How waterproof a jacket is, as well as its breathability, are measured in two ways. Each one can help give you an idea of which jacket will be best for your activity.
This measures how waterproof a jacket is. It is calculated by applying a column of water an inch in diameter against the fabric, and seeing how high the water will rise before it starts to seep through the jacket.
So, a 5000mm rating means the water rises to 5 metres before water penetrates the fabric. For a fabric to be called waterproof, it must have a rating of at least 1500mm. However, for everyday use, you need a much higher rating to deal with pressure from wind, elbows and brushing up against rocks.
A rating of 5,000mm-10,000mm is ideal for spending long days on a mountain in all weather conditions, while 10,000mm-20,000mm is best for much wetter climates.
MVT (Moisture Vapour Transfer)
This measures how breathable a jacket is. It is calculated by how much moisture can pass through the fabric in 24 hours. You may see this displayed as 10,000 gr/m²/day, or 10,000gr.
Breathability ratings don’t need to be worried about too much as a good waterproof membrane will be highly breathable. For highly active use, where you will sweat no matter what, look for features such as underarm zips to increase ventilation.