How to use a hydrometer

A hydrometer is a sealed glass tube weighted at the bottom. It floats higher in denser liquids and lower in less dense liquids — the same principle that keeps a ship afloat deeper when it is empty and higher when it is heavy. In meadmaking, the density of your liquid changes as sugar is converted to alcohol: a sugary must is denser than water, and a dry finished mead is close to or even slightly below water's density.

The scale printed on the paper inside the tube shows Specific Gravity (SG)— the ratio of your liquid's density to the density of pure water. Pure water measures exactly 1.000. A typical mead must before fermentation might read 1.090–1.120. A finished dry mead typically reads 0.995–1.010.

A standard brewing hydrometer has three parts:

1. 1

   The bulb — The weighted bottom that keeps the hydrometer floating upright. The weight is usually lead shot or small ballast pellets sealed inside the glass.

2. 2

   The stem — The narrow tube that extends above the liquid surface. The scale is printed on a paper insert inside the stem.

3. 3

   The scale — Usually shows three scales side by side: Specific Gravity (SG), Brix (% sugar by weight), and potential ABV. The SG scale is the one you will use most. Brix and potential ABV are derived from SG and are printed as a convenience.

Most hydrometers are calibrated at 20°C (68°F) — readings taken at significantly different temperatures will require a small correction (see the temperature correction section). Hydrometers typically come with a trial jar — a tall, narrow plastic cylinder used to take readings without disturbing your fermenter.

1. 1

   Sanitise your hydrometer and trial jar before use. Anything that touches your must can introduce contamination. A standard no-rinse sanitiser (Star San or equivalent) is sufficient.

2. 2

   Draw a sample using a wine thief or turkey baster. Fill your trial jar to about 3/4 full. Never put the hydrometer directly into your fermenter — you cannot sanitise it in place, and it will disturb your fermentation.

3. 3

   Remove bubbles. If your must is actively fermenting, CO₂ bubbles will cling to the hydrometer stem and give a false low reading. Spin the hydrometer gently between your fingers to dislodge them, then let it settle before reading.

4. 4

   Let it float freely. Make sure the hydrometer is not touching the sides of the trial jar. Give it a gentle spin and let it come to rest in the centre of the liquid before reading.

5. 5

   Read at eye level. Crouch down so your eye is level with the surface of the liquid. Do not read from above — looking down at an angle causes parallax error and produces a reading that is too high.

6. 6

   Read at the bottom of the meniscus — surface tension causes the liquid to curve upward where it meets the hydrometer stem. Read the scale at the lowest point of this curve, not the raised edges.

7. 7

   Note the temperature of your sample and apply a correction if it differs significantly from 20°C (68°F). See the temperature correction section below.

Surface tension causes liquid to curve upward where it contacts the hydrometer stem. This curve is called the meniscus. When you take a reading, you must decide where on that curve to read — and the answer matters.

Always read at the bottom of the meniscus — the lowest point of the liquid surface at the centre of the hydrometer. Reading at the top of the curve (at the raised edges where the liquid meets the glass) will give a reading that is consistently too high. The error is small — typically 0.001–0.002 — but it accumulates across OG and FG readings and will make your ABV calculations unreliable.

For dark or opaque meads — bochet, dark fruit melomels — it can be difficult to see the meniscus clearly against the liquid. Shine a torch or flashlight behind the trial jar to backlight the scale and make the meniscus more visible.

Hydrometers are calibrated at 20°C (68°F). If your sample is at a different temperature, the density of the liquid itself changes slightly, giving you an inaccurate reading.

The rule of thumb is simple: a sample warmer than 20°C will read lower than the true gravity (add the correction); a sample cooler than 20°C will read higher (subtract the correction).

For most mead situations, if your sample is within 5°C of 20°C the correction is small enough to ignore. For accurate readings, take your sample at or near room temperature — draw the sample and let it sit for a few minutes before reading.

OG is the gravity reading taken before fermentation begins — after all your honey and water (and any fruit) have been combined, but before pitching yeast. It tells you how much fermentable sugar is in your must, which determines predicted ABV and YAN requirement.

Take your OG reading carefully and record it. Once fermentation starts, the reading will never return to this value — you cannot measure OG retrospectively.

Use Wyvern's calculator to calculate your target OG →

FG is the gravity reading when fermentation is complete. A dry mead typically finishes at 0.990–1.010. A sweet mead — where fermentation was stopped intentionally, or the yeast hit its alcohol tolerance limit — finishes higher: 1.020–1.040 or above.

How to confirm fermentation is complete: take a gravity reading on two consecutive days. If the SG has not changed between readings, fermentation is done. Never rely on airlock activity alone — a mead can appear completely still while still fermenting slowly.

Attenuation is the percentage of sugar that has been consumed:

Once you have both OG and FG, ABV is calculated with a single formula:

Note: this formula assumes a dry finish. If your mead finishes sweet (FG above 1.020), the actual ABV will be lower than the formula suggests — Wyvern's calculator uses the yeast's ABV tolerance as a cap to account for this.

A refractometer measures Brix (sugar concentration) using light refraction. It only needs a few drops of liquid — making it very convenient for taking readings without drawing a full sample from your fermenter.

The limitation is significant: once fermentation begins, the alcohol in the must affects how light refracts through the liquid, producing readings that are lower than the true gravity. Refractometer readings mid-fermentation and at FG are therefore inaccurate unless you apply a correction formula — which adds complexity and reduces precision.

The practical rule: use a refractometer for OG (pre-fermentation, when there is no alcohol to distort the reading). Use a hydrometer for all mid-fermentation readings and for FG. If you only own one instrument, own a hydrometer.

• Hydrometers are fragile glass — handle with care, especially when placing them in trial jars.
• Always sanitise before use with a no-rinse sanitiser.
• Rinse thoroughly with clean water after each use to prevent sugar residue from drying inside the tube and altering buoyancy.
• Dry carefully and store in the plastic tube it came in, or somewhere it cannot roll off a surface.
• Check calibration periodically: fill your trial jar with pure water at exactly 20°C (68°F). A correctly calibrated hydrometer reads exactly 1.000. If yours reads 0.998 or 1.002, note the offset and add or subtract it from all your readings. Most brewing hydrometers are accurate to ±0.001 out of the box; a consistent small offset is normal.