How do I tell if fermenation is finished?

[CraigMac]

Hello,

My first brew is bubbling away quite happily. The instructions which came with my kit say to fermentation should be over in 5-7 days but it's day seven and it's still bubbling through the air lock now and again. Should I leave it for a bit longer before siphoning it off for secondary fermentation?

Thanks,
Craig

 

[tubby_shaw]

The only reliable way to tell if fermentation has abated is to get two gravity readings of the same value at a constant temperature over a couple of days.
An indication can be had by observing the airlock, if there are no bubbles through the airlock or are at a rate of less than one every couple of minutes then fermentation has stopped or stalled.
Either way leaving your brew for an extra couple of days under airlock will not hurt and will give time for some of the yeast to drop out of suspension :thumb:

 

[Wez]

Wot TS said

Also, if you know the OG (original gravity) of the wort before you pitched your yeast you can work out what FG (Final Gravity) it should get down to.

For example Safale S04 yeast will reduce a beers OG by around 73% (73% attenuation) so if the OG of your wort was 1.050 (ignore the 1.0 bit) you would expect the yeast to drop it by 73% giving you an anticipated FG of 1.013 (and a half)

Sounds complicated but it's not really, useful calculation to do once you've taken your OG reading :thumb: