Compute bulk fermentation, proof time, and starter feeding schedule from temperature, hydration, flour type, and inoculation ratio. Or reverse-calculate the entire chain from your target bake time. Free.
Or try the web calculator →Most sourdough "calculators" are ratio calculators — they convert between hydration and ingredient weights. That's useful. But the real timing question — "given my conditions, when will my dough be ready?" — requires actual fermentation science. S.D Timer is a timing calculator, built around the Q10 temperature rule, per-flour multipliers, and inoculation-ratio non-linearity.
Four interconnected timing problems, all solved from the same input set:
Two operating modes:
Forward mode: Enter feeding time, temperature, ratio, and flour. Get back: estimated peak time, projected mix window, projected bulk fermentation duration, shape and proof timing, and total bake time.
Reverse mode (the differentiator): Enter your target bake time and all conditions. The calculator solves backward: when to feed, when to mix, how long the bulk will take, when to shape, when to proof, and when to bake. This is what most sourdough calculators don't offer.
Yeast and lactic acid bacteria activity scales exponentially with temperature. The Q10 coefficient — the multiplier per 10°C rise — is approximately 2 to 2.5 for sourdough. Practical effect:
| Temperature | Rate vs 22°C baseline | Bulk fermentation @ 80% hydration white |
|---|---|---|
| 18°C | 0.65× | 8–10 hours |
| 20°C | 0.80× | 6–8 hours |
| 22°C | 1.00× | 5–6 hours |
| 24°C | 1.25× | 4–5 hours |
| 26°C | 1.50× | 3.5–4 hours |
| 28°C | 1.85× | 2.5–3.5 hours |
| 30°C | 2.25× | 2–3 hours |
| Flour | Fermentation multiplier vs white |
|---|---|
| White bread flour | 1.00× (baseline) |
| Spelt | 1.35× |
| Whole wheat | 1.50× |
| Rye | 2.00× |
For blends: weighted average. 80% white + 20% whole wheat = (0.8 × 1.0) + (0.2 × 1.5) = 1.10× — 10% faster than pure white.
Higher ratios (1:1:1, 1:2:2) ferment fast because there's more existing yeast biomass. Lower ratios (1:5:5, 1:10:10) ferment slow because the population has to multiply across more substrate first. The relationship is non-linear because of the lag-vs-exponential phase split:
Imagine you want bread Saturday morning. With a standard timing rule of thumb you'd reason:
That's manual math. Now imagine you check the weather Friday morning and realize the kitchen is 25°C, not 22°C. Everything in your chain shifts. Recalculate:
That's the math the calculator does instantly. Change one input, see the entire schedule update. No mental gymnastics.
A fridge retard (typically 12–24 hours at 4°C) changes the math significantly. At 4°C:
This means: minimal additional rise during the retard, significant ongoing flavor development, and accumulating sugars for the bake-day Maillard reaction. The calculator builds cold retard in as an optional step in the chain. Use it to push your mix earlier and let the dough develop in the fridge overnight.
Specify the mix percentage (e.g. 60% white, 20% whole wheat, 20% rye). The calculator uses mass-weighted multipliers automatically.
If you bulk at 24°C for the first 2 hours then move to a cooler room at 20°C, the calculator can split the bulk into two temperature phases and compute the effective progress.
Higher hydration (85–90%) doughs ferment slightly faster than 65% hydration doughs at the same temperature — the substrate is more accessible to enzymes. The calculator includes this 5–10% adjustment.
Every starter has personal rhythm. Set the "starter activity" parameter to slow/normal/fast based on observation, and the calculator scales all yeast-driven times accordingly.
Free on iPhone, Android, and the web. No login.
It computes how long fermentation takes given your conditions. S.D Timer specifically reverses the calculation: enter your target bake time, get back when to feed your starter, when to mix dough, how long bulk fermentation will take, and when to shape, proof, and bake.
Hydration calculators convert between ratios and weights (e.g. "for 100% hydration starter, how much water for 50g flour?"). Timing calculators compute when each stage happens. S.D Timer is the latter — it tells you when, not how much.
Yes. You can specify a cold retard step in your schedule. The calculator accounts for yeast activity dropping to ~10% of room-temp rate at 4°C and shifts the chain backward to accommodate it.
For a calibrated starter at known temperature, expect total timing within ±30 minutes. Your starter's personal rhythm and your specific flour matter. Use the output as a starting point; refine based on observation over 3–5 bakes with the same recipe.
Yes. Web app at sdtimer.com is fully free, no account. iOS and Android apps are also free.
Yes. Both ferment by the same biological process. For pizza (60–65% hydration, often 1:10:10 cold ferment), use the cold retard mode. For focaccia (85–90% hydration, often shorter bulk), adjust hydration and inoculation accordingly.