What does waterproof rating (water column) actually mean?
The number tells you how waterproof the fabric is, but seams, zippers and fit also decide how dry the child stays.
When you see “10,000 mm water column” on a jacket or pair of trousers, it doesn't automatically mean the garment is perfect for every rainy day. It means the fabric itself has been tested for how much water pressure it can take before water pushes through.
Picture a column of water pressing down on the fabric. The higher the number, the more pressure the material can withstand in the test. That's why water column ratings are common on rainwear, shell clothing and winter garments. But the number only tells part of the story.
In real life, kids' clothes are more complicated. A garment can have a high rating but still let in moisture at zippers, seams or cuffs, or where the fit lets water run inside. When the child sits on wet ground, crawls in sand or wears a backpack, the pressure on the fabric can also be much higher than ordinary rain. Washing, wear and tumble-drying can affect performance over time too.
For preschool, the whole picture matters more than a single number. Check the water column, but also look for taped or welded seams, adjustable cuffs, good leg endings, sturdy fabric and clear care instructions. A garment that fits well and is used correctly can outperform one with an impressive number but weak details.