Waterproof Ratings Explained: What mm and Breathability Numbers Mean

Quick answer

Waterproof jacket ratings use millimeters (mm) to show how much water pressure a fabric can hold back before leaking, while breathability ratings show how much moisture vapor can escape. For everyday city rain, a fully waterproof shell with taped seams like the Rayu Rain Jacket is enough. For sustained storms, high-output activities, or cold-weather layering, look for higher mm and breathability numbers and plan to layer.

Waterproof ratings can look like a secret code: 10K/10K, 20,000 mm, 5,000 g/m². They are simpler than they seem. The first number tells you how much rain pressure the fabric resists. The second tells you how much sweat vapor it lets out. Neither number alone tells you whether a jacket will keep you comfortable.

This guide explains what those numbers mean in real conditions, when a lower rating is genuinely fine, and when you should pay for more. It also covers how fit and layering change the equation, with concrete examples from Cotopaxi jackets and bags.

What the mm waterproof rating actually measures

A fabric's waterproof rating is measured by placing a column of water on it and seeing how tall the column can get before water pushes through. A 10,000 mm rating means the fabric resists a 10-meter column of water. In practice, that handles steady rain and moderate wind-driven rain. A 20,000 mm fabric resists twice that pressure, which matters in prolonged storms, heavy snow, or when a backpack strap presses water against the fabric.

The number says nothing about seam construction. A high-mm fabric with untaped seams will still leak at the stitching. For any jacket you plan to wear in real rain, taped or sealed seams matter as much as the mm rating.

• Look for the mm rating and a seam-sealing statement together, not either one alone.
• A 10,000 mm shell is usually enough for commutes, travel, and light hiking.
• Above 20,000 mm is for sustained exposure, not casual city use.

Breathability ratings: the number that keeps you from getting clammy

Breathability is measured in grams of moisture vapor per square meter per 24 hours (g/m²/24h). A 5,000 g rating means the fabric lets out a moderate amount of sweat vapor. A 20,000 g rating moves much more. If you hike uphill in a rain shell, a low breathability rating traps moisture inside and you end up wet from sweat even though the rain never got in.

Breathability and waterproofing pull against each other. More waterproofing usually means less vapor escape, which is why high-output activities often use a layered system: a breathable midlayer under a waterproof shell. The Pacaya 2.0 Insulated Hooded Jacket is built as that kind of breathable, movement-friendly layer, and its recycled shell and recycled insulation make it a practical midlayer under a waterproof shell on stormy days.

• Choose higher breathability for running, skiing, or fast hiking.
• For standing around in town, moderate breathability is fine.
• Pit zips and vents can compensate for a lower breathability rating.

Match the rating to your climate, not to the marketing

A 5,000 mm jacket is a reasonable choice for a dry climate with occasional showers. A 10,000 mm jacket covers most rainy cities and three-season hiking. A 20,000 mm jacket is for places where rain comes sideways for hours, or for winter sports where snow sits against the fabric. Buying more waterproofing than your climate demands usually means paying for stiffness and weight you do not need.

The same logic applies to bags. A lightweight packable daypack like the Empacable 16L Daypack is made from 100% recycled fabric and stuffs away easily, which suits travel where rain is brief. A commuter pack like the Allpa 18L Daypack uses highly durable fabric built to handle getting roughed up on trains and subways, with a laptop and tablet sleeve and mesh exterior water bottle pockets for the daily. Neither is a substitute for a waterproof shell, but the durability and organization differences matter for how you carry gear in wet weather.

• Dry climate, short showers: 5,000-10,000 mm is plenty.
• Rainy city, regular commuting: 10,000 mm with taped seams.
• Sustained storms, snow, or alpine use: 20,000 mm and up.

Fit and layering change how a rating performs

A waterproof rating assumes the fabric is not stretched tight or compressed. If a jacket is too small, movement pulls the fabric and can stress seams. If it is too large, cold air and water can pool in gaps. The right fit leaves room for a midlayer without excessive bulk, and lets the hood cinch down around your face.

The Rayu Rain Jacket is a fully waterproof shell with two zippered hand pockets and a cinchable hood, built for slick city streets. It has all the features you need and none that you do not, which makes it a good example of matching a rating to a real use case rather than chasing the highest number. For cold-weather layering, the Pacaya 2.0 Insulated Hooded Jacket is designed to work under a waterproof shell, so you get warmth and breathability without doubling up on waterproofing.

• Try the shell on over your thickest midlayer before buying.
• Check that the hood cinches and does not block peripheral vision.
• Make sure wrist cuffs close snugly over or under gloves.

Decision rules for picking a waterproof rating

• If you mostly walk city blocks and wait for buses: A 10,000 mm waterproof shell with taped seams and a cinchable hood covers nearly all of it. The Rayu Rain Jacket fits this rule with a fully waterproof build and two zippered hand pockets. Do not pay for 20,000 mm unless your commute regularly involves hours in driving rain.
• If you run, ski, or hike uphill in the rain: Prioritize breathability ratings over maximum mm. Pair a breathable midlayer like the Pacaya 2.0 Insulated Hooded Jacket with a waterproof shell so sweat vapor escapes and rain stays out. High breathability plus a solid shell beats one jacket trying to do both poorly.
• If you travel with one bag and unpredictable weather: Choose a packable layer and a pack that suits your carry style. The Empacable 16L Daypack stuffs away easily for sudden showers, while the Allpa 18L Daypack gives you a laptop sleeve and durable fabric for daily transit. Match the bag to how you move, not to a single weather forecast.
• If you are layering for cold and wet conditions: Use a waterproof shell as the outer barrier and an insulated, breathable layer underneath. The Pacaya 2.0 is designed as that under-shell layer, with recycled insulation that regulates heat. Two purpose-built layers outperform one overbuilt jacket.

How these Cotopaxi pieces fit the ratings conversation

• Rayu Rain Jacket - Men's: A tried and true rain shell built for slick city streets, fully waterproof with two zippered hand pockets and a cinchable hood. It is a practical everyday shell when you need reliable rain protection without paying for alpine-level specs you will not use.
• Pacaya 2.0 Insulated Hooded Jacket - Women's: A movement-friendly, breathable layer with a 100% recycled shell and recycled insulation made from pre-consumer wool scraps that regulates heat; an ideal layer under a waterproof shell on stormy days. It solves the breathability side of the equation, so you can wear a simpler waterproof shell on top and stay warm without overheating.
• Empacable 16L Daypack - Del Día: Super lightweight and 100% recycled, easy to stuff away and pop back out in a cinch. A packable option for travel and errands where you want a bag that disappears when the weather clears.
• Allpa 18L Daypack: A traditional panel-loader with a laptop and tablet sleeve, mesh exterior water bottle pockets, and highly durable fabric that handles getting roughed up on trains and subways. Better for daily commutes and organized carry when you need structure and durability more than packability.

Fit guidance for waterproof layers

Best for

• Commuters and travelers who need a reliable shell for regular rain
• Cold-weather users who layer an insulated midlayer under a waterproof shell
• Anyone choosing between a packable daypack and a structured commuter pack

Not for

• Alpine climbers or expedition users who need the highest available mm and breathability ratings
• People who want one jacket to handle both extreme rain and high-output activity without layering

Waterproof ratings are a starting point, not a verdict. Match the mm number to how hard and how long rain hits you, match breathability to how much you move, and let fit and layering do the rest. A well-chosen shell and a breathable midlayer will outperform a single overbuilt jacket in almost every real-world situation.