Temperature guidelines
In principle, an unclipped horse has enough coat to protect itself from the cold and rain. If an unclipped horse has little or no shelter outdoors, you can choose to use a rug to keep it dry. However, bear in mind that a rug flattens the hair, meaning your horse is less able to keep itself warm.
Clipped horses do need a rug. But which rug do you need at which temperature?
Guidelines for rug filling by temperature
Above 15 °C: No rug, or only an unlined rain rug (0 grams) in heavy rain.
10 to 15 °C: No rug up to 50 grams (0–50g for unclipped horses, 0–100g for clipped horses).
5 to 10 °C: 50 to 100 grams for clipped horses; unclipped horses often do not need any filling yet.
0 to 5 °C: 100 to 150 grams for unclipped horses, 200 to 300 grams for clipped horses.
Below 0 °C (freezing point): 150 to 200 grams for unclipped horses and 300 to 400 grams for clipped horses in severe frost
Guidelines
The information above consists only of guidelines and averages. Every horse is and remains different. If your horse is shivering, it is cold. This is a sign that it needs a warmer rug to keep itself warm. If a horse feels clammy or is sweating under its rug, it is wearing a rug that is too thick and needs a lighter one.