Equipment
The hydrometer and the trial jar
How do I use a hydrometer to measure cider?
In short
A hydrometer floats at a depth determined by the density of the liquid it is in. Sugary juice is denser than water, so the hydrometer floats high; as fermentation converts sugar to alcohol the density falls and it sinks.
The reading is taken where the liquid surface crosses the stem, in a tall narrow vessel called a trial jar rather than in the fermenter itself.
Two readings — before fermentation and after — are what makes it useful. A single reading tells you the density and almost nothing else, because juice and part-fermented cider can share a gravity while being entirely different liquids.
Taking a reading properly
Fill a trial jar — a tall cylinder, usually of glass or a rigid polymer, deep enough that the hydrometer floats free — with enough sample that the instrument does not touch the bottom. Spin the hydrometer gently as you release it, which knocks off the bubbles that otherwise cling to the stem and lift it. Wait for it to settle and stop moving.
Read at the bottom of the meniscus, the lowest point of the curved surface, with your eye level with the liquid. Reading at the top of the meniscus is a common and consistent error that makes every gravity read a little high. Some instruments are calibrated to be read at the top; the maker’s instructions settle it, and the important thing is to be consistent.
Every hydrometer is calibrated at a stated temperature, marked on the paper scale inside the stem — commonly 20 °C, sometimes 15 °C or another value. A sample read well away from that temperature needs a correction, and published correction tables exist for the purpose. On a cold day in an unheated cider house the correction is not negligible.
Return the sample to the batch or discard it, but do not put a hydrometer into the fermenting vessel. It is a source of contamination, it is difficult to read against a curved wall, and it breaks.
What the numbers mean
Specific gravity is the density of the liquid relative to water, so pure water reads 1.000. Apple juice typically reads somewhere above that, the excess being mostly dissolved sugar. Some hydrometers carry additional scales — Brix or degrees Oechsle — which express the same measurement in different units; Brix is nominally percentage sugar by mass, and is exact only for a pure sucrose solution.
The drop in gravity across fermentation is proportional to the sugar consumed, and there are standard formulae relating that drop to the alcohol produced. Those formulae are approximations: they assume a particular relationship between sugar consumed and ethanol produced, and cider deviates from it because yeast also makes glycerol and other products, because acids and unfermentable material contribute to the density, and because apples contain sorbitol, which yeast does not ferment.
The last of these has a specific consequence. A dry cider that has genuinely used all its fermentable sugar can finish below 1.000, because ethanol is less dense than water — final gravities in the region of 0.995 to 1.000 are ordinary. It is also why perry, with its higher sorbitol content, can finish at a higher gravity than an apple cider of the same dryness, and why a perry gravity reading over-states the residual fermentable sugar.
Where a hydrometer misleads
- Suspended solids raise the apparent density. A cloudy sample straight off a vigorous ferment reads high; the same cider settled reads lower.
- Dissolved carbon dioxide raises the reading and sticks bubbles to the stem. Degas the sample by swirling or standing it before reading a fermenting cider.
- Temperature error is systematic and often larger than the difference the maker is trying to detect.
- A hydrometer that has been dropped, or whose paper scale has slipped inside the stem, reads confidently and wrongly. Check it in distilled water: it should read 1.000 at its calibration temperature.
- A single reading cannot distinguish a stuck ferment from a slow one. Only a series of readings over days can.
The trial jar
The trial jar is not an accessory but part of the instrument. Its diameter must be large enough that the hydrometer does not stick to the wall by surface tension, and its height enough that the instrument floats clear of the base at the lowest gravity expected. A narrow jar drags the float against the side and gives a reading a few points out.
It also defines the sample volume, which matters because every reading removes cider from the batch. At demijohn scale a hundred and fifty millilitres taken repeatedly is a real loss, and it is one reason small-batch makers reach for a refractometer at the start and reserve the hydrometer for the readings that matter.
Trial jars are cleaned and sanitised like anything else that touches the cider, and the hydrometer with them. A hydrometer that lives in a drawer between uses goes into the sanitiser before it goes into the sample.
Also answered on this page
Questions this page covers, so you can tell at a glance whether it is the one you want.
- How do you read a hydrometer?
- What should the final gravity of cider be?
- Why is my final gravity below 1.000?
Related
What people ask next
Questions readers ask about the things this page mentions. Each one goes to the section that answers it rather than to a page written to receive the question.
- How do i know when my cider has finished fermenting — Take two hydrometer readings several days apart: if the gravity has not moved, the ferment is over. Airlock activity is not a reliable test, because a slow ferment can produce gas too slowly to see.
- How long does cider take to ferment — A warm ferment with cultured yeast can finish in one to two weeks; a cool wild ferment in a cellar may take three months or more. Slow is not the same as stuck — the test is whether gravity is still falling.
- How many calories are in cider — Roughly 40 to 60 kcal per 100 ml for most ciders, so a UK pint falls somewhere around 200 to 250 kcal. Alcohol contributes about 7 kcal per gram and residual sugar about 4, so both strength and sweetness matter.
- What is perry — Perry is an alcoholic drink made by fermenting the juice of pears, traditionally of specific perry pear cultivars rather than eating pears. It is to pears what cider is to apples, and has its own history in the three counties of England and in Normandy.
- Why did my cider stop fermenting — The usual causes are a shortage of yeast-available nitrogen, a temperature that has dropped, too much sulphite at the start, or a yeast that has reached its alcohol limit. Check the gravity before assuming anything is wrong: many ciders simply finish.
- Can i use a refractometer to track cider fermentation
Sources
What this page rests on. Where a source is marked as registered rather than read, CiderHQ is recording that the body is authoritative on the subject without claiming to have worked through the document itself. See our evidence policy for what each state means.
The New Cider Maker’s Handbook: A Comprehensive Guide for Craft Producers
Claude Jolicoeur, Chelsea Green Publishing, 2013. ISBN 9781603584739 · reference work · bibliographic record verified, not opened 2026-08-25
Bibliographic record verified on 2026-08-25 against the Open Library union catalogue: Chelsea Green Publishing, 2013, ISBN 9781603584739, one edition recorded. That establishes the citation points at a real book in a stated edition, which is what a citation needs and is all it establishes. No copy was opened and nothing is quoted from it. The book itself is in print and not digitised in any open collection; where CiderHQ needs a figure from this territory it uses an accessible research source instead and says so.
The Science of Cidermaking and associated technical writing
Andrew Lea · reference work · passage verified 2026-08-24
Written by a food chemist who worked at Long Ashton on apple phenolics. Unusual among specialist cider writing in that it is primary-research-adjacent: the author is describing work he did, and cites the literature. This is why it is registered at tier 1 for chemistry while a general cider book is not.
Cider and perry production technical training material
Peter Mitchell / Cider and Perry Academy · reference work · registered as competent for this subject
Searched again on 2026-08-25 and it remains the one printed source here that a bibliographic check cannot fix, because it is not a published work with an edition. It is course material issued to participants of a training programme, so there is no catalogue record to verify, no ISBN, and no lawful public copy. Its state stays `registered` for that reason rather than through inattention.
Where to go next
- Cider making equipment — The rest of the equipment material, grouped by what each piece is for.
- How cider is made — The methods this equipment exists to carry out.
- Troubleshooting — The faults that are traced back to equipment and to cleaning.