How to Convert Instant or Active Dry Yeast to Sourdough Starter (1 Packet = 100g Chart)

The Conversion Is Exact — If You Account for What Is Hidden

Most published "yeast to sourdough" tables are wrong in one specific and consequential way: they treat a 100g sourdough starter as a single monolithic ingredient. It is not. A 100% hydration starter is exactly half flour and half water by weight, and when you stir 100g of it into a dough, you are adding 50g of flour and 50g of water that never appear in the recipe.

Ignore that and your hydration silently drifts upward by several percentage points — the same "apparent versus true hydration" error that collapses so many high-hydration loaves. Ignore it and you also inflate your total flour, so the loaf comes out heavier than planned.

The conversion itself is a two-line calculation. The subtraction is the part most recipes forget.

The Packet-to-Gram Anchor

One standard US packet of dry yeast is 7g, or roughly 2¼ teaspoons. That single anchor is what ties the entire commercial yeast world to the sourdough world, because a 7g packet of instant yeast delivers the same leavening power as 100g of a ripe 100% hydration levain in a standard 500g-flour loaf.

The relationship runs through a normalisation factor that expresses dry yeast in instant-yeast equivalents:

The type factor $k$ is the entire reason a packet of active dry yeast is not interchangeable with a packet of instant yeast at the same gram weight:

Active dry yeast carries roughly 20% less leavening power per gram than instant yeast because it is a live, partially dehydrated culture that must be rehydrated in warm water before it can bloom. Fresh cake yeast carries roughly one third, because it is only about 33% solids.

From Instant Equivalent to Sourdough Starter

Once the yeast is normalised to instant equivalent, the starter weight follows directly from the packet anchor:

Worked against the standard 500g flour, 300g liquid artisan dough:

Notice the final column. Because the starter's flour and water are added back into the totals, the true hydration of the converted dough is identical no matter which yeast type you started from — 60.0% in every row. That invariance is the proof that the conversion is mathematically sound rather than a rule of thumb.

The Subtraction, Written Out

Start with a plain commercial recipe and convert it. Take a 500g flour, 300g water, 7g instant yeast, 10g salt pizza dough:

The flour and water deductions are the whole trick. Skip them and you end up with a 60g heavier, several points wetter dough than the recipe you were trying to convert.

Why the Starter Must Be Ripe

A packet of commercial yeast is a standardised, industrially dried culture with a known cell count. Sourdough starter is a living community whose leavening power depends entirely on when you used it. Converting a recipe is therefore only half the task; the other half is the timing.

The converted dough also ferments on a different clock. The converter returns the expected bulk window alongside the weights:

If your converted dough feels sluggish, the cause is almost always that the starter was not fully ripe. A levain used at the flat, barely-doubled stage carries far fewer active yeast cells than the 7g packet it replaced.

Going the Other Way: Sourdough Back to Instant Yeast

The reverse conversion is equally useful when you want to make a sourdough loaf on a commercial timeline — a two-hour rise for a dinner roll, or an overnight cold ferment for a convenience bake.

Start with a sourdough formula of 500g flour and 300g water containing 100g of 100% hydration starter. The starter contributes 50g of flour and 50g of water, so the total system is 550g flour and 350g liquid, a true hydration of 63.6%. Remove the starter and restore its flour and water into the main dough, then dose commercial yeast against the total flour:

Note how the yeast dose collapses as you extend the fermentation window. A 0.4 teaspoon of instant yeast for a cold overnight ferment is not a rounding error — it is the difference between a long, slow, flavourful ferment and a loaf that tastes like a sweet bun.

Practical Checklist for the Conversion

• **Weigh, never scoop.** Volumetric measures of commercial yeast vary by up to 25% between brands, and that error compounds through the whole conversion.
• **Hydrate active dry yeast first.** Dissolve it in a small portion of the recipe water at 38–43°C (100–110°F) for 10 minutes before combining, or it will sit as a dry clump in a wet dough and fail to bloom.
• **Always subtract.** Flour and water deductions are non-negotiable if you want the converted dough to match the original recipe's hydration.
• **Feed for ripe levain.** Plan the starter build 4 to 8 hours ahead; the conversion is only as good as the culture you put into it.
• **Check the dough, not the clock.** Judge the converted dough by the same rise percentage and sensory gates you would use for a sourdough bake.