We've observed a trend toward increased water resistance in mobile devices, but "water resistant" is not a one-size-fits-all term and it does not mean the same thing as "waterproof". Fortunately, those degrees of protection are codified. Here's a handy reference guide to those ratings.
You've probably seen a few IP codes pop up on some major mobile flagships – they always start with the letters IP (which stands for Ingress Protection) followed by two other variable digits. The iPhone 7 and 7 Plus are IP67 rated, for example, and the Samsung Galaxy S7 series has a rating of IP68.
The third character in the code doesn't actually have to do with water at all – it refers to the level of protection against infiltration from solids. This may not be something we think about very often when it comes to wearables or mobile accessories, but keep in mind that IP codes apply to a number of industries.
The final character in the four-digit code refers to the level of protection against liquids i.e. water. In the case of our examples above, the IP68-rated Samsung series is just a little more protected than the iPhone.
Protection against solids
Levels of ingress protection against solids are 0-6. These levels denote the size of particles that the object is able to keep out. If it can only keep out large objects, it will have a lower rating. As the device becomes increasingly capable of keeping out tiny particles, the rating gets higher.
0 (zero): No protection against contact or ingress.
1: Effective against objects > 50 mm. Protects against large surfaces of the body (such as incidental brushing with the back of a hand) but no protection against deliberate contact.
2: Effective against objects > 12.5 mm. Fingers or similar objects are kept out.
3: Effective against objects > 2.5 mm. Protects against tools, thick wires, etc.
4: Effective against objects > 1 mm. Keeps out wires, small hardware, large insects, etc.
5: Dust protected. Some dust might get in, but not enough to hinder operation.
6: Dust tight. Complete protection against the ingress of dust.
Protection against liquids
Levels of ingress protection against liquids are 0-9K. These levels denote the movement, depth and pressure of water the device is capable of withstanding. The higher the number, the greater the water resistance. In mobile technology, we generally see ratings 0-8, without any "K" designations, which denote increased water pressure.
0 (zero): No protection.
1: Protects against vertically dripping water, an equivalent of 1 mm rainfall per minute for 10 minutes.
2: Protects against dripping water when tilted up to 15 degrees. Even if the device is slightly tilted, it is still safe against the same amount of vertically dripping water as above.
3: Protects against spraying water. The device is safe from water lightly spraying at any angle up to 60 degrees.
4: Protects against splashing water. There will be no damage from brief splashing from any direction.
5: Protects against water jets. Water projecting from a 6.3 mm nozzle from any direction will not cause damage.
6: Protects against more powerful water jets. Water projecting from a 12.5 nozzle from any direction will not cause damage.
7: Protects from immersion up to 1 meter in depth. The device remains unharmed during immersion in water up to 1 meter deep for 30 minutes.
8: Protects from immersion more than 1 meter in depth. Duration and exact depth may vary.
It's occasionally necessary for individual manufacturers to go beyond the code itself and offer further specifics on the duration and depth of the water resistant rating. In the case of the Samsung Galaxy S7, Samsung has stated that the phone is protected in up to 5 feet (1.54 meters) of water for up to 30 minutes.
Occasionally, you may see an "x" stand in for one of the numerals. An X means that no official protection rating has been given. This is different than a complete lack of protection (which would be a zero). For example, the first-generation Apple Watch was rated IPX7. Surely there were some solids that could not make it inside its enclosure, but there was no official guidance on what, if anything, could get through.
These codes and measurements are published by the International Electrotechnical Commission (IEC) as a global standard.