How to Use This Tool
Start by entering the amount of active sourdough starter your recipe requires, selecting the correct weight unit (grams or ounces). Next, input the hydration percentage of your active starter and discard — most home bakers use 100% hydration for both, which is pre-filled as the default. Click "Calculate Ratios" to see exactly how much discard to use, plus the flour and water contributions from the discard. Use the "Reset" button to clear all inputs and start over, or click "Copy Results" to save the breakdown to your clipboard.
Formula and Logic
This calculator uses standard sourdough hydration math to ensure accurate substitutions:
- Starter flour content = (Starter weight) * 100 / (100 + Starter hydration %)
- Starter water content = (Starter weight) * Starter hydration % / (100 + Starter hydration %)
- Required discard weight = (Starter flour content) * (100 + Discard hydration %) / 100
- Discard flour and water contributions are calculated using the same formula as starter, using the discard weight and hydration.
The hydration ratio compares your discard's hydration to your active starter's hydration, to help you understand how the substitution will affect dough consistency.
Practical Notes
Keep these real-world sourdough baking tips in mind when using your results:
- Discard is unfed starter, so it may be more acidic than active starter — this can affect fermentation time, so plan for longer rise times if substituting discard in yeast-free recipes.
- If your discard has been stored in the fridge for over a week, it may have separated (hooch on top) — stir it well before measuring to get an accurate hydration reading.
- For volume measurements (cups), note that 1 cup of 100% hydration starter weighs approximately 240 grams, but this varies based on how packed the starter is in the cup.
- Always subtract the discard's flour and water contributions from your recipe's total dry and wet ingredients to avoid overly sticky or dry dough.
Why This Tool Is Useful
Home bakers often accumulate excess sourdough discard and want to use it in recipes that call for active starter, but guessing substitution ratios can lead to failed bakes. This tool eliminates guesswork by calculating exact measurements, reducing food waste and saving you from trial-and-error batches. It works for any sourdough recipe, from pancakes and waffles to bread and pizza dough, making it a versatile addition to your baking routine.
Frequently Asked Questions
Can I use this calculator for discard with different hydrations?
Yes — simply enter your discard's actual hydration percentage (e.g., 80% if your discard is stiffer, 120% if it's more liquid) to get accurate results. Most home bakers keep discard at 100% hydration to match their active starter, but the tool supports any hydration between 1% and 200%.
What if my recipe uses volume measurements instead of weight?
Weight measurements are far more accurate for sourdough baking, but you can convert volume to weight using the approximate guide: 1 cup of 100% hydration starter = 240 grams, 1 ounce = 28.35 grams. Enter the converted weight into the tool for the best results.
Do I need to adjust fermentation time when using discard?
Discard is unfed, so it has less active yeast than active starter. For recipes that rely on sourdough for leavening (like bread), you may need to add a small amount of commercial yeast or extend fermentation time by 1-2 hours. For recipes that use baking soda/powder (like pancakes), no fermentation adjustment is needed.
Additional Guidance
Store your sourdough discard in an airtight container in the fridge for up to 2 weeks — any longer and it may develop off flavors. Label the container with the date and hydration percentage to make future calculations easier. If you bake sourdough regularly, keep a small jar of discard at room temperature for same-day use, and store the rest in the fridge to slow fermentation. Always use a digital kitchen scale for measuring starter and discard, as volume measurements are inconsistent and can throw off your ratios.