Waterproof Breathability Ratings: mm and g/m2/24h

What the numbers mean

When you see a shoe labeled with a waterproof rating, two numbers usually appear. One is expressed in millimeters (mm) and describes water resistance. The other is expressed in grams per square meter per 24 hours (g/m2/24h) and describes breathability. Each number measures a different property, and neither one alone tells you how comfortable the shoe will be in real conditions.

The millimeters value comes from a hydrostatic pressure test. A column of water is placed over the fabric, and the height of the column is increased until water begins to penetrate. A higher millimeter value means the fabric can withstand a taller column of water, which translates to greater resistance against rain and puddles. The breathability value measures how much moisture vapor can pass through one square meter of the material in a day, which relates to how well sweat can escape.

These two properties are in tension. A material that blocks water completely also tends to trap moisture vapor. Materials that let sweat out easily often let rain in. Understanding this tradeoff is the first step to choosing the right pair for your needs.

• Millimeters (mm): the waterproof rating, higher is more rain resistant
• Grams per square meter per 24 hours (g/m2/24h): the breathability rating, higher is more breathable
• Both numbers matter: a very waterproof shoe with poor breathability can cause sweaty feet

Waterproof rating in millimeters

The millimeter rating is a measure of hydrostatic pressure. It answers a simple question: how much water pressure can the shoe material withstand before water gets through? The test is standardized, but the number you see on a product page is reported by the manufacturer and may be measured slightly differently from one brand to another.

For light rain or dry conditions, a lower rating may be sufficient. For walking through puddles, wet grass, or steady rain, a higher rating matters more. The key is to match the rating to the conditions you actually encounter. A shoe with a moderate waterproof rating can keep your feet dry in a drizzle but may leak in a heavy downpour or when you step into deep water.

Waterproofing also depends on more than the fabric. Seams, zippers, laces, and the tongue opening are common leak points. A shoe with a high millimeter rating on the upper can still let water in if the construction allows it. When comparing, look for a design that integrates waterproofing into the entire upper, not just the outer layer.

For context on how waterproof height affects coverage, see our guide to boot waterproof height and our comparison of waterproof leather vs membrane.

Breathability in g/m2/24h

Breathability is expressed as moisture vapor transmission rate (MVTR). It measures how many grams of water vapor can pass through one square meter of material over 24 hours. A higher number means the material releases sweat more efficiently, which keeps your feet drier on the inside.

When your feet sweat, the moisture needs to escape. If the shoe material blocks vapor, sweat builds up, making your socks damp and your feet feel clammy and cold. This is why a shoe that is highly waterproof but has low breathability can still make your feet wet, just from your own perspiration rather than from the rain.

Breathability is especially important for activities that make you sweat, such as walking long distances, working on your feet, or hiking. If you mostly wear shoes for short trips in cool weather, breathability matters less because your feet produce less moisture.

The tradeoff between waterproofing and breathability is central to choosing shoes. Materials that are highly waterproof often need a membrane to let vapor out, and the quality of that membrane determines the g/m2/24h number. For a closer look at how materials affect breathability, see our guide to shoe breathability materials and our explanation of waterproof vs water-resistant shoes.

Finding the balance

The ideal shoe balances both numbers. For rainy climates, a higher mm rating gives confidence that your feet stay dry in downpours. For active use, a higher g/m2/24h rating keeps your feet comfortable when you are moving. The difficulty is that no material excels at both maximum waterproofing and maximum breathability simultaneously.

A practical approach is to decide which condition you face more often. If you commute in a rainy city and spend most of your time indoors, prioritize waterproofing. If you walk for exercise or stand and move all day, prioritize breathability and accept a lower waterproof rating.

Wearing the right socks also affects your perceived comfort. A breathable sock with moisture-wicking properties can make a less breathable shoe feel more comfortable, while a thick cotton sock can trap moisture even in a highly breathable shoe. See our guide to sock thickness and shoe size for how socks interact with fit.

For wet conditions, the boot or shoe height plays a role too. A higher shaft keeps water from entering at the ankle, which is a common leak path. Read our guide to rain boot thickness and our guide to waterproof boot leak testing for more on how construction affects real-world performance.

What to pick for your use

Your activity and environment determine how much of each number you need. There is no single rating that works for everyone, so focus on your worst-case conditions. If you are rarely out in heavy rain, a moderately waterproof shoe with high breathability is more comfortable. If you stand in rain or walk through wet terrain, a high waterproof rating with moderate breathability is the safer choice.

Waterproof membranes add a layer that can affect flexibility and weight. If you do not need waterproofing, a breathable non-membrane upper may be lighter and more comfortable. The decision, ultimately, is about where you are willing to compromise.

For specific recommendations, our best waterproof shoes guide compares options with high mm ratings, while our best rain or snow shoes guide focuses on boots for wet weather. If you work outdoors, the best work boots guide covers boots that balance waterproofing with durability.

What to pick for your use

Questions

Is a higher mm waterproof rating always better?

Higher mm means better resistance to water pressure, but it is not always necessary. For light rain, a moderate rating is enough. A very high rating often comes with lower breathability, which can make your feet sweat. Choose the rating that matches the rain and puddles you actually encounter.

What does g/m2/24h mean for sweaty feet?

This number tells you how much moisture vapor passes through the material in a day. A higher value means more sweat escapes, keeping your feet drier. If you tend to have sweaty feet, look for a higher g/m2/24h rating, especially if the shoe also has a waterproof membrane.

Can a shoe be both fully waterproof and breathable?

No material is perfectly waterproof and perfectly breathable at the same time. Waterproof membranes allow some vapor out, but the amount is limited. A shoe can be very waterproof and adequately breathable, but the two properties are a tradeoff.

Why do my feet get wet inside a waterproof shoe?

Sweat that cannot escape is the most common cause. If the shoe has a high mm rating but low g/m2/24h, moisture builds up inside. Other causes include water entering through the tongue, laces, or seams that are not fully sealed.

Do I need a waterproof membrane?

Only if you encounter rain or wet ground. If you mostly wear shoes indoors or in dry conditions, a membrane adds weight and reduces breathability without a benefit. For wet climates, a membrane with a decent g/m2/24h rating keeps rain out while letting some sweat escape.

How do I know if a shoe is actually waterproof?

Manufacturer ratings give a baseline, but real conditions can defeat a high rating. Check whether the seams, tongue, and zippers are waterproofed, and consider the shaft height for deep water. A home test with a damp cloth can tell you about the upper, but check the waterproof boot leak test for a systematic approach.