The Float Test
The float test drops a spoonful of sourdough starter into water: if it floats, the starter is full of fermentation gas and near its peak, ready to raise bread. It's a useful quick check, but not infallible — stiff or whole-grain starters can sink even when they're perfectly ready.
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What it is
An active starter at its peak is riddled with carbon dioxide bubbles, which make it less dense than water — so a gently scooped spoonful floats. A starter that has just been fed, or one that peaked long ago and collapsed, has lost that gas and sinks. The test is really a rough density reading of how much fermentation gas the starter is holding right now.
How to read it
Scoop gently — stirring or squashing the sample knocks the gas out and gives a false sink. A confident float suggests the starter is at or near peak: domed, bubbly, aromatic and ready to leaven a dough. A sink after gentle handling usually means too early or too late in the cycle, and the starter's look and smell will tell you which.
Its honest limitations
The float test can lie in both directions. Stiff starters and whole-grain starters are naturally denser and can sink while being fully active; conversely, a collapsing starter can still trap enough gas to float briefly. Treat it as one clue among several — the rise in the jar, the bubble structure and the aroma are more reliable witnesses than the water glass.
Related terms
Frequently Asked Questions
What does it mean if my sourdough starter floats?
Floating means the starter is holding a lot of fermentation gas, which usually means it's at or near its peak activity and ready to raise bread. Pair it with other signs — a visible rise in the jar, plenty of bubbles and a pleasantly tangy aroma.
My starter sinks but looks active — is it dead?
Almost certainly not. Stiff and whole-grain starters are denser and often sink even when fully ready, and rough scooping can knock the gas out of any starter. If it's rising predictably in the jar, full of bubbles and smells right, trust those signs over the float test.
Is the float test the same as the poke test?
No — they check different things at different stages. The float test asks whether a sourdough starter is active enough to begin a dough; the poke test asks whether a proofed dough is ready to bake. Both are quick sensory checks, used at opposite ends of the bread-making process.
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