How this yeast starter calculator works
This tool computes the **cells required** from batch size, gravity (°P) and pitch rate (M cells/mL/°P). It then estimates your **viable cells on hand** (liquid packs by manufacture date or dry grams) and, if needed, sizes a **starter** using an empirically grounded growth-yield model based on the work of Kai Troester (Braukaiser).
Formulas
1) Convert SG → °P (ASBC polynomial)
\[ ^\circ P = -616.868 + 1111.14\,SG - 630.272\,SG^2 + 135.997\,SG^3 \]
Accurate to typical brewing ranges.
2) Cells required
\[ \text{Cells}_\mathrm{req} = \underbrace{PR}_{\text{M cells/mL/}^\circ P} \times 10^6 \times \underbrace{V_\mathrm{mL}}_{\text{batch volume}} \times \underbrace{^\circ P}_{\text{gravity}} \]
3) Liquid yeast viability (default)
\[ \text{viability} \approx \max\!\bigl(0,\,1 - 0.007 \times \text{days since mfg}\bigr) \]
≈0.7% loss/day (~21%/month). You can override with your own viability.
4) Starter growth (Braukaiser-style)
\[ \text{Cells grown} \approx \underbrace{Y}_{\text{B/g}} \times \underbrace{(100 \,\text{g/L} \times V_\mathrm{starter,L})}_{\text{grams of DME at ~1.040}} \]
Typical yields: no stir ~0.6 B/g; shaking ~1.0 B/g; stir-plate ~1.4 B/g.
Why these pitch rates?
- Industry heuristics: ~0.75 M cells/mL/°P for ales; ~1.5 for lagers—originating from George Fix and widely adopted (MrMalty, Wyeast, AHA).
- Adjust for style & goals: lower pitch rates can increase esters (some ales); higher rates promote cleaner profiles, quicker fermentation, and better attenuation (esp. lagers).
Glossary
| Symbol | Meaning |
|---|---|
| SG | Specific Gravity. |
| °P | Degrees Plato (extract % w/w). |
| PR | Pitch rate in million cells per mL per °P. |
| B | Billion cells (109 cells). |
| Y | Growth yield (B cells per gram of DME). |
Worked example
Target: 20 L of 1.050 ale (≈12.3 °P) at 0.75 M/mL/°P.
- Cells required: \(0.75\times10^6 \times 20{,}000 \,\text{mL} \times 12.3 \approx 184.5 \text{ B}\).
- On hand: 1 White Labs pack (100 B), manufactured 60 days ago ⇒ viability ≈ \(1-0.007\times60 = 58\%\) ⇒ ~58 B.
- Deficit ≈ 126.5 B. On stir-plate with Y ≈ 1.4 B/g ⇒ DME needed ≈ 90 g ⇒ starter ≈ 0.9 L (at ~100 g/L).
Authoritative sources (named & citable)
- Pitch rates (ale 0.75 / lager 1.5 M/mL/°P): George Fix via MrMalty and AHA/BYO guidance; Wyeast professional pitch-rate resource.
- SG↔°P conversion (ASBC polynomial): documented widely by Brewer’s Friend / BYO and ASBC-aligned tables.
- Starter growth modeling: Kai Troester, “Step Up Your Starters” (NHC 2013), implemented by Brewer’s Friend and YeastCalc.
- Liquid yeast viability decay: rule-of-thumb ≈21%/month (~0.7%/day) used by Brewer’s Friend/MrMalty; cross-check manufacturer guidance (Wyeast, White Labs).
FAQ
Should I always make a starter?
For fresh **dry yeast**, usually no—manufacturers provide high viable counts (often 18–22 B/g). For **liquid yeast**, a starter is recommended for most 5 gal / 20 L batches above ~1.045 unless using higher-cell brands or multiple packs.
Is over-pitching bad?
Mild over-pitching is generally fine but can reduce ester character, body, and perceived malt complexity. Large over-pitching may shorten growth phase and impact flavor development.
What starter gravity is best?
~1.037–1.040 (≈9–10 °P) balances growth vs. cell health. Very high-gravity starters can impair viability.
Do stir plates really help?
Yes. Stir-plates improve oxygenation and CO₂ stripping, raising growth yield (often ~1.3–1.6 B/g vs. ~0.6–1.0 B/g with no stir / shaking).
References
- Wyeast: Professional Pitch Rates & Activator™ guidance (industry rates ~0.5–1.5+ M/mL/°P; 6-month viability warranty).
- MrMalty / J. Zainasheff: Pitching rate primer; Fix recommendations (0.75 ale / 1.5 lager).
- AHA & BYO: Standard 1.0 M/mL/°P guideline and SG↔°P background.
- K. Troester (Braukaiser): Step Up Your Starters, NHC 2013—growth vs. yield data and practical implications.