How Many Calories do I Need?


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This post is the beginning of a series on determining how many calories, carbohydrates, protein and fat you need per day. Enjoy!

Determining how many calories you need can be tricky. If you google “how many calories do I need” you get hundreds of results with all sorts of different calculators. How do you know which is right for you? The thing to remember is that the calculations are a best guess. If you really want to nail down your calorie needs, then you can have your Resting Metabolic Rate (RMR) measured. However, if you can not have it measured, the formulas are a good place to start.

When I’m calculating calorie needs, I prefer the Mifflin-St. Jeor equation. This equation has been found to be most accurate in a variety of people.  The formula is below. The weight is in kg and height in cm. To convert your weight to kg, divide your weight in pounds by 2.2. To convert your height to cm, multiply your height in inches by 2.54. Here is the Mifflin-St. Jeor formula:

Men: 10 x weight (kg) + 6.25 x height (cm) – 5 x age (y) + 5
Women: 10 x weight (kg) + 6.25 x height (cm) – 5 x age (y) – 161

This gives you an estimate of your RMR – which is, basically, how many calories you would need if you hung out on the couch all day. Since that is not the case for most of us, I multiply the RMR by an activity factor. The way I do it for my clients is to choose an activity factor for their day that does not include their workouts. Here are the activity factors:

  • Sedentary (little or no exercise) = 1.2
  • Llightly active (light exercise/sports 1-3 days/week) = 1.375
  • Moderately active (moderate exercise/sports 3-5 days/week) = 1.550
  • Very active (hard exercise/sports 6-7 days a week) = 1.725
  • Extra active (very hard exercise/sports and physical job) = 1.900

Finally, I add in the calories for workouts. Most athletes have a Garmin or heart rate monitor that gives them calories expended in a workout.

Here’s an example. Sandy is a marathoner who has a desk job. When she is home and not working out, she is playing with her two small children and doing house work, washing clothes, etc. She doesn’t sit much when she is at home. She is 32 years old, 5’5″ tall and weighs 140 pounds. She does not want to change her weight or body composition.

  • 65 inches  2.54 = 163.15 inches
  • 140/2.2 = 63.6 kg

The Mifflin-St. Jeor equeation for women is: 10 x weight (kg) + 6.25 x height (cm) – 5 x age (y) – 161

Sandy’s RMR would be (10 x 63.6) + (6.25 x 163.15) – (5 x 32) – 161 = 1337 calories per day. Given her work and at home (non-workout activities) I would multiply her RMR by 1.35 (1337 x 1.35 = 1805 calories per day). Her workouts range from short runs when she burns 400 calories to long runs when she burns 1200 calories. This means that:

  • On Sandy’s off day, she should eat about 1800 calories.
  • On her short workout days, she should eat about 2200 calories.
  • On her long workout days, she should eat about 3000 calories.

For most people, this gets a little overwhelming. To make it easier, we look at a seven day average. It is OK to move some calories (200 – 500) from your long workout day to the day before or after to pre-fuel and get you over being extra hungry on your day off.

I hope this helps you determine how many calories you need. If you have questions, let me know.


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