Reading Gravity, ABV & Efficiency

Three numbers tell you most of what happened on brew day and in the fermenter: original gravity (OG), final gravity (FG), and the efficiency you got from your grain. From OG and FG you get ABV and attenuation; from OG and volume you get brewhouse efficiency.

The last one is the one worth caring about in a club, and the reason is simple: a recipe’s stated efficiency is the author’s, not yours. Brew a house recipe or another member’s beer on your own kit and you will not hit their OG unless your systems happen to match. That is not the recipe being wrong, and it is not you doing it wrong — it is the single most common reason a shared recipe disappoints the first time.

Taking a reading you can trust

Hydrometer

A weighted float; the more sugar, the higher it sits. Cheap, accurate, and works right through fermentation.

• Temperature. Calibrated at one temperature, usually 20 °C. A hot sample reads low. Cool the sample or correct (~+1 point per 6 °C above 20 °C)
• Read at the bottom of the meniscus, with a CO₂-free sample. Bubbles cling to the float and read high
• Resolution. A glass hydrometer resolves maybe half a point. That matters when you are watching for a small change rather than a big one

Refractometer

Reads sugar from two drops as °Brix — ideal on brew day because it needs almost no sample and no cooling.

• Wort correction. Most read slightly high on wort; divide Brix by ~1.04 before converting
• Alcohol breaks it. Once fermentation starts, alcohol bends the light and the reading is wrong. Refractometer for brew day, hydrometer for fermentation — or use a refractometer-FG calculator that corrects using OG plus current Brix

Comparable readings

Most gravity confusion comes from comparing readings that were never comparable. Take them the same way each time: same tool, same temperature correction, same point in the process. When you are watching a slow change — the tail of a fermentation, or a possible hop creep — take readings a day apart rather than hourly. Sampling too often just gives you noise, and it costs you beer.

ABV

ABV (%) ≈ (OG − FG) × 131.25

OG 1.052, FG 1.010 → 0.042 × 131.25 ≈ 5.5% ABV. A more accurate form exists for strong beers; at normal ale strengths this is within a rounding error.

Attenuation

Apparent attenuation (%) = (OG − FG) / (OG − 1.000) × 100

1.052 → 1.010 gives 0.042 / 0.052 ≈ 81%. Compare against your yeast’s published range — every strain page under yeast lists one. Landing well below it usually means a stuck or incomplete fermentation rather than a recipe that was meant to finish sweet. Landing well above it, on a dry-hopped beer, is worth a second look: that is what hop creep looks like on paper.

Efficiency — and why your kit is not their kit

Efficiency is how much of the grain’s available sugar you actually extracted. There are three figures and people mix them up constantly:

• Conversion efficiency — did the mash convert the starch (crush, temperature, time, pH)
• Mash/lauter efficiency — conversion plus how well you rinsed the sugar out
• Brewhouse efficiency — the whole-system figure into the fermenter, after the boil and all losses. This is the one recipe software wants, and homebrew systems typically land between 65–80%

That spread is the important part. Across the club we brew on all-in-ones, three-vessel rigs and BIAB setups, with different mills, different crushes and different dead space. A 15-point spread in brewhouse efficiency on the same grain bill is an OG difference of several points — enough to move a beer out of style.

So when you brew a club or member recipe: check what efficiency it was built at, compare it to your own, and scale the grain bill rather than accepting a low OG. If the recipe does not say, assume it was built at the author’s number and treat your first attempt as the calibration run.

Working out your own number

Work in points × litres, which are conserved — boiling concentrates gravity but does not change total sugar.

Collected  = (measured SG − 1) × 1000 × volume (L)
Possible   = Σ (grain kg × ~300 points·L/kg for base malt)
Efficiency = Collected / Possible × 100

Worked example. 5.0 kg base malt, into the fermenter 21 L at 1.050:

• Collected = 50 × 21 = 1050
• Possible ≈ 5.0 × 300 = 1500
• Efficiency ≈ 1050 / 1500 = 70%

Tell your software 70% and it will predict your OG closely from then on. Do this across three or four batches and take the average — one brew day is not a measurement, it is a data point.

When to take readings

Always record the volume with each reading. Gravity alone does not give you efficiency; gravity × volume does. This is the single most common gap in a brew log.

Common issues

• OG under target — low efficiency (coarse crush, mash pH off, under-sparge) or too much volume. Tighten the crush, check pH, and set the recipe’s efficiency to what you actually get rather than what it came with
• OG over target — boiled down too far. Top up with sanitised water to hit gravity and volume
• FG too high — under-attenuation: under-pitched, too cold, or stuck. Warm and rouse; see Yeast Handling
• FG lower than the yeast should manage — a diastatic strain, or a dry hop that restarted fermentation. See Diacetyl & hop creep
• Refractometer FG looks wrong — it is. Alcohol skews it; use a hydrometer

See also: the brew-day walkthrough for where each reading fits in the day, and Brewing Water Basics for the mash pH side of conversion efficiency.