Two people brew the same recipe, take gravity readings, run the numbers, and report different ABVs — sometimes by a tenth of a point, sometimes by nearly a full percent. It's tempting to assume one of you made an arithmetic mistake. Almost always, neither of you did. The disagreement comes from upstream of the arithmetic, in how the two readings were actually taken, and it's worth tracing through with real numbers rather than shrugging it off as "close enough."
Start with the readings, not the formula
Say you and a friend brewed the identical recipe, both landing on an OG of 1.062. You read your FG at 1.014, cool, well-settled, off a hydrometer calibrated and checked against distilled water beforehand. That gives 6.30% ABV by the simple formula, 6.54% by the advanced one. Check it here.
Your friend read theirs at 1.0155 — a genuinely tiny difference, four tenths of a gravity point, well within what a slightly warm sample or a slightly-off hydrometer produces. Their reported ABV: 6.10% simple, 6.35% advanced. Check that here. A gap of two-tenths of a percent, from a starting-point disagreement that's genuinely tiny and easy to produce by accident.
That's the first, and usually the biggest, source of "my ABV and my friend's ABV don't match": the two of you are quietly feeding the calculator two different FG readings, not disagreeing about the arithmetic at all.
Where a 0.4-point gap like that actually comes from
A few completely mundane things produce exactly this size of discrepancy:
- Sample temperature. A hydrometer calibrated at 60°F reading a sample at 80°F reads a few thousandths low uncorrected — see the hydrometer temperature correction chart for the full table. If one of you corrected and the other didn't, that alone accounts for a gap this size.
- Where on the hydrometer you read the meniscus. Reading at the top of the curved liquid surface versus the bottom is a genuinely common source of a point or two of reading error, especially for someone newer to using one.
- Hydrometer calibration itself. Cheap hydrometers vary batch to batch, and a hydrometer that's never been checked against plain water (which should read almost exactly 1.000 at its calibration temperature) can carry a small, consistent offset nobody's ever caught.
- CO2 still in solution. A sample pulled straight from an actively fermenting batch, with dissolved CO2 still fizzing out, reads artificially and inconsistently — degassing the sample (a gentle swirl, or drawing it a few minutes before reading) removes this source of noise.
None of these is a big, obvious mistake. All of them are exactly the kind of small, easy-to-miss inconsistency that produces two "correct-looking" readings that are quietly a few thousandths apart.
Then there's the formula itself, and it matters more at higher gravity
Even with identical OG and FG readings, two calculators can still disagree if one defaults to the simple ABV formula and the other to the advanced one — see why does my ABV differ from my recipe software for that specific gap worked through in detail. The short version: at moderate gravity the two formulas track closely, but the gap widens as OG climbs. At OG 1.085 to FG 1.016 — a strong pale ale or light imperial territory — the simple formula gives 9.06% ABV, the advanced formula 9.74% — a gap of nearly seven-tenths of a percent, purely from formula choice, on top of whatever gap the readings themselves introduce. Check this example.
Stack a reading discrepancy and a formula discrepancy together on a big beer, and it's entirely possible for two honest, careful brewers working from the same recipe to report ABVs a full percentage point apart, with neither one having made an obvious error anywhere.
The refractometer wrinkle, if either of you used one
If one of you checked FG with a refractometer instead of a hydrometer, there's a third, much larger source of disagreement waiting: a refractometer reading fermenting or fermented beer needs a correction for alcohol's effect on refractive index before the number means anything as an SG value at all. Skip that correction and the raw reading overstates the true remaining gravity substantially — see refractometer correction, explained for a worked example showing exactly how far off an uncorrected reading can be. This is worth checking first if the gap between your two numbers is large rather than small — a big, confusing gap points at a missed correction step far more often than it points at a genuine brewing difference between the two batches.
Attenuation tells the same story from a different angle
If you want a second, independent check on whether a reading discrepancy is the real culprit, compare apparent attenuation rather than ABV — it's derived from the same two numbers, but it's a percentage rather than a small decimal, and small reading differences show up more visibly as a percentage than as a fraction of a gravity point. The 1.062/1.014 reading above works out to 77.4% apparent attenuation; the 1.062/1.0155 reading works out to exactly 75.0% — a genuinely noticeable 2.4-point gap in attenuation from what looked, on the gravity readings themselves, like barely any difference at all. If your friend's attenuation reads suspiciously round (75.0%, 80.0%, 70.0%) while yours doesn't, that's sometimes a sign their FG reading was itself rounded to a convenient-looking number rather than read precisely off the instrument — worth asking about directly rather than assuming it's a coincidence.
It shows up in calories too, if that's what you're comparing
The same OG/FG discrepancy that shifts ABV by two-tenths of a percent shifts the calculator's calorie estimate by a comparable margin — roughly 178.5 kcal per 12 oz serving for the 1.014 reading against roughly 180.0 kcal for the 1.0155 reading. It's a small difference in absolute terms, but it's worth knowing that every downstream number this site's ABV calculator reports — ABV, attenuation, alcohol by weight, calories — all move together off the same two input readings, so a small disagreement anywhere upstream shows up consistently across every figure downstream, not just the one you happened to be comparing.
When "close enough" actually is close enough
None of this means every ABV discussion between two homebrewers needs to become a forensic investigation. For casual comparison — bragging rights, or just curiosity about how a shared recipe turned out for each of you — a gap of a couple tenths of a percent is well within normal reading-to-reading noise and not worth chasing further. It's worth actually tracking down the source of a disagreement when the gap is large (most of a percentage point or more), when you're trying to hit a specific style's ABV range precisely, or when you're troubleshooting a batch that seems to have underperformed and want to rule out "the reading was wrong" before assuming something went wrong in the brewing itself.
Compare readings before you compare formulas
Don't try to reconcile two ABV numbers by adjusting the formula until they match — that's solving the wrong problem. Instead, compare your actual OG and FG readings directly, side by side, before either number gets run through any formula at all. If the readings themselves agree, any remaining gap is a formula-choice question you can resolve by simply reporting which formula each of you used. If the readings don't agree, that's the real disagreement, and it's almost always explained by one of the reading-technique differences above — worth chasing down not because the exact percentage matters enormously, but because the habit of catching a bad reading before it feeds into a calculation is the same habit that catches a genuinely stalled fermentation, a genuinely off recipe, or a genuinely miscalibrated instrument later on, when it matters much more than bragging rights over a tenth of a percent.