Handwashing put this number on the map. The World Health Organization and most national health agencies advise scrubbing with soap for around twenty seconds, which is roughly how long it takes to hum a familiar chorus twice through. The same span shows up anywhere a product needs contact time before it can do anything useful.

What people use this timer for

Scrubbing with soap, covering backs of hands, thumbs and fingertips
Rubbing alcohol gel in until your hands are genuinely dry
Letting a disinfectant wipe stay wet on a worktop before you dry it
Shaking a bottle of salad dressing so the oil and acid actually combine
Shaking a can of spray paint or primer before the first pass
Rinsing a contact lens in solution before it goes back in

Pick one song chorus and use it every single time, rather than counting. Counting drifts fast when your hands are busy, but a tune you know keeps a steady pace and gives children an obvious finish line to aim for.

Questions about this timer

Does the twenty seconds include rinsing and drying?

No. The twenty seconds refers to scrubbing with soap and lather. Rinsing under running water and drying properly come afterwards, and drying matters more than people expect because damp hands pick up and pass on far more.

Is twenty seconds also right for hand sanitiser?

Roughly, though the honest measure is dryness rather than the clock. Use enough gel to coat both hands, keep rubbing every surface, and if your hands are dry well before twenty seconds you almost certainly used too little.

Why twenty seconds and not ten or sixty?

It is the point where scrubbing removes the bulk of what is on your skin without asking for a wait that people will skip. Longer is marginally better, but a target nobody follows protects nobody at all.