Tool

Is my cider still fermenting, stuck, or finished?

Enter your gravity readings as you take them and this reads the shape of the log — still working, slowing in the ordinary way, collapsing early, or stopped — judged against the ferment’s own pace rather than a fixed speed, with what to check, in order, if it has stalled.

Fermentation progress

Fruit

Starting gravity

The reading before fermentation began. It counts as day 0.

Temperature now (°C, optional)

Of the cider, if you know it; otherwise of the room it is in.

Readings since fermentation began

One row per reading: the day it was taken, counted from day 0, and the gravity, corrected for temperature if you can.
Day	Gravity	Remove
Day of reading 1	Gravity of reading 1
Day of reading 2	Gravity of reading 2
Day of reading 3	Gravity of reading 3
Day of reading 4	Gravity of reading 4

What the log shows

Stopped with sugar left

The gravity has held at 1.018 for 4 days. Either this is deliberate — a keeved or arrested ferment, which is meant to stop sweet — or it is stuck. If it was not meant to stop, the checks below are in the order worth working through.

Gone so far: 34 points
Apparent attenuation: 65.4%
Alcohol so far, about: 4.5% ABV
Latest rate: 0.3 points a day (peak 3.5)

Do not seal this in bottles yet. Fermentable sugar may remain, and a ferment that restarts in a sealed bottle builds pressure the glass was not made for. Bottle only once the gravity has held steady for a week or more at the finish you intend — or use a method designed to stop or contain refermentation.

What to look at, in order

Rule out first: Was it meant to stop?. Keeving and arrested fermentation both stop a cider sweet on purpose. If either was the plan, this is the intended outcome and the question is stability, not restarting. Read more
Rule out first: The readings themselves. Correct each one for temperature, spin the hydrometer to shed gas bubbles, and take the sample from mid-depth. A stall that appears between two carelessly taken readings sometimes disappears on the third. Read more
Likely: Yeast nutrient: apple juice is usually short of it. In one survey of 108 apple juices, 94% carried less assimilable nitrogen than wine practice treats as the minimum for a complete ferment. A population that runs out of nitrogen stops taking up sugar even while sugar is plentiful — the commonest cause of a stuck cider. Read more
Possible: Temperature, and whether it has changed. The commonest cause after nitrogen, and the easiest to check. A cellar that has cooled in the last week explains a lot of sudden slowing. Read more
Possible: Sulphite added before or during the ferment. An addition that was high for the juice’s pH inhibits the yeast as well as what it was meant to suppress. Read more

What a healthy ferment looks like on paper

Plotted, the readings of a ferment that is going well make a lopsided S. They hardly move for the first day or few while the yeast population builds; they fall fastest through the middle, when most of the sugar is still there and the population is at its largest; and they flatten as the sugar runs out, so that the last ten points can take as long as the forty before them. That flattening is the ferment ending, not failing, and it is the reason a projected finish date is always too early.

How fast the middle runs varies more than most guides admit. A cultured yeast at cellar-warm temperatures can take several points a day; a traditional uninoculated ferment in a cold barn may never exceed one, and take months, and be entirely sound. That is why this tool judges slowing against the ferment’s own peak rather than against a fixed speed — a fixed rule would call every traditional ferment stuck from the start.

Stopped, stuck and finished are three different things

A finished cider has used its fermentable sugar: an apple ferment ends at or a little below 1.000, because alcohol is lighter than water. A stuck one has stopped with sugar left and will not restart on its own. An arrested or keeved one has stopped with sugar left because the maker meant it to. The gravity log cannot tell the second from the third — only the maker knows what they did — which is why the first question the tool asks of a ferment that stopped sweet is whether that was the plan.

Perry is the exception to the rule about 1.000. Pears carry sorbitol, a sugar alcohol that yeast does not ferment, so a perry can be finished several points above where a cider from the same gravity would stop, and taste soft rather than sugary for it.

The airlock is not a measurement

Bubbles through an airlock report gas leaving the vessel, and gas leaves for reasons other than fermentation: a warming cellar drives dissolved carbon dioxide out of a cider that finished a week ago, and a leaking seal lets a vigorous ferment vent silently. The only measurement of whether sugar is being used is the gravity, taken on a sample corrected for temperature and spun to shed the bubbles that hold a hydrometer up.

The standard that matters most is the last one: the same reading over a week or more, at the finish intended, before anything is sealed in a bottle. Sugar left in a bottled cider is the cause of over-pressurised bottles, and glass that fails under pressure injures people.

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Where this reading comes from

Guides that reach this calculator, and what the number decides once you have it.