You might find this more helpful than just going by waist measurement alone, Radders -
http://www.bmi-calculator.net/body-fat-calculator/
Have a look at the waist to hip calculator while you're there as well. Apparently women like me who are apple shaped as opposed to pear shaped (tendency to store fat around the middle rather than around the hips) are more at risk of health problems, as Jenny says. So although I'm a petite size 10-12 with a BMI of 20.28 my body fat level is only at an "acceptable" rather than fit level and my waist to hip ratio means my health risk is moderate rather than low.
I'm not sure what we're supposed to do about it though, without going underweight ...
🙄