Waterproof Rating (mm) Explained for Jackets and Tents

When a jacket or tent is labeled "10,000mm waterproof," it is not telling you anything is ten meters tall. That number is a hydrostatic head measurement, and it is the single most useful spec for predicting whether you will stay dry — once you know how to read it. Here is what the millimeter rating means and the threshold that actually matters.

What the Millimeter Number Measures

The waterproof rating comes from a hydrostatic head test: a sealed tube of water is placed over the fabric, and water is added until pressure forces a drop through. The height of the water column at that point, measured in millimeters, is the rating. A 10,000mm fabric holds back a 10-meter column of water before it leaks.

That sounds like overkill until you realize that body weight, a backpack strap, or kneeling on a tent floor concentrates pressure and effectively raises the water column pushing against the fabric. A static drizzle exerts almost nothing; a knee on wet ground exerts a lot. The rating has to cover the worst-case pressure, not the average.

The Numbers and What They Survive

Rating	Holds up to
0–5,000mm	Light rain, dry snow; not for sustained weather
5,000–10,000mm	Moderate rain, light pressure; the everyday standard
10,000–15,000mm	Steady rain, heavier use
15,000–20,000mm+	Hard storms, high pressure, mountaineering

The widely accepted floor for a garment to be called genuinely "waterproof" rather than "water-resistant" is around 10,000mm. Below that, fabric handles a passing shower but wets through under sustained rain or pressure. For a rain jacket you will actually rely on, 10,000mm is the number to clear; for serious mountain weather, 20,000mm.

Tents Use the Same Scale Differently

A tent has two numbers, and they are not equal:

• The flysheet (the outer canopy) is exposed to rain but little pressure. 1,500–3,000mm is often enough here.
• The floor takes your body weight, knees, and gear pressing the fabric into wet ground. It needs a much higher rating — 5,000mm or more — because that pressure is exactly what the hydrostatic test simulates.

This is why a tent can advertise a modest flysheet figure and still keep you dry: the floor, where pressure concentrates, is rated far higher. If a tent lists only one waterproof number, check whether it is the floor or the fly before trusting it.

Why Waterproof Is Only Half the Story

A high hydrostatic head keeps water out, but a fully sealed fabric also traps sweat in. That is why good rain gear pairs its waterproof rating with a breathability rating (often in grams: how much water vapor escapes per square meter in 24 hours). A 20,000mm jacket that cannot breathe will leave you soaked from the inside on a hard hike. The two specs are a balancing act, and the right mix depends on whether you are standing at a bus stop or climbing uphill.

Also worth knowing: the rating measures the fabric, not the seams. Water finds needle holes first. Taped or sealed seams are what turn a waterproof fabric into a waterproof garment, so check for them regardless of how high the millimeter number climbs.

Frequently Asked Questions

What does 10000mm waterproof mean? It is a hydrostatic head rating: the fabric withstands a 10-meter (10,000mm) column of water before leaking. Around 10,000mm is the common threshold for a garment to count as genuinely waterproof rather than just water-resistant.

Is a higher waterproof rating always better? Higher keeps out more water under more pressure, but a fully waterproof fabric traps sweat, so for active use you also need breathability. For everyday rain, 10,000mm is plenty; chase higher only for storms or heavy loads.

What waterproof rating does a tent floor need? Higher than the flysheet — typically 5,000mm or more — because your body weight and gear press the floor into wet ground, concentrating pressure the way the hydrostatic test does. Flysheets can be lower since they face little pressure.

Why does my waterproof jacket leak at the seams? The millimeter rating measures the fabric, not the stitching. Needle holes at the seams let water in unless they are taped or sealed. A high rating with untaped seams will still leak.

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The rating is one factor. Our hardshell jacket buying guide covers breathability, membranes, and seam construction together.