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. Measuring them is what turns brewing from guesswork into something you can repeat on purpose.

What specific gravity is

Specific gravity (SG) is the density of wort or beer relative to water (1.000). Sugar makes wort denser than water (say 1.050); as yeast eat the sugar and make alcohol — which is lighter than water — gravity falls toward the FG (say 1.010).

Brewers talk in gravity points: the digits after the decimal × 1000. So 1.050 = “50 points”. Points make the maths easy.

The two tools

Hydrometer

A weighted float — the more sugar, the higher it sits. Cheap, accurate, and works right through fermentation. Two things to get right:

• Temperature. It’s 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).

Refractometer

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

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

ABV

The simple, good-enough formula:

ABV (%) ≈ (OG − FG) × 131.25

Example: OG 1.052, FG 1.010 → 0.042 × 131.25 ≈ 5.5 % ABV.

For stronger beers a more accurate form exists, but at normal ale strengths the simple one is within a rounding error.

Attenuation — how dry it finished

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

Example: 1.052 → 1.010 gives (0.042 / 0.052) ≈ 81 %. Compare to your yeast’s published range: landing well below it usually means a stuck or incomplete fermentation, not a recipe that was meant to finish sweet.

Efficiency — why a recipe hits (or misses) its OG

Efficiency is how much of the grain’s available sugar you actually extracted. It’s the single most useful number to log, because every system has its own consistent figure, and feeding the right one into your recipe software is what makes predicted OG match reality.

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

How to calculate brewhouse efficiency

Work in points × litres, which are conserved (boiling concentrates gravity but doesn’t 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 % next time and it’ll predict your OG closely.

When to take readings

Always record the volume with each reading. Gravity alone doesn’t give efficiency — gravity × volume does.

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.
• OG over target — boiled down too far / low volume; top up with sanitised water to hit gravity and volume.
• FG too high — under-attenuation: under-pitch, cold, or stuck. Warm and rouse.
• Refractometer FG looks wrong — it is; alcohol skews it. Use a hydrometer.