Purchased in addition to a yellow surveste when I found that my backpack was a large dark spot in the middle of the night my back, negating the whole point of the yellow surveste! It fits perfectly into my bag (30cm wide 40 high and 10 deep) and there is still margin. Its use is really easy and quick: - Around the bag is an elastic bag cover is positioned as a cover - Then on the bottom and on the top, 2 straps (sewn on one side and a snap on the other end) allow to "close" the bag on the back cover. - Tested every day, I confirm that protects much of the rain. Bought a second for my 15 year old son who uses no problem then that is another story for yellow jacket ....