10 grams of fresh yeast and 125 grams of sourdough starter produce the same rise in 500 grams of flour. One takes 90 minutes. The other takes 8 hours. The difference is not just time - it is flavor, shelf life and a flour-water correction that most recipes never mention.
That correction is the part that trips people up. Sourdough starter is not pure leavening. A 100% hydration starter at 125 grams contains 63 grams of flour and 63 grams of water. Add it to a recipe without adjusting, and the dough is suddenly 63 grams too wet and 63 grams short on structure. The loaf spreads flat. The crumb goes gummy.
The same kind of hidden math shows up in homebrewing. A hydrometer reads 1.065 before fermentation and 1.012 after. The gap is 0.053 SG points. Multiply by 131.25 and you get 6.96% ABV - a solid IPA. But most brewers also want to know: is 57 IBU enough bitterness for that gravity? The BU:GU ratio answers that. At 0.88 it is hop-forward. Drop to 0.44 and the same beer becomes a hazy, malt-forward NEIPA.
Three calculators went live today. They cover the math behind all of this.
The flour and water hiding inside your sourdough starter
The most common mistake in sourdough conversion is treating the starter like a drop-in replacement for yeast. It is not. A starter at 100% hydration is half flour, half water by weight. When you add 125 grams of starter to a recipe, you are also adding 63 grams of flour and 63 grams of water that were not in the original.
The Yeast vs Sourdough Converter handles this automatically. Enter the flour mass, the amount of yeast your recipe calls for, pick a flavor intensity - and it calculates the exact starter amount plus the flour and water corrections.
Three intensity levels produce three different results from the same 500-gram flour base:
| Flavor | Starter | Flour correction | Rise time |
|---|---|---|---|
| Mild (17.5%) | 88 g | -44 g flour, -44 g water | 8-14 h |
| Medium (25%) | 125 g | -63 g flour, -63 g water | 5-9 h |
| Intense (35%) | 175 g | -88 g flour, -88 g water | 3-6 h |
The conversion works in reverse too. A sourdough recipe calling for 200 grams of starter at 100% hydration contains 100 grams of hidden flour and 100 grams of hidden water. Remove the starter, add those amounts back to the recipe and pitch 12 grams of fresh yeast. Rise time drops from half a day to under two hours.
The trade-off is real though. Yeasted bread goes stale faster. Sourdough bread keeps for days because the lactic acid acts as a natural preservative. No calculator can change that.
Two gravity readings and 131.25 - where ABV comes from
Homebrewers do not measure alcohol directly. They measure sugar. A hydrometer dipped into wort before fermentation reads the original gravity - OG. After the yeast finishes, it reads the final gravity - FG. The difference between those two numbers, multiplied by 131.25, gives you ABV.
The Homebrew ABV Calculator runs both the standard formula and the Balling formula (which corrects for ethanol density at high ABV). It also shows apparent attenuation and calories per 330 ml serving.
Here is what typical OG readings produce at 75-80% attenuation:
| Beer style | OG | Typical ABV | Calories / 330 ml |
|---|---|---|---|
| Session ale | 1.035 | 3.3-3.9% | ~107 |
| Pilsner / Lager | 1.048 | 4.7-5.3% | ~148 |
| Pale Ale | 1.055 | 5.5-6.2% | ~170 |
| IPA | 1.065 | 6.5-7.5% | ~198 |
| Belgian Tripel | 1.080 | 8.0-9.5% | ~260 |
| Imperial Stout | 1.100 | 10.0-12.0% | ~305 |
The standard formula works well below 8% ABV. Above that, the relationship between gravity and alcohol becomes non-linear. The Balling formula corrects for ethanol density and gives a more accurate result. For a barleywine at OG 1.100 and FG 1.022, the standard formula reads 10.2% while Balling reads 11.3%. That 1.1 percentage point gap matters for labeling and for knowing what you are actually drinking.
Attenuation tells you how thoroughly the yeast did its job. Most ale strains finish at 72-78% apparent attenuation. Belgian strains often push past 85%. If your reading sits below 65%, fermentation probably stalled - check your temperature and consider repitching.
57 IBU and a BU:GU of 0.88 - what hop bitterness actually means
IBU stands for International Bitterness Units. It measures the concentration of iso-alpha acids - the compounds that make beer taste bitter. The Tinseth formula calculates how much of the alpha acids in your hops actually dissolve into the wort, based on three things: hop weight, alpha acid percentage and boil time.
The IBU Calculator handles up to five hop additions per batch. Each addition contributes differently depending on when you drop it in.
A typical West Coast IPA recipe for 23 liters at OG 1.065 might use three additions:
| Addition | Hop | Weight | Alpha | Boil | IBU |
|---|---|---|---|---|---|
| Bittering | Columbus | 35 g | 14% | 60 min | 42.9 |
| Flavor | Simcoe | 25 g | 13% | 15 min | 14.1 |
| Whirlpool | Citra | 30 g | 12% | 0 min | 0.0 |
The 60-minute addition does 75% of the work. The 15-minute addition adds both bitterness and flavor. The 0-minute whirlpool addition contributes zero IBU but loads the beer with tropical aroma from Citra's essential oils.
The BU:GU ratio puts that 57 IBU in context. GU is gravity units: (1.065 - 1) x 1000 = 65. Divide 57 by 65 and you get 0.88. That is hop-forward territory. A balanced lager sits around 0.5. A NEIPA can drop to 0.3 despite using twice as many hops - because those hops go in at 0 minutes or as dry hop, not at 60 minutes.
One thing the calculator cannot account for: the human palate stops distinguishing bitterness above roughly 100 IBU. Adding more hops beyond that point changes flavor and mouthfeel, but not perceived bitterness.
Five rules from today's calculators
1. Sourdough starter is not a yeast substitute - it is flour, water and yeast in one package. Always subtract the flour and water hidden inside the starter from your recipe totals. A 125 g starter at 100% hydration contains 63 g of each. 2. The standard ABV formula breaks above 8%. For imperial stouts, barleywines or anything with OG above 1.080, switch to the Balling formula. The difference can exceed 1 percentage point. 3. Bitterness depends on when you add hops, not just how much. 30 grams of Citra at 60 minutes yields 42 IBU. The same 30 grams at 0 minutes yields 0 IBU but floods the beer with mango and passion fruit aroma. 4. BU:GU ratio matters more than raw IBU. 40 IBU in a 1.040 lager (BU:GU 1.0) tastes aggressively bitter. 40 IBU in a 1.080 imperial stout (BU:GU 0.5) tastes balanced. Context is everything. 5. Feed your starter 8-12 hours before you need it. A cold, unfed starter straight from the fridge will not raise dough on schedule. Plan ahead or switch to yeast - the converter handles both directions.Tools discussed in this article
Yeast vs Sourdough Converter - Convert any bread recipe between yeast and sourdough with automatic flour and water correction, fermentation time estimate and three flavor intensity levels.
Homebrew ABV Calculator - Calculate alcohol by volume from original and final gravity readings. Standard and Balling formulas, SG and Plato units, apparent attenuation and calories per serving.
IBU Calculator - Calculate hop bitterness in IBU using the Tinseth formula. Supports up to 5 hop additions per batch with per-addition breakdown, BU:GU ratio and beer style classification.
ABV by original gravity - quick reference
Jump straight to the ABV estimate for your wort gravity: OG 1.035 | OG 1.040 | OG 1.045 | OG 1.048 | OG 1.050 | OG 1.055 | OG 1.060 | OG 1.065 | OG 1.070 | OG 1.075 | OG 1.080 | OG 1.090 | OG 1.100
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