How to Calculate Your Calorie Deficit for Weight Loss

Frequently people seem to take an approach to weight loss that essentially amounts to wishful thinking. Here’s how it goes: person picks a given amount of weight that she wants to lose in a given amount of time (usually the timeframe is completely unrealistic) and breaks it down by how much she needs to lose by month/week/day. She then decides on an arbitrary number of calories (usually way too low) that she somehow concludes will enable her hit these targets. And then gets frustrated when it doesn’t work.

Well, it would be nice if you could will yourself into losing whatever amount of weight you wanted, but unfortunately it doesn’t work that way. You need to do the math and use some common sense to figure out how much weight you can reasonably expect to lose in a given period, and then set a timeframe for achieving your goals that’s actually feasible. The calculations here aren’t perfect, but they’re better than guesswork and they’re the best place to start. So here’s how to do it:

Step 1: Basal Metabolic Rate (BMR)

Whether you want to lose, gain, or maintain weight, the first thing you need to know is your daily calorie maintenance needs: the number of calories you need to consume in a day in order to maintain your current weight.

First, you need to determine your Basal Metabolic Rate (BMR). This is the number of calories that you burn everyday just by existing—literally the amount you’d need to maintain your weight if you were in a coma. You can find this number by inputting your height, weight, age, and gender into an online calculator such as this.

For example, as a 5’8 140lb 21 year old female, my BMR is 1485 calories.

***Note: BMR doesn’t take into account lean muscle mass versus body fat, but it does have an impact. The more muscle you have, the higher your BMR (which is why weight lifting helps you lose weight!!) I can’t find an exact number for how it affects your BMR, but it appears to be about 5-10%—say +/- 100 calories. Also, long-term low calorie diets can LOWER your BMR by up to 30% (which is why crash dieting is a bad thing!)

Step 2: Daily Calorie Maintenance Needs

Your BMR equates to the number of calories your body would use if you were lying in a coma all day. Obviously, you’re not. The number of calories that your body uses is affected by your activity level. Therefore, you need to multiply your BMR by the following activity level multipliers in order to determine your daily calorie maintenance needs:

  1. If you are sedentary (little or no exercise) : BMR x 1.2
  2. If you are lightly active (light exercise/sports 1-3 days/week) : BMR x 1.375
  3. If you are moderately active (moderate exercise/sports 3-5 days/week) : BMR x 1.55
  4. If you are very active (hard exercise/sports 6-7 days a week): BMR x 1.725
  5. If you are extra active (very hard exercise/sports & physical job or 2x training) : BMR x 1.9

Right away you can see there’s going to be some estimation involved—how do you know if you’re lightly versus moderately active, for instance? My advice is to calculate your daily calorie maintenance for the two levels that you think best apply, which will give you a range. No matter what, your daily calorie needs will be at LEAST your BMR x 1.2, so you should use this number as a baseline minimum.

For example, let’s say I fall somewhere between moderately and very active. My daily calorie needs with a BMR of 1485 would be between 2300 and 2560. If I was completely sedentary, my maintenance would be 1780.

***Note: Chances are that this number is higher than you expect. In my experience, most people underestimate both the number of calories they need AND the number that they’re actually consuming. People who don’t diet/track calories will usually say they’re eating a “normal” amount of food and estimate their intake around say 1800-2000 calories, when in reality they’re often eating much more. People who do diet/track calories will often aim for an intake that’s way too low on the days that they track, but then fail to factor in the non-tracked binges/”cheat days” that make their average intake much higher than they think it is (I did this for years). And even if you’re scrupulously tracking, often servings will have more calories by weight than by volume and even packaged foods can be incorrect, so unless you’re weighing 100% of what goes in your mouth, you’re probably underestimating. I’m definitely not saying you should weigh all your food (I never have!), but just be aware that you CAN eat and probably ARE eating more than you think!

Step 3: Calorie Deficit

It takes a ~3500 calorie deficit to lose a pound, or a 500 calorie deficit per day to lose a pound per week. For most non-obese people, a 500-1000 calorie deficit is a good place to start, which should produce 1-2 lbs of weight loss. Greater than 1000 deficit and you probably won’t be able to sustain it for long enough to produce significant weight loss, less than a 500 calorie deficit and the results won’t show up fast enough to keep you motivated.

For example, let’s say I want to lose 1lb per week, so I need to create a 500 calorie deficit. If my maintenance level is between 2300 and 2560, I should aim for 1800-2060 calories to lose one 1lb per week.

No matter what, you shouldn’t go below an average daily intake of 1200 calories (and believe me, if you’re REALLY eating 1200 calories per day it’s not much food!) So if your daily calorie needs are 1800 but you want to lose 2lbs a week, you need to either adjust your expectations, or increase your activity level to raise your maintenance calories. And if you’re finding it impossible to create a deficit, that’s probably a sign that you’re close to your ideal weight and don’t have much to lose, so you’re better off focusing on exercise to make you thinner and leaner rather than cutting calories further which could slow your metabolism and trigger a binge.

***Note: You should be able to see why it’s not feasible for most people to lose, say, 5 lbs of REAL weight (as opposed to water weight) in one week. That would require a deficit of 17,500 calories, or 2500 per day, and the average person isn’t even burning that much to start out with! When doctors and health officials say that you shouldn’t lose more than 2 lbs per week, it’s not just because it’s something they’re *supposed* to say (like the whole “don’t have more than 1 alcoholic drink per hour” warning—yeah right), it’s because most non-obese people simply CAN’T lose more REAL weight than that on a weekly basis.

Conclusion: Weight Loss!

Now that you have a starting point, you can use this number as a target for your weight loss goals. Measuring food and tracking your calorie intake (as well as macronutrients, which I will discuss later) on a website/smartphone app such as LoseIt, SparkPeople, or My Fitness Pal is very beneficial when you’re first losing weight as well as when you’re transitioning into maintenance because it will give you hard data that you can analyze for patterns in your intake and eating habits.

if you notice that you’re not losing weight, you may need to adjust your calorie goals downward. But be patient—just because the scale isn’t moving as fast as you want doesn’t mean that you’re not making progress! Similarly, if you find it impossible to stick with your calorie goals, you should increase your target upward. Even if you aren’t losing weight as quickly, you’re still losing weight—that’s better trying to eat so little that you end up bingeing every other day and canceling out your deficit, or giving up altogether. Also, you’ll want to periodically recalculate your BMR and maintenance needs as you lose weight (but you don’t have to do it THAT often—a 10 lb weight loss only lowers your BMR by about 50 calories).

You may also need to change your activity multiplier if your activity level changes. However, important to note that you should NOT eat back exercise calories, because they’re already accounted for in the activity level multiplier you choose. (Unless it’s a one-time exception—ie you’re normally just lightly active, but one day on vacation you go on a 10 mile hike). 

Finally, even if you hit on the perfect plan and follow it to a T, that doesn’t mean that you can exactly predict your weight loss down to the month, week, and day. Our bodies are not calculators, weight loss is not perfectly linear, the scale does not perfectly reflect weight loss, and small changes in real weight can easily be disguised by temporary fluctuations and water weight. You need to be patient, think about your progress in the long-term not the short term, and use other measures of weight/fat loss such as measurements, pictures, and how your clothes fit so you don’t get too wrapped up in the scale.

Hope this is helpful, and as always message me if you have questions!!

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3 Responses

  1. […] factor, then subtract to create a deficit if I want to lose weight. (I explain how to do this here). I eat that number of calories on a daily basis, whether I exercise that day or not. My general […]

  2. Reblogged this on InspiredWeightloss! and commented:
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