Note to Self…Sourdough Maths

I don’t know about you, but I am always looking up standard equivalents up on the internet as I switch between recipes from many different places. Different standards are used – particularly between the US and those other societies that employ metric measurements. Historical recipes can also throw a spanner in the works with odd drams, pinches, tea cups (sometimes called gills), smidgens and scruples – non-standard, informal measures. I do have handy conversion apps on my phone, on my computer, and even on my iPad for conventional measurement conversions. Plus, I’ve bookmarked any number of specialist websites – for specific ingredient conversions, descriptions of informal measures and their approximations to conventional ones, and also sites for substituting ingredients. Sometimes it is hard work looking up sites and calculating measures when I want to make something.

I’d like to make a note of the following information that I commonly use. It comes from multiple sources, so it is handy to have all in one place.

How to Convert a Yeast Bread Recipe to Sourdough

Bread recipes will have yeast, flour and water as the basic ingredients. Of course, it is possible that milk or other liquids can be used instead of water. These are the ones affected. Other ingredients such as salt, butter, oil, egg, sugar, etc. (if they are used) in the original yeast bread recipe will remain the same.

Before we get into maths and numbers, there is one thing to bear in mind: there are no exact measurements in making bread, only guidelines. It requires knowledge of ingredients and experience with the texture of your dough. The following numbers are approximates. Despite this inexactness, I find It useful to have a template – a recipe – to understand the balance of ingredients.

For this exercise in sourdough maths, start with the yeast. You will need to convert the measurements for yeast by type. There are two basic varieties of yeast (other than your sourdough starter). These are packets of dry yeast and cakes of fresh yeast.

Basic unit Teaspoons Ounces Grams
1 packet dry yeast 2 1/4 (2.25) 1/4 (0.25) 7
1 cake fresh yeast 1/2 (0.5) 14

For each basic unit of yeast, there is one addition (+) and two subtractions () to the original yeast bread recipe.

Action Ingredient US cups Ounces Grams Note
+ Sourdough Starter 1 8 225 Weight conversions will vary with the starter because of the CO2 activity. In other words, if very bubbly, it will be lighter. These weight conversions are basic ones for 100% hydration starter.
Water (or other liquid) 1/2 (0.5) 4 118 Other common liquids include milk, and fruit or vegetable juices that replace all or part of the water.
Flour 3/4 (0.75) 3 1/3 (3.33) 94 Weights may vary according to the type of flour used – plain (all-purpose), strong (bread flour), whole wheat, rye, etc.

= One loaf of sourdough bread.

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Another Note: Bread making – particularly sourdough bread making is an art as well as a science. Three years ago, I wrote a post on Variables of a Sourdough Kind which laid out the diversity of sourdough creations based on variables in the basic ingredients. Bread making is an exercise in experimentation – usually all edible.


  1. Also, I just had a thought… All this will depend on the hydration of your starter – mine is 100% whereas Celia’s is (I think) 133%).


    • You are absolutely right. Mine is 100%. One of the reasons I indicated this was approximate and you needed to go by feel, adjusting flour or water to get the right texture. One of the things I like about bread baking – its an art as well as a science.


  2. That’s really useful. Being English, I tend to avoid any recipe using cups – way too complicated. Conversion tables rule! However, the main thing I have taken from this post is that I MUST get a sourdough starter going again. It’s been too long…..


  3. I’m fortunate I was still a school kid when Australia converted from imperial to metric measures so I can both both systems. Thanks for working this out


  4. i know what you mean deb. i am constantly looking up measurements! and i have various sites bookmarked. it’s a right royal pain sometimes isn’t it? 🙂 cheers sherry

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  5. I’m sure if maths didn’t make you weep this would make sense. I really wanted to ‘get’ this as it would be very handy but sadly not. I’ll keep chipping away and hopefully the penny will drop Took me 2 years to grasp bakers percentages and now wouldn’t work without them 😦


    • I do understand! Luckily, I’m fairly comfortable around math. Bakers’ percentages are certainly the most professional measurements used in bread baking and I ran across a really good explanation of them in Daniel Leader’s book, Local Breads. There is also a lot of other useful information in there on ingredients, procedures and equipment. I really should read it again and start using bakers’ percentages. (Maybe even write a post on them?). This “sourdough maths” post is based on the simple idea of converting non-sourdough bread recipes (i.e. conventional recipes) to sourdough ones. Basically – a starter is yeast with a bit of flour and water, so it was just a matter of altering the ingredients of a “conventional” one to fit. Hope this helps!

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      • I think would have been put on the ‘special’ list if at school these days. I love using bakers %, took me ages to conquer but I got there. I just can’t see the correlation to the +/- values. I’ll persist, when it does happen, it stays locked in!


  6. Sorry, I’m going to annoy you until I crack this. If the water and flour add up to 212g, and the starter taken off is 225g, is that allowing for 7g of yeast too or is the starter a different hydration?


    • You’ve just hit the flaw in this conversion. I deal with 100% hydration (or close enough since I am never that precise in refreshing the starter). As I qualified, these are guidelines not strict rules. Just remember you have to subtract liquid (i.e. water) and flour when you use a starter in place of fresh or dried yeast. The web-sites I used indicated that 7gr dried yeast is the same as using 14gr fresh yeast. So, I am assuming that the 1 cup of starter (which is what measurements those sources were using) contains a similar amount of yeast. I weighed my 1 cup starter and it came to 225gr. I know there is a discrepancy (between 212 and 225) of about 13g and that seems about right for fresh yeast. Naturally more or less flour/liquid can be added when you start making the bread since you are familiar with the texture. Hope this helps!


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