To convert kilograms (kg) to stones (st), you need to divide the weight in kilograms by the conversion factor of 0.15747, which is the number of kilograms in one stone.

Formula: Stones = Kilograms / 0.15747

Using this formula, we can calculate the number of stones in 91 kilos:

Stones = 91 kg / 0.15747

Stones ≈ 581.08

Therefore, there are approximately 581.08 stones in 91 kilos.