Brewing step
Beer fermentation: temperature, time, gravities
The wort is ready; the yeast is going to make the beer. Fermentation is the same with a kit, with extract or in all grain, and it weighs the most on flavour: a stable temperature suited to the yeast counts for more than the brewing method.
Updated on 18 October 2026
What yeast does
Yeast consumes the sugars in the wort and produces alcohol, carbon dioxide and aromas. It works in three phases:
- The lag phase, 6 to 24 hours: the yeast multiplies by consuming the oxygen in the wort. Nothing shows yet.
- Primary fermentation, 3 to 7 days: a thick foam forms (the krausen), the airlock bubbles, the gravity drops fast. The wort temperature rises 2 to 4 °C above that of the room.
- The end of fermentation and conditioning, a few more days: the yeast finishes the remaining sugars and reabsorbs unwanted compounds, such as diacetyl (buttery flavour).
Pitching
- Cool the wort to fermentation temperature before adding the yeast, never above 25 °C.
- Aerate the wort by pouring it from a height or by shaking the closed fermenter: yeast needs oxygen to multiply.
- Pitch enough yeast. For 20 litres at 1.050 with a dry ale yeast, the pitching rate calculator recommends 186 billion cells, that is 2 sachets of 11.5 g. A single sachet often works, with a longer lag. A lager needs about twice as much yeast.
- With a liquid yeast, a yeast starter prepared one or two days ahead multiplies the cells.
Temperature
| Yeast type | Wort temperature | What happens if it is too warm |
|---|---|---|
| Ale (US-05, S-04, English yeasts) | 16 to 20 °C | Fruity esters, fusel alcohols, solvent flavour |
| Belgian and saison yeasts | 18 to 26 °C, rising | Sought-after aromas, but strong phenols beyond |
| Wheat (weizen) | 17 to 22 °C | Very marked banana above 22 °C |
| Lager (W-34/70, S-23) | 9 to 13 °C | Esters and sulphur, loss of the clean character |
| Kveik | 25 to 37 °C | Little affected by heat |
It is the wort temperature that counts, not the room temperature. In the first days, fermentation warms the wort by several degrees: a room at 18 °C gives a wort at 20-22 °C.
To hold the temperature without equipment: a cellar, an interior cupboard, a tub of water in which you stand the fermenter with bottles of frozen water. With equipment: an old fridge or freezer driven by a thermostat (with a heat belt or heat pad for winter), or a regulated double-walled fermenter.
Following the gravity
- Measure the original gravity before adding the yeast.
- During fermentation, the gravity drops. An airlock that has stopped bubbling proves nothing: the lid may leak, and fermentation may continue slowly.
- Fermentation is finished when the gravity stays the same two to three days in a row, at a value close to the one expected. The estimated final gravity calculator predicts it from the yeast and the recipe.
- Read the hydrometer at 20 °C or correct the reading (gravity correction for temperature).
With the original gravity and the final gravity, the ABV calculator gives the strength: from 1.050 to 1.010, the beer is 5.25%. The attenuation shows what share of the sugar the yeast has consumed, 80% here.
How long
- An ale: one to two weeks in all, including three to seven days of primary fermentation.
- A lager: two to three weeks of cold fermentation, often a two-day rest at 16-18 °C at the end of fermentation to remove diacetyl (diacetyl rest), then several weeks of cold conditioning (lagering).
- A strong beer (above 1.070): two to four weeks.
Leaving an ale a few more days after fermentation has finished does it no harm and helps the yeast clean up unwanted aromas. A transfer to a second fermenter (secondary fermentation) is not needed for most beers.
Dry hopping and clarification
- Dry hopping adds hops directly to the fermenter, at the end of fermentation, for aroma: 3 to 5 days are enough; beyond that, the aromas can turn grassy.
- A cold crash of 2 or 3 days at around 2 to 4 °C before bottling drops the yeast and particles out and clears the beer.
Depending on your equipment
Fermentation does not depend on the brewing method: the wort from a kit, an extract brew or an all-grain brew ferments the same way. Only two differences:
- With a hopped kit, the yeast supplied is often old: replacing it with a fresh dry yeast of the same style improves consistency.
- With a 5 litre kit, the glass demijohn is kept out of the light, and a quarter to half a sachet of dry yeast is enough.
Common problems
- Nothing happens after 48 hours: check the gravity before concluding, the lid may be leaking. If it does not move, the yeast was dead or the wort was too warm when pitching: add a new sachet.
- The gravity gets stuck above the expected value: raise the temperature by 2 to 3 °C and gently swirl the fermenter to put the yeast back in suspension.
- A rotten egg smell: common with lagers and some yeasts, it disappears at the end of fermentation.
- A white film on the surface after fermentation: often a contamination. Taste before throwing it away, and review your sanitation.
Depending on your equipment
The other steps
Frequently asked questions
What temperature should you ferment beer at?
Between 16 and 20 °C for an ale, between 9 and 13 °C for a lager, measured in the wort. Belgian, saison and kveik yeasts tolerate or need warmer. A stable temperature counts as much as its value.
How long does beer fermentation take?
One to two weeks for an ale, two to three weeks for a lager, more for strong beers. Fermentation is finished when the gravity is stable two to three days in a row, not when the airlock stops.
How do you know fermentation is finished?
Measure the gravity with the hydrometer two or three days in a row. If it no longer changes and is close to the expected final gravity, fermentation is finished and you can bottle.
Do you need a secondary fermentation?
Not for most beers. Leaving the beer in the primary fermenter for two to three weeks gives the same result with less risk of oxidation and contamination. A second fermenter is only useful for long conditioning or adding fruit.
Ready for your next brew?
Create your account, describe your first recipe to Krausi, and let him do the maths.