Sourdough Feeding Calculator

Temperature is the biggest factor in fermentation speed. At 24°C a 1:5:5 feed might peak in about 9 hours, but at 18°C the same feed could take several hours longer. Each degree below 24°C slows fermentation by roughly 10%. Because a cool kitchen is already slower, this calculator suggests a lower feeding ratio (less flour and water) so the starter still peaks by your target time; in a warm kitchen it suggests a higher ratio. If your kitchen is very cool, you can also move the jar somewhere warmer like an airing cupboard or on top of the fridge.

The ratio depends on when you need the starter to peak (figures are for white flour at about 24°C). For a quick turnaround (~4 hours) use 1:1:1 (equal parts starter, flour, water). Around 7 hours use 1:3:3. For overnight (~9-13 hours) try 1:5:5 up to 1:10:10. Higher ratios mean more dilution and a longer, milder ferment. This calculator works backwards from your target time, temperature and flour type to suggest the right ratio.

Yes. Wholemeal and rye flours ferment faster than white flour because they contain more nutrients, enzymes and wild yeast. Rye is the fastest — a rye-fed starter can peak around 20% quicker than white at the same ratio. Spelt falls between white and wholemeal. Because these flours are faster, the calculator suggests a slightly higher feeding ratio (more dilution) so they still peak at your target time rather than racing ahead.

100% hydration (equal weight flour and water) is the most common and easiest to manage. This creates a thick, pancake-batter consistency. Some bakers prefer a stiffer starter at 60-80% hydration for more complex flavours and slower fermentation. Liquid starters at 125%+ hydration are sometimes used for ciabatta and focaccia.

A peaked starter has doubled or tripled in volume, has a domed surface, and is full of bubbles. If the surface has started to sink or flatten, it has passed its peak. Use a rubber band around the jar at feed time to track the rise. The float test (a spoonful floats in water) also indicates peak readiness.