I originally wrote a derivation of this message for a woman who wanted to know whether a certain amount of activity would result in her losing weight. She asked that I ignore her diet (which isn’t the most effective way of losing weight). I found that it was a good general guide, besides being good for her. So, I thought I would share. Just so you know where my numbers were coming from, she was 188 pounds, 5′6″, and 17 years old, walking 2.2 miles/day for 30 minutes, and wanting to lose 40 pounds in 5 months (as with most people, she’s setting an awfully fast weight loss goal; the turtles keep the weight off better than the hares!).

1.) Calculate your basal (or resting) metabolic rate and your daily needed calories. This is the amount of calories your body burns just from keeping your lungs breathing and heart beating without including any exercise you get. There are lots of online calculators that can do this for you (using a couple different formulas), but the formula I usually use is based on weight, height, gender, and age. The formulas are below. In your case, your BMR is going to be aprox. 1700 calories.

Men: BMR = 66 + (13.7 X wt in kg) + (5 X ht in cm) - (6.8 X age in years)

Women: BMR = 655 + (9.6 X wt in kg) + (1.8 X ht in cm) - (4.7 X age in years)

Now, if you do anything other than lie in bed 24 hours per day, you are going to end up burning more due to walking around, cleaning your house, shopping for groceries, climbing stairs, etc. To calculate this simply multiply your BMR by the appropriate number below. So, even if you are sedentary, your body is burning at least 2,000 calories/day. I like to calculate based on sedentary (daily activities and such), and then calculate intentional exercise and workouts based on actual calories burned.

sedentary (little or no exercise): BMR x 1.2
lightly active (light exercise/sports 1-3 days/week): BMR x 1.375
moderatetely active (moderate exercise/sports 3-5 days/week): BMR x 1.55
very active (hard exercise/sports 6-7 days a week): BMR x 1.725

2.) Determine proper nutrition and healthy/safe calorie deficit in your diet. Generally, you should only make a maximum deficit of 500-1000 calories (lightly overweight stick to the 500 side and obese can slide towards the 1000 calorie side). In this case, you said to ignore this. In fact, you can’t really ignore this if you are serious about your weight loss. However, let’s assume you eat about 2,000 calories per day (which is fairly easy unless you are eating junk food; Avoid soda/chips/fried foods, since these will easily sky rocket your calories). Then, your weight loss will be completely determined by your exercise.

3.) Now, calculate the calories you burn during exercise. Again, there are lots of online calculators you can use. Most need the speed of your exercise, though. So, first determine your walking speed. In your case, 2.2 miles/.5 hour equals about 4.4 miles/hour. This actually seems to be a very fast walk (light jog, even), so check to see if this is correct. Input this, along with your weight (and maybe gender/height) into an online calculator to see how much you burn. SO, you are burning about 258 calories in each half hour workout. Now, if you do this seven days per week, you are burning about 1,800 calories/week.

4.) Determine speed/amount of weight loss and set goals. So, at your current activity level, you will lose 1 pound (3,500 calories) about every two weeks (13.5 days). Now, your time frame is about 5 months (6 if you are talking about the end of June). So, you have about 20 weeks in your schedule. At 1 pound per two weeks, that means you will lose 10 pounds by the start of June. However, losing weight by exercise instead of diet means you may lose faster than the calculation (whereas dieting can make you lose slower than the numbers due to the metabolism slowing). Also, by exercising, you will lose more inches (and tone up your body) more than you would by dieting.

5.) Realize that it isn’t completely about the calculations. Even if you calculate the number of calories burned during the exercise, it doesn’t count how much your metabolism will be speed up due to increasing muscle and doing cardiovascular exercise (or how your ratios of carbohydrates, proteins, and fats will contribute). So, consider the numbers to be conservative.

So, I do think you should keep at least an overall general idea of your daily calorie intake. Even if you don’t count them all, make sure you aren’t way overdoing it with junk food and “empty” calories. Also, make sure you eat plenty of fruits/vegetables of many different colors! Now, if you double your exercise every night to an hour of exercise each night, you will end up losing 20 pounds by June. If you make it two hours each night, you will hit your 40 pound goal. It may or may not be realistic for you to exercise this much. I mean, two hours per night is quite a bit. You could also increase your speed/incline/intensity to help increase your caloric burn and burn more calories in less time.

In general, you shouldn’t push too fast to lose weight. As I said before, if you do want to make it faster, doing it with exercise is MUCH better than doing it by eating hardly any calories, but you should still be careful of going too fast (my disclaimer is that you should consult your doctor before beginning any diet/exercise program). Losing weight too fast can cause stretch marks and saggy skin (not to mention that overall “bag-like” look). Keep in mind that the exercise will tone you and make you look a lot smaller (losing inches) at whatever weight you get to as compared to dieting alone. With exercise, you may find that losing 25 pounds and toning your body makes you look as good as someone who lost 40 by over dieting and losing water and muscle mass.