Start with the soil, not the forecast icon
A millimetre of rain is a depth of water spread across a flat surface. In a garden, some of it runs off, some is held near the surface, and some reaches deeper roots. That is why 5 mm can refresh a shallow pot without fully soaking a dry bed.
A practical rule of thumb
Under 2 mm is usually a light sprinkle. Around 5 mm can be useful for established plants if the soil was already a little damp. Around 10 mm or more is a proper drink for many garden plants, although very dry soil can still shed the first rainfall before it soaks in.
Why our forecast looks back and forward
We count recent rain because yesterday’s shower still matters today. We also look ahead because watering just before a substantial rainfall is often unnecessary. The safest final check is still to push a finger into the soil and feel what is happening below the surface.