- Which formula should I trust — simple or Miller?
- Below 6% ABV, they agree to within a few hundredths of a percent. Above 6%, the simple formula starts under-reporting because it assumes a fixed conversion factor (131.25) that doesn't account for ethanol's lower density compared to water. The Miller formula corrects for this: as more sugar converts to alcohol, the math compensates. For a session beer or pale ale, simple is fine. For an imperial stout, barleywine, or anything above 7%, use Miller — it's the number competition judges and commercial brewers calculate from. Both are estimates from gravity readings; the only way to measure ABV directly is with gas chromatography or a distillation rig.
- What's a 'good' apparent attenuation?
- It depends on the yeast and the wort. Most ale yeasts (Wyeast 1056, Safale US-05, London Ale III) attenuate 73–80%. Lager yeasts run similar. Saison strains push 85–95% — that's the point of a saison. Belgian yeasts vary widely (65–85%). If you brewed a pale ale and got 65% attenuation, the fermentation likely stuck: too cold, under-pitched, mineral-deficient wort, or wort too sweet for the yeast strain. If you got 90% on a malty beer, something fermented that shouldn't have (wild yeast, brett contamination). The number is diagnostic — it tells you whether the yeast did its job.
- Why does mash efficiency not change the ABV?
- Efficiency measures how much of the sugar potentially in your grain bill ended up in the wort. ABV is calculated from the wort that's actually in your fermenter (your OG reading) and what's left after fermentation (your FG reading). If your efficiency was low, you got less sugar than you planned for — your OG will be lower than your recipe predicted. The calculator can't see your grain bill; it only sees the readings. Efficiency matters for recipe scaling and grain-cost analysis, not for ABV from finished gravity readings. Track it in your brewing notes; the tool records it for that purpose.
- Hydrometer or refractometer — does it matter?
- For OG, either works — both give the same number on un-fermented wort, with corrections for temperature (hydrometers) or temperature plus a refractive-index adjustment (refractometers). For FG, hydrometers are the safer choice. Once alcohol is present, refractometer readings are skewed by the ethanol and require a correction formula. Most refractometer calculators apply this automatically, but if you're reading a raw °Brix value off the eyepiece during fermentation, convert it before plugging into this calculator. When in doubt, use a hydrometer for FG — it measures density directly and isn't fooled by alcohol.
- My FG is higher than my OG. What went wrong?
- Either you swapped the numbers (easy fix, just swap them back), or you actually wrote them down wrong at some stage. Real fermentation always lowers gravity — yeast eats sugar (dense) and produces alcohol (less dense) plus CO2 (escapes). If your readings genuinely show FG > OG, double-check both: confirm the hydrometer was clean and at the right temperature, confirm the readings are in specific gravity units (1.050) and not °Plato or °Brix (12, 12.4) which are different scales. The calculator returns zero when FG > OG because the formulas don't have a physical meaning in that case.
- My ABV came out lower than I expected. Why?
- Three common reasons. (1) Stuck fermentation: your FG is higher than the yeast was supposed to push it. Try rousing the yeast, raising fermentation temperature a few degrees, or adding a fresh yeast pitch. (2) Low OG: your mash didn't extract as much sugar as the recipe planned for. Check your efficiency calculation. (3) Bad reading: a refractometer FG without alcohol correction over-reports the gravity, making the apparent ABV look lower. Calibrate your hydrometer in distilled water at 60°F (it should read exactly 1.000) before trusting any reading.
- How accurate is the calorie estimate?
- Within about 10% of measured values for typical beers. The Ray Daniels formula uses real extract (a weighted average of original and apparent extract that estimates residual unfermentables) and ABV by weight. It's the same formula BeerSmith and BrewersFriend use. Sources of error: high-adjunct beers (corn, rice) have different unfermentable profiles than all-malt; lactose-laden beers (milk stouts, pastry stouts) have residual lactose the formula doesn't see; brett-fermented beers have very low residual sugar but unusual flavor compound profiles. For a regular all-malt ale or lager, the estimate is good enough for nutrition labels. For anything weird, treat it as a starting point.
- What's the difference between apparent and real attenuation?
- Apparent attenuation is what this calculator returns — it uses the FG reading directly, which is depressed by the alcohol now in solution (ethanol is less dense than water, so the wort reads lighter than its actual sugar content would suggest). Real attenuation corrects for this and is a few percentage points lower (e.g. 80% apparent ≈ 65% real). Real attenuation is the actual fraction of fermentable sugar the yeast consumed. Apparent attenuation is what every homebrewer reports and what yeast manufacturers print on their datasheets — so it's what we publish here. The difference is real but mostly academic for recipe design.
- Can I use this for wine, mead, or cider?
- The simple formula works as a rough estimate for mead and cider (similar gravity ranges to beer). It's less accurate for wine, where the starting gravity is much higher and the Miller formula becomes more important. For wine specifically, professional winemakers use °Brix readings (sugar by weight) and a different correction; this calculator is calibrated to beer's typical 1.030–1.120 OG range. For mead and cider in that same range, it's fine. For grape wine starting at 1.080–1.110, prefer the Miller (advanced) result — it's the more honest number.
