The Float Test: Does It Actually Work?

You have probably seen this advice everywhere: drop a spoonful of starter into water, and if it floats, it is ready to bake. If it sinks, feed it again and wait. It is one of the most repeated tips in the sourdough world. But after years of baking and testing, I have a more nuanced answer than most guides give you.

What the Float Test Actually Measures

The float test works on a simple principle: when sourdough starter is actively fermenting, it is full of carbon dioxide bubbles. Those gas bubbles make the starter less dense than water, so it floats. When the starter is flat and inactive (either underfed or past its peak), it has fewer gas bubbles and sinks.

In theory, this makes it a perfect readiness test. In practice, it is more complicated.

When the Float Test Fails

Whole wheat and rye starters: These are naturally denser than white flour starters. A whole wheat starter can be perfectly active and at peak fermentation but still sink because the heavier flour makes the overall mixture too dense to float. This leads people to think their starter is not ready when it absolutely is.

Stiff starters (less than 100% hydration): A starter maintained at 60% or 75% hydration is thicker and denser by nature. Even at peak activity, it may not float.

Over-ripe starter: A starter that has peaked and just begun to fall may still float because it retains enough gas. But it is past its ideal point for baking. The float test cannot tell you the difference between "rising" and "just starting to collapse."

So Should You Use It?

If you have a white flour starter at 100% hydration, the float test is a reasonable quick check. But it should not be your only tool. The best indicator of starter readiness is understanding your starter's behavior over time, how long it takes to peak, what it looks and smells like when it is ready, and how it responds to your kitchen's temperature.

For more on maintaining a healthy starter, check out my feeding guide and my article on rescuing a neglected starter.