· Nora Whitfield
What size jar for a sourdough starter? The arithmetic, worked through
"Leave room to rise" is the standard advice and it is useless, because it does not give you a number. Here is the number, worked out from the only two facts that matter: how much starter you keep, and how far it rises.
The rule, and where it comes from
A healthy starter doubles. On a good day, warm and well fed, it triples. Anything beyond that is unusual enough to plan around rather than for.
So the safe capacity is three times your working volume. Keep 200 ml, own a 600 ml jar minimum. Keep 300 ml, own 900 ml. It really is that simple, and everything below is just applying it.
Two ml conversions worth having: 900 ml is about 30 fl oz, and 200 ml is about 6.8 fl oz. American jars are often sold in quarts — a quart is roughly 950 ml, which lands in the same territory.
How much starter should you keep?
This is the question behind the question, and the answer surprises people: less than you think.
A typical loaf uses 50 to 100 g of starter. If you bake one or two loaves a week, keeping 200 ml on hand is generous — it covers a bake with plenty left to carry the culture forward.
Keeping more means discarding more at every feed, washing a bigger jar, and using more flour to maintain something you never draw on. The instinct to keep a large starter "just in case" costs you flour every single day for a benefit you use once a week.
Worked through, in a 900 ml jar
| You keep | Doubled | Tripled | Verdict in a 900 ml jar |
|---|---|---|---|
| 100 ml | 200 ml | 300 ml | Very comfortable — suits small, frequent feeds |
| 200 ml | 400 ml | 600 ml | The sweet spot — room at both |
| 300 ml | 600 ml | 900 ml | Fine doubled; a triple reaches the rim exactly |
| 450 ml | 900 ml | over the rim | The hard ceiling |
| 600 ml | over the rim | over the rim | Too much for this size |
Read the bottom two rows carefully. At 450 ml you are relying on the starter not having a good day — which is a strange thing to hope for from a culture you are trying to keep vigorous. If you genuinely need more than 450 ml on hand, that is a second jar rather than a fuller one.
Why not just buy a huge jar?
Because a starter spread thinly across a wide base is harder to read than a taller column, and because you end up washing a large jar to maintain a small culture.
There is also a practical limit on the counter. A jar you have to move to open a cabinet is a jar that ends up somewhere less convenient, and a starter you do not see is a starter you forget to feed.
And a jar sized well above what you need makes the rise harder to judge, not easier. A 200 ml starter in a 2-liter vessel occupies a tenth of the height; doubling takes it to a fifth, which reads as almost nothing even though it is the full event. The same starter in a 900 ml jar climbs from the 200 mark to the 400 mark, and the graduations make that unmissable. Matching the jar to the culture is partly about spillage and mostly about being able to read what is going on.
Feeding ratio changes the answer
Here is the wrinkle most size guides miss: the headroom you need depends on how you feed, not just on how much you keep.
A 1:1:1 feed — one part starter, one part flour, one part water — triples the contents of the jar the moment you make it. Start with 50 g of starter and you have 150 g in the jar before any rising has happened at all. Then it rises on top of that.
A 1:5:5 feed, used in warm weather to stretch the timing, multiplies the volume by eleven. From 20 g of starter you get 220 g, then it rises. Same jar, very different occupancy.
| Feed | 50 g of starter becomes | After a double |
|---|---|---|
| 1:1:1 | 150 g | ~300 ml — comfortable |
| 1:2:2 | 250 g | ~500 ml — fine |
| 1:5:5 | 550 g | over 900 ml — too much for this jar |
So if you feed big ratios, keep a smaller seed. Twenty grams of starter on a 1:5:5 gives you 220 g in the jar and doubles to around 440 ml — well within range. The mistake is keeping a large seed and feeding a large ratio, which is how a jar that seemed generous ends up on the counter.
Height, width and why proportions matter
Two jars can hold the same volume and be very different to use.
Wide beats narrow. A 900 ml jar that is 3.9 in across and 5.9 in tall gives you room to get a hand and a spatula inside. A taller, narrower one of the same capacity shows a bigger column for the same rise — which looks more impressive and means exactly the same thing — and is considerably harder to scrape.
Straight beats shouldered. Where a jar narrows toward the top, a ring of starter sits above the reach of any spatula and dries there. Straight walls let one pass take the whole side down.
The container page goes into both in more detail, and what makes a jar worth owning covers the full list of criteria.
The part size cannot solve
A big enough jar stops overflow. It does not tell you how far the starter rose, and that is the measurement you actually need.
The difference between a rise from 200 to 400 ml and one from 400 to 500 ml is the difference between a doubling and a quarter — yet the second climbs further up the glass. Judged by height alone you would read it backwards. This is why a printed scale matters as much as capacity, and why an unmarked jar of the right size still leaves you guessing.
Some numbers
a capacity that suits home baking: 200 ml of starter with room for a triple, and a practical ceiling of 450 ml
— Technical sheet, 2026
starters sequenced across four continents in the largest survey of sourdough microbiomes to date. Rise behavior varies widely between healthy cultures, so size your jar for the good days rather than the average ones
— Landis et al., eLife, 2021
Where to go next
Building a starter from scratch? Making one covers the first two weeks. For the routine, feeding explains ratios and timing — and note that a bigger feeding ratio needs more headroom, so the two questions are linked. Storing a starter matters here too: a fridge starter needs less room than a counter one. And sourdough tools lists everything that lives alongside the jar.