Is the 5-Second Rule True?

Should you really abide by the famous 5-second rule?

Sabrina Stierwalt, PhD
3-minute read
Episode #213

Most of us have heard it: if you drop food on the floor, it’s still okay to eat it, as long as you act quickly and pick it up within five seconds of it hitting the ground. But does the so-called “five-second rule” have any scientific backing? Anecdotally, most of us would agree—it depends on the kind of food. I’m more likely to eat a cracker that I’ve dropped on the floor than I am, say, a buttered bagel that lands spread-side down. But does timing also make a difference? Is there really a delay or grace period before which germs can find their way to your fallen food?

The Science Behind the 5-Second Rule

Avid listeners know that I’m not against eating moldy food, but sometimes even I have to draw the line at food that has fallen on the floor. Living in a house with dogs and children means my floors are never going to satisfy the June Cleavers of the world, so is there any amount of time that is OK for food to sit on the floor?

In a study recently published in the Journal of Applied and Environmental Microbiology, microbiologists subjected the five-second rule to some rigorous scientific tests. The scientists dropped four different foods (watermelon slices, strawberry gummy candy, bread and buttered bread) onto four different kinds of surfaces (steel, ceramic tile, wood, and carpet) that were contaminated with bacteria. They then left the foods there for <1, 5, 30, or 300 seconds before testing them to see if they had also become contaminated. With 128 possible combinations of food type, surface type, and contact time each replicated 20 times each, the researchers took > 2500 measurements.

As predicted by the five-second rule, the longer the food was in contact with the germy surface, the more contaminated it became. However, anything that fell on the contaminated surface itself became contaminated, no matter how long it was left there. Moisture also played a role: watermelon was most easily contaminated, while gummy candy had the lowest chance of picking up the bacteria.

The surface also mattered: non-absorbent surfaces like steel and tile reliably transferred bacteria more effectively than carpet and the more porous wood. So what you are dropping and where you are dropping it should be considered just as much if not more than how long your dropped item has lingered on the floor.


Please note that archive episodes of this podcast may include references to Ask Science. Rights of Albert Einstein are used with permission of The Hebrew University of Jerusalem. Represented exclusively by Greenlight.

About the Author

Sabrina Stierwalt, PhD

Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.