You always need fat to function normally. How much body fat can or should you have? This depends on several factors. Women need more body fat than men. Breasts and hips of women contain much more fat than those of men. If a woman has too little body fat, the menstrual cycle can stop.
Another important factor about body fat is the age: with age we have naturally more fat.
A fat percentage between 15 and 20% for men and 25-30% for women is generally a nice value. (Other sources mention other values​​.)

Fat % women
Age        too low                normally              too high               much too high
20-39     < 21%                   21% - 33%           33% - 39%           > 39%
40-59     <23%                    23% - 34%           34% - 40%           > 40%
60-79     <24%                    24% - 36%           36% - 42%           > 42%

Fat percentage for men
Age        too low                normally              too high               much too high
20-39     <8%                       8% - 20%             20% - 25%           > 25%
40-59     < 11%                   11% - 22%           22% - 28%           > 28%
60-79     < 13%                   13% - 25%           25% - 30%           > 30%

Easier to measure is the waist circumference (over the belly button). Which is normally smaller than
94 cm for men and
80 cm for women.
The risk of a heart attack increases by 2% per cm. (My tape-measure does not like me.)
In many do- it-yourself shops there hang paper tape measures of 100 cm. They feel warmer than those plastic things on your bare skin. If they are too short, don’t blame the meter.

(Actually, they should be useful without you glue a second part on it.) Or simply take a piece of string with a knot at 100 cm.

We can calculate the ideal weight with the formula of Lorenz Vandervael :
For women : ideal weight in kg = 50 + [ (stature in cm - 150) x 0.6 ]
For men : ideal weight in kg = 50 + [ (stature in cm - 150) x 0.7 ]

Moderately overweight is between 110 and 130% of ideal weight.

The fat percentage can be measured (by specialists) and calculated by

• Skin fold Measurement: measure several skin folds with nonius calipers and compare with tables
• Electrical Diagnosis: measure the speed of a weak electric flow of foot to wrist. Some scales have a similar (not exact) function from foot to foot. The upper body then is not involved in the measurement.
• BMI (Body Mass Index) measurement: BMI (kg / m²) = body weight in kg / height in meters squared.

Example: With a weight of 80 kg and a length of 1.72 m
BMI = 80 / 1.72 x 1.72 = 27.0 (kg / m²) with

- 18.5 to 24.9 : normal weight
- 25.0 - 29.9 : overweight
- 30.0 to 34.9 : obesity
- 35.0 to 39.9 : adiposity
- > 40.0 : extreme adiposity

A British study by City University London gives a clear and simple guideline in 2014: let your waist circumference become not larger than half your height.

Research shows that our fat molecules approximately are 1.3 years old, 2.1 years for fatties. So you do not drag your whole life with the same fat, it’s only a relatively fresh stock. Probably intended for one winter.
Scientists at Oxford University have found that fat from food three hours after a meal already converted into two or three teaspoons of fat tissue around the abdominal area.
This adipose tissue around the waist is used only as short-term storage, so that it can be addressed at efforts. As people continue to excessive eating, fat spreads to the surrounding tissues including the buttocks and thighs for long-term storage.

A typical amount of belly fat can assist in regulating the resistance and the repair of damaged tissue via the omentum, a peritoneum in which organs and a layer of fat are retained. The cells from the omentum can transform themself into lung cells and bone cells. That helps in the rehabilitation and reconstruction of tissue. (Dr. Makio Iwashima Loyola University Chicago Stritch School of Medicine).

He had a beautiful figure before his belly began an own career. (Paraphrasing Margaret Hasley)