Only partly. When a spoonful of starter floats in water, it is holding enough gas to be lighter than the water, which usually means the culture is active and somewhere near its peak. That much is useful. But the test can’t tell you whether the starter is still rising or already past its best, and a sinking spoonful proves very little, because many healthy starters sink even at their peak. Treat a float as one encouraging sign, and judge readiness mainly by how the starter rises and behaves after each feed.
How the test works
Water weighs about 1 g per millilitre. A freshly mixed starter of flour and water is heavier than that and sinks. As yeast produce carbon dioxide, the gas gets trapped in a network of gluten and starch, and the starter swells. Once enough gas is held inside, a spoonful becomes less dense than water and bobs to the surface.
So the float test really measures one thing: how much gas a small sample is holding at the moment you scoop it. That depends on the culture’s activity, but also on the starter’s texture and on how you handle it.
Doing it properly
If you want to use the test, give it a fair chance:
- Wait until the starter has clearly risen, domed and filled with bubbles.
- Don’t stir it first. Stirring knocks out a lot of the gas.
- Take a spoonful gently from the top, where the gas is most concentrated.
- Let it slide off the spoon into a glass of water at room temperature, rather than dropping it from a height.
- Watch for a few seconds. A float or a slow hover near the surface counts as a pass.
Use a fresh glass each time and tip the sample away afterwards. A spoonful is all it costs.
Why a sinking starter can still be ready
Plenty of perfectly good starters fail the float test. The usual reasons:
- High hydration. A starter at 125% or more is too runny to hold large bubbles. The gas escapes as it forms.
- Whole wheat or rye in the feed. Bran cuts through the gluten network and the flour is heavier, so these starters often sink at their peak. Rye has very little gluten-forming ability at all.
- Handling. A stirred or squeezed sample loses gas before it reaches the water.
- A small spoonful. Tiny samples lose a bigger share of their bubbles at the surface.
If your starter rises predictably after a feed and raises your bread well, a sinking spoonful doesn’t matter.
Why a floating starter can still mislead
A pass isn’t a guarantee either. A starter well past its peak often still holds enough gas to float for an hour or two after it has started to fall. It floats, but it is hungrier and more acidic than you want for a loaf. A very stiff starter traps gas so well that it can float even when its activity is fading.
The test also says nothing about strength over time. A starter that floats on Saturday but took ten hours to get there is still a sluggish one.
Better signs of a ready starter
A ready starter, fed at your usual ratio and temperature, shows several of these at once:
- It has risen by at least double, often more, in a predictable time.
- The top is domed, or just beginning to flatten.
- Bubbles run through the whole jar, not only the surface, when you look from the side.
- It smells pleasantly yeasty and mildly tangy rather than sharply of nail polish remover.
- A spoonful pulled up looks stringy and web-like, not soupy.
A rubber band or a strip of tape on the jar at the starting level makes the rise easy to read. What your sourdough starter’s smell is telling you goes into the smell side in more detail.
Knowing when to look
The most useful thing you can bring to any readiness check is timing. If you know roughly when the starter should peak, you can check at the right moment instead of guessing.
The sourdough starter feeding calculator estimates the peak from the feeding ratio and room temperature. For 100 g of starter plus 20 g to keep, at 100% hydration, it gives:
| Feed | Room at 20 °C (68 °F) | Room at 24 °C (75 °F) |
|---|---|---|
| 1:1:1 | about 6 h to 9 h | about 4 h to 6 h |
| 1:2:2 | about 8 h to 12 h | about 5 h 15 min to 8 h |
| 1:5:5 | about 11 h to 16 h 30 min | about 7 h 15 min to 11 h |
Check the starter at the start of that window. If it is still climbing, check again an hour later. If it has already peaked and started to fall, note the time, because that is your real peak for those conditions.
If your starter regularly takes much longer than these ranges, it may need a few days of steady feeding in a warm spot. Sourdough starter not rising? works through the common causes.
Where the float test helps most
The test is most useful as a quick confirmation when you’re unsure about timing, especially with a white-flour starter at 100% hydration. If the jar looks risen but you can’t remember when you fed it, a float suggests there is plenty of gas. A sink with a whole grain starter means very little.
It is less helpful with a brand-new culture. Young starters often go through an early burst of gas from bacteria that later die off, and a spoonful from that phase can float even though the culture isn’t ready to raise bread. How to make a sourdough starter from scratch describes that misleading early stage.
Putting the checks together
A practical routine uses the float test as the last of several checks rather than the first. Feed the starter at your usual ratio and note the time and the temperature. When the feeding window opens, look at the jar from the side: has it doubled, and are bubbles visible all the way through? Then smell it and look at the surface. If all of that points to a peak, the float test is an optional confirmation.
If the signs disagree, trust the rise. A starter that has doubled and domed but sinks is usually ready. One that floats but has barely grown is probably just trapping gas near the surface. After a few feeds, you will know which signs your own starter shows at its best, and the float test will matter less and less.
Float test mistakes
- Stirring before scooping. You lose the gas you’re trying to measure.
- Repeating the test until it floats. If the starter has to be tested five times, it isn’t ready.
- Ignoring the clock. A float three hours after the peak doesn’t mean a fresh, sweet culture.
- Judging a rye starter by it. Rye starters rarely float and are often perfectly ready.
Float test questions
Does the temperature of the water matter?
Not much. Room-temperature water is easiest. Very cold water can make the sample contract a little, but the result usually doesn’t change.
Can I float-test a levain or a dough?
You can test a levain the same way as a starter, with the same limits. Some bakers drop a small piece of dough into water near the end of bulk, but the size and texture of the dough make that even less reliable than a starter test.
My starter floats, so why is my bread still dense?
The starter is only one part. A dense loaf usually comes from bulk fermentation ending too early, weak gluten or loose shaping. Look at those steps before blaming the culture.