Ideal is relative, but healthy is anywhere between 20-25% for the general population. Between 26-29, you may be slightly overweight, and above 30% is where it starts getting into ‘obese’ territory. Athletic girls (ideally with muscle), may fall to 16-18% or so. Anything under 14% is dangerous territory and qualifies most people to be ‘underweight’.
We NEED fat for survival, and women need more than men (why we have higher percentages). When we’re over-restricting with diet & exercise, the body will start cannibalizing muscle and not fat for energy. It needs it. To lose fat on an already slim body, you need to eat enough for your body to feel comfortable dipping into it’s fat stores.
I find there’s a dangerous trend with young girls & women to glamorize or seek the lowest possible percentage, thinking that’s somehow ‘better’ or more ‘desirable’. It’s not a contest, and you can’t guage how YOUR body will look at any percentage. My body is at it’s best around 20-23%: lower and I begin looking a little sickly. That’s a personal preference, and it’s fine with me. As long as you fall within a healthy range, and your body likes being there, you’ll be okay. Different percentages look different on compared individuals: don’t compare someone else’s to yours.
Body fat percentage is NOT B.M.I, but I prefer to use it as a measure of overall health. Many people are at a normal weight and have a high body fat percentage: essentially putting them in an ‘obese’ category internally. A friend of mine was 140lbs - very healthy for her height - but at a 30% body fat percentage. That qualified her for the ‘obese’ category, despite being at a relatively healthy weight according to B.M.I. (see her results!)
You can also lower your body fat percentage without the scale moving at all: it’s about the amount of body fat you have in your body, which takes up more room than muscle. Two bodies of the same weight & height, but with different body fat percentages, will appear VERY different. Another reason why no one should be fixated on the numbers on the scale. Measuring body fat percentage is a better indicator of how you’re doing.
I don’t recommend using online calculators (unless they ask for measurements) to determine body fat percentage. You can get yourself tested at the gym, or you can do it at home with a measuring kit. AccuMeasure MyoTape MT05 and AM-3000 Fitness 3000 Personal Body Fat Tester Kit.
Body fat scales are popular too. They work by sending electrical currents through your body to measure bone density, muscle, fat etc. Not exactly sure how accurate they are, but I’ve had friends rave about them. Do your research: read reviews before you invest in body fat scales. The measuring tape is just as good (if you don’t mind 5 minutes to do it).
Hope this helps!