If you know your weight in stone, take the whole stones (i.e if you're 14 st 10.5lbs, take the 14) and x it by the amount of pounds in a stone (which is also 14). Then add on the extra pounds that you weigh (10.5 in this case). You'll have a large number of pounds then (206.5). That's the amount of pounds you weigh. 10% is just moving the decimal point one way to the left so 10% would be 20.65, or 20lbs and 1 half. So a person starting at 14st10.5lbs would want to lose 20.5lbs to have lost 10% of their body weight, which would take them to 13st4lbs.