Bulk fermentation guide
Use the numbers — then read the dough
Temperature, starter percentage, and flour type are the three biggest variables that control bulk fermentation timing. The calculator above gives you a time window. The guide below helps you understand what's happening inside the dough and how to recognise when it's actually ready.
Slow, acid-forward fermentation. Good for overnight bakes and complex flavour.
Balanced timing that suits most same-day and next-morning bake schedules.
Fast, active fermentation. Watch closely — dough can overproof in under 3 hours.
For every 8°C (14°F) increase in dough temperature, fermentation speed roughly doubles. Halving starter percentage has a similar effect — it approximately doubles bulk time at the same temperature.
How to use this bulk fermentation calculator
Enter your dough temperature using the slider — toggle between Celsius and Fahrenheit to match the thermometer you use. Set your starter percentage: this is the weight of starter as a percentage of total flour in the recipe. Finally, choose your flour type to account for differences in fermentation speed between white, wholemeal, and rye.
The calculator outputs an estimated bulk fermentation window, a midpoint estimate, and a visual timeline from mixing to shaping. A reference table below the calculator shows how temperature and starter percentage interact across a practical range so you can plan bakes ahead even when you change one variable.
What is bulk fermentation?
Bulk fermentation is the first and most important fermentation stage in sourdough baking. It begins the moment you mix your dough and ends when you divide or shape it. During this time, yeast produces carbon dioxide that raises the dough, and lactic acid bacteria produce the organic acids that give sourdough its depth of flavour.
The word "bulk" refers to the fact that the entire batch of dough ferments together as one mass, before being divided into individual loaves. Getting bulk fermentation right is the single biggest factor in determining whether a sourdough loaf will have good structure, a healthy oven spring, an open crumb, and a complex taste.
Why dough temperature controls bulk fermentation
Temperature is the most powerful variable in bulk fermentation because yeast and bacterial activity roughly double or halve with every 8°C (14°F) change in dough temperature. A dough that takes 8 hours at 20°C will take closer to 4 hours at 28°C. This relationship is not perfectly linear at extreme temperatures, but it holds well in the practical baking range of 18–30°C.
Dough temperature is not the same as room temperature. Friction from mixing warms the dough, and the temperature of your flour, water, and bowl all contribute. Serious bakers use a probe thermometer to measure the actual dough temperature rather than relying on ambient air temperature. The difference can be 2–4°C, which translates directly into bulk fermentation time.
How starter percentage changes bulk time
Starter percentage — the weight of starter as a proportion of total flour — directly controls how much active yeast and bacteria you introduce at mixing. More starter means more microbial activity from the start, so fermentation accelerates. The relationship is roughly inverse: halving starter percentage approximately doubles bulk time at the same temperature.
Most sourdough recipes use between 10% and 25% starter. Lower percentages (5–10%) are common when bakers want a longer, more acidic fermentation, often overnight in the fridge. Higher percentages (20–30%) are used when baking same-day or when working in a cool kitchen where fermentation would otherwise stall.
How flour type changes bulk fermentation
White bread flour is the slowest to ferment because the bran and germ have been removed. Wholemeal flour retains more of the wheat kernel, including bran particles that carry native yeast and bacteria as well as enzymes that break down starch more readily. This typically speeds fermentation by around 15% compared to an equivalent white formula.
Rye flour has an even more dramatic effect. It contains pentosan sugars that absorb water aggressively, a high enzyme load, and different starch behaviour. Even small amounts of rye — 10–20% of total flour — can measurably accelerate bulk fermentation. At 100% rye, bulk times can be 30–40% shorter than white flour at the same temperature, and dough requires careful monitoring.
How to judge when bulk fermentation is done
The calculator gives a time range, but the final call should always be the dough itself. Experienced bakers look at four things: volume increase, surface appearance, container behaviour, and dough feel. Together these give a reliable picture of fermentation progress.
Aim for 50–75% volume increase. A flat-bottomed, straight-sided container with a rubber band at the start height makes this easy to measure. The surface should dome slightly — not be perfectly flat — and you should see small bubbles breaking through the dough. When you shake the container, the dough should wobble loosely. When you pull a small piece, it should stretch without tearing.
Troubleshooting bulk fermentation
Under-fermentation produces dense, tight dough that tears when shaped. The loaf has poor oven spring, a thick crust, and an overly chewy crumb. If your dough hasn't risen after the expected time, check whether your starter is active — it should double within 4–8 hours of feeding at room temperature. Also check your actual dough temperature with a probe thermometer.
Over-fermentation produces slack, sticky dough that spreads rather than holding its shape. The loaf flattens in the oven, the crust browns unevenly, and the crumb can be gummy. Once bulk is obviously over, do not try to recover it with more folding — shape immediately, skip final proof, and bake straight away or put in the fridge. Keep notes so you can adjust timing on the next bake.
Bulk fermentation and cold retarding
Many bakers complete bulk at room temperature, then move the shaped dough to the fridge (cold retard) for 8–16 hours before baking. This gives flexibility: you can bake on your own schedule rather than when the dough demands it. The cold slows fermentation dramatically but does not stop it — a shaped loaf in a 4°C fridge is still slowly fermenting.
Some recipes extend bulk fermentation itself into the fridge — a long, cold bulk followed by a shorter final proof at room temperature. This produces a more acidic loaf with a more complex flavour. The calculator models ambient temperature bulk; if you plan to cold-retard during bulk, reduce the estimated time by 60–70% and rely more heavily on dough signs.
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