You didn't mention your height! Any BMI below 18.5 is considered underweight. A BMI below 16 means you are significantly underweight and at risk of suffering various health conditions caused by a low body weight and low-nutrient consumption.
To understand how underweight you are with a BMI of 16, it is helpful to know how small the range is for BMI categories. A healthy BMI is between 18.5 to 24.9, with anything over that considered overweight and anything under considered underweight. To be more specific about your level of health risk at a BMI of 16, the underweight category is split into sections. A BMI of 17 to 18.49 is considered mild thinness, a BMI of 16 to 16.99 is considered moderate thinness and a BMI under 16 puts you in the severely thin category, meaning you at high risk of suffering health effects associated with being underweight.
With a BMI of 16 you are likely not getting enough nutrients, vitamins and minerals, including calcium, to remain healthy. Your body requires calcium to keep your bones healthy and strong, therefore a lack of calcium puts you at risk of developing osteoporosis. Osteoporosis is a condition where your bones become porous, which makes them brittle and weak. This condition increases your chances of suffering bone fractures, commonly in the hip, wrist and spine. In severe osteoporosis your bones are so brittle, coughing or falling can cause them to break.
Having a BMI of 16 can leave you feeling tired and drained, with little energy, and one of the causes of this may be anemia. Anemia is a condition where your body does not have an adequate amount of healthy red blood cells. If you are underweight, your diet likely lacks in proper amounts of iron, found in eggs, meats, green leafy vegetables as well as iron-fortified foods. Your body uses iron to make hemoglobin, an iron-rich protein which makes your blood red and carries oxygen throughout your body. Anemia can make you feel dizzy, irritable and cold.
BMI is traditionally calculated using the metric measurement system, using kilograms to measure weight and meters to measure height, according to William D. McArdle, Frank I. Katch and Victor L. Katch, authors of the textbook "Sports and Exercise Nutrition." Use the following formula:
BMI = weight in kilograms / height in meters squared
If you only know your height in centimeters, then simply divide your height by 100 to determine your height in meters. For example, if you are 170cm tall, then divide 170 by 100, which equals 1.7m.
As an example, using this formula, if you weigh 75 kg and are 1.7 m tall, your BMI is 75 / 1.7^2, which equals 26.0 when rounded to the nearest tenth.
BMI of less than 18.5 is considered underweight. At a BMI of 16, you're bordering on dangerously low. You mentioned that you're eating habits aren't good. Try & eat lots of carbs -- pasta, rice, potatoes -- as they'll help you put on weight.