Our vet had a great cheat sheet for blanketing. Wish I still had it. Maybe yours does. To make it simple: A rain sheet if the temps are over 30 degrees, a lightweight blanket 20-30 degrees and the heavy weight blanket under 20 degrees. However, every horse is an individual and you can start with that but check your horse at the warmest part of the day. If your horse feels too warm or any sweat then you have blanketed too heavy. If in doubt go for the lighter weight blanket. Since I let my horses grow some winter coat they still have only rain sheets on. They are happy and the temps are 20- 30 here right now. I make sure they have plenty of grass hay to munch on at night when it gets the coldest though. You will know if your horse is too cold because he will shiver or look all hunched up and miserable, then blanket heavier at that temperature. My husband is not a big fan of blanketing the horses because it is not "natural" and they are range animals meant to be out in the weather. But, lets face it, they are no longer wild, being the over bred Quarter Horses they are, and they do not live in a natural environment. He is a fan of how clean they stay and easy to cool out. I check the weather forecast every day to give me an idea if I might need to switch out their blankets. You need to remember that you will have to launder the blankets with special water proofing detergent and then re-spray them with a can of Camp Dry (from Walmart) to keep up the water proof quality. Nothing worse than a blanket that leaks and holds the wet against a horses body! So, blanketing makes your life more pleasant if you ride often in the winter but it is an extra expense and you must manage it correctly. Not hard at all once you get accustomed to the habit of it though. Also, once you start blanketing you must stick with it through the winter.