, so the ABV and priming calculators work the same way. The mash water calculator is the one that only matters once you are mashing your own grain.
How long does a batch take?
A brew day is four to six hours, fermentation is one to two weeks, and bottle conditioning is two weeks more at room temperature. The gravity readings, not the calendar, are what tell you fermentation has finished.
Why did my beer come out weaker than the recipe said?
Almost always efficiency. A recipe written for 75% brewhouse efficiency and brewed at 62% gives roughly a percent less alcohol on an ordinary-strength beer. Put your measured gravity into the mash water calculator and it will tell you the efficiency you actually got.
Is it legal to brew beer at home in the UK?
Yes, for your own consumption, with no licence and no duty. Selling it is a different matter entirely and brings HMRC's approval and duty regime with it. Distilling at home is not lawful here without a licence, and this site has nothing to say about it.
What should I brew first?
Something ordinary in strength and forgiving in temperature: a pale ale around 1.045 with a clean ale yeast. Strong beers and lagers both punish a first brew day, the first for the gravity you will not hit and the second for the temperature control you probably do not have yet.
Mashzo Pro
Keeping the readings, brew day after brew day
Pro keeps your recipes and your readings, so the efficiency and boil-off you measured last time are already in the boxes next time.
- Email a brew sheet to whoever is brewing with you
- Take payment if you sell a batch
- Send what a batch cost through to your books
£3per month, whole team
Start Mashzo Pro PricingMashzo Pro is £3 a month. It renews on the same day each month at the same price until you cancel, and the price and the renewal date are shown again before you pay.
Ask us about the Brew your own beer
Tell us what you were brewing and which number did not look right.