How Many Calories Do I Need In A Day? From A Board Certified Physical Therapist

How do I know how many calories I actually need to eat in a given day?  As a physical therapist, this is a common question that I am asked, especially from people who are recovering from trauma and injury, but REALLY, it is a question that is relevant to EVERYONE.

Hi there!  I'm Dr. Kelly Sadauckas, a Board Certified Doctor of Physical Therapy, here to demystify your amazing body. Today, I am going to teach you about fueling your body, and help you learn how to decide how many calories you need to eat in a given day. 

Starting with the basics, let's define what a calorie is, and why this matters.

A calorie, scientifically, is the amount of energy needed to raise the temperature of one gram of water by one degree Celsius.  In the context of food, it represents the amount of energy your body can obtain from what you eat and drink (Osilla, 2022). 

Say what?!?  

Jacqueline Marcus simplifies this, in her book Culinary Nutrition: The Science & Practice of Healthy Eating, when she says that a calorie is a measure of the potential energy in any food or drink.  

I, Dr. Kelly, will simplify this further and explain a calorie as a description of how my fuel or energy a food has to provide your body with.  It's equivalent to putting fuel into your car.  You KNOW that YOUR CAR needs "X" number of gallons to run "Y" number of miles.  

So think of calories, not as “good” or “bad”, perse…but as the equivalent of that gasoline that you put into your car, to keep your car running.  You need the right amount and quality of gasoline for the task at hand, consistently, to keep the car running and in top form.

On "normal use" days or weeks, you need "a normal amount" of gasoline.  

On "big trips", you need "more fuel"...and it's the same with calories...we need a "normal number of calories" for ourselves on "normal days"....and "more calories" on days when we do "more things". 

Conversely, on "less active days"...ironically, on days when we might use more actual gasoline in the car...we probably need less physical calories...does this make sense?

A "junker-car" might get "junk-gas", whereas a "sports car" gets fancy gas...you get to decide the "quality of the fuel you put in "your car", ie', your body...but that's for a later post :) 

But the one way to guarantee less than optimal physical or mental health, is to drastically exceed, or miss, your caloric targets...but to meet, exceed, or miss those targets...we have to first KNOW what those targets are...hence this post.   

 Do I have your attention? Ready for more? Let’s go. 

The Problem with a “2000 calorie a day Guideline”

It turns out, 2000 calories a day, as a guideline, was selected by a committee from the FDA, because it was considered “a reasonable average” for humans…and it was an easy number to work with...but it was never actually rooted in any real science.

In reality, all humans are DRASTICALLY different in size, activity level and body composition as far as the size of our muscles and how much physical activity we get on a daily basis. 

My dear friend Kaitlyn, regularly rides something like 400 miles a week on her bike.  I’m lucky if I get 20…and some folks will never ride 20 miles in their entire lives…so the daily caloric needs of Kaitlyn versus myself versus the person who will never ride 20 miles in their entire life, versus a Sumo Wrestler, will all look drastically different.  

So how do I determine the number of calories I need in a day?

If you were my Physical Therapy Student, and you asked me that question, I would tell you, "to estimate daily caloric needs, you would calculate your Basal Metabolic Rate (BMR) using the Harris-Benedict equation OR the Mifflin-St. Jeor Equation, then adjust for activity level to determine your Total Daily Energy Expenditure (TDEE). Don't forget that factors like age, sex, weight, height, and activity level significantly influence these calculations." 

I would tell you that. 100% I would.  Aren't I a meany-pants :) 

But since you are NOT my PT student, I'll be nice and break it down for you.  Aren't you lucky.  

Let's start with Defining Basal Metabolic Rate, or BMR

Basal Metabolic Rate (BMR) is the number of calories your body needs to do nothing but survive.

To stick with our car analogy, because I love it, your BMR would be the gasoline cost of just idling your car in the driveway ALL DAY.  You'd still need a certain amount of gasoline to do that, right?  

From your body's perspective, imagine lying in bed all day, not moving, not even scrolling your phone (I know, tragic). Your body still has work to do behind the scenes—it still has to work to keep your heart beating, your lungs breathing, your brain thinking, and to keep your cells doing their job. All of that work takes energy.

That energy comes from food that you eat—and it’s measured in calories...and that is your Basal Metabolic Rate...cool, huh?!  

Even cooler...the MORE muscles you have...the MORE calories you'll burn at rest...JUST like the bigger the car you have, the more gasoline that car would burn at rest, idling in the driveway (don't kill me, environmentalists...I'm simply making an analogy, here!!). So a Gian MUSCLY-HUMAN will burn more calories at rest than a wispy one...so the former needs MORE CALORIES to exist than the latter.  

Do you want to figure out how many calories you need in a day?  Or do you want to hire a professional, who does this for a living?

 The Basal Metabolic Rate (BMR) and is calculated (most commonly) by something called the Harris Benedict Equation OR the Mifflin-St. Jeor Equation.

You can consult a personal trainer, or physio (who knows how to do this–remember not all physios learn this, and not everyone uses it daily)…so you want a fitness professional who does this all the time to help you.  If you're in Teton Valley, I love Lacey Vandebutte at https://www.wildfirecoach.com/. (she does my strength programming, and I can do pull ups because of her). Frankie at Barefoot Fitness & Pilates in Driggs, & Dayne & all her trainers at Targhee Athletics are all great (& SORRY IF I FORGOT YOU!!) & if you go to Lacey, tell her I sent you, so she goes easy on my next round of Bulgarian Split Squats :) Please....I can't do any more...I might actually be stuck at the bottom of this rep...

If you're not in Teton Valley, do Google Searches of YOUR area, search mom groups on FB, and The Gram.  Ask your friends who they work out with.  Ask an athlete WHO YOU RESPECT, who coaches them.  And when you meet with the coach or trainer, DON'T BE SHY about stating your personal goals for strength, functional fitness AND nutrition.  TAKE UP SPACE. MAKE SURE YOU TWO ARE A GOOD FIT FOR EACH OTHER.  

K...that being said...if you want to do it on your own, or at least have a basic understanding of how your coach or trainer is doing it, the two major formulas to figure out how many calories you are burning in a day are the Harris-Benedict Equation, and the Mifflin-St. Jeor Equation.  

🧮 Harris-Benedict Equation

Think of it as the OG calculator.
This formula was created way back in 1919 to estimate how many calories you need each day, based on your:

• Age
• Sex
• Height
• Weight
• Activity level 

BUT, because of its tendency to overestimate required calories required, I do tend to recommend Mifflin-St Jeor, so that's what I'll use for the demonstration purpose in this blog, AND IN THE SAMPLE CALCULATOR that I created for you later in this post (you're welcome, and yes, bookmark this and share it with friends and other PT students, because it's AWESOME!!   :). If you are a geek like me, though, and you have time...I do recommend playing around with both, for yourself or for your clients, and see which calculator you find to be more accurate.  Better yet, come up with your own, and you can be featured in my next blog!!   K-let's move on to the Mifflin-St Jeor Equation!!

🧮 Mifflin-St Jeor Equation

The Mifflin-St Jeor Equation  for determining BMR is*:

• For Men: 

  BMR = (10 × weight in kg) + (6.25 × height in cm) - (5 × age in years) + 5 
• For Women: 

  BMR = (10 × weight in kg) + (6.25 × height in cm) - (5 × age in years) - 161

   

  *Multiplied by a modifier for Activity Level for Total Caloric Expenditure
  

This is the newer, more updated version.
Created in the 1990s (see Mifflin, et al, 1990) and I dare you not to picture Steve Carrell and Scranton, PA when you read Mifflin), it uses the same factors (age, sex, height, weight), but is more accurate for modern bodies and lifestyles.

Studies show that Mifflin-St Jeor is more reliable for most people today—especially if you're not extremely lean or muscular. 

Let's Use the Mifflin-St Jeor to Calculate How Many Calories You Need in A Given Day!!

Psst:  Spoiler alert: There's a fillable form at the end of this blog post...but I want you to know HOW we came to the numbers...

Step 1: Grab your info. You’ll need:

• Your weight (in kilograms or pounds)
• Your height (in centimeters or inches)
• Your age (in years)
• Your sex (because the formula is slightly different for men and women because of hormones and muscles mass--YES, I know, very 1960s...)
• Your activity level (to adjust for how much you move)

✏️ Step 2: Plug into the equation

🔸 For women:

BMR = 10 × weight (kg) + 6.25 × height (cm) – 5 × age (years) – 161

🔸 For men:

BMR = 10 × weight (kg) + 6.25 × height (cm) – 5 × age (years) + 5

That gives you your Basal Metabolic Rate—the number of calories your body needs just to stay alive (even if you stayed in bed all day).

🏃‍♀️ Step 3: Multiply by your activity level to get Total Daily Caloric Need (Total Daily Energy Expenditure, as we call it in the BIZZ!!)

To estimate your Total Daily Calorie Needs, multiply your BMR by one of these numbers:

🧠 TL;DR Version:

(yes, my middle schoolers are my new social media and blog advisers–can you tell?? THey added the TL;DR Version, which stands for Too Long; Didn’t Read…apparently, this is what the “youngsters” will jump to for the 20-second summary…I’m not sure if I’m keeping it, but am giving them a 2-month trial to see if people LOVE it :)

1. Use the formula to find how many calories your body needs at rest.
2. Multiply that by your activity level to get how many you need in a real day.
3. That’s your daily calorie target for maintenance (not weight loss or gain). 

🔍 So What’s My Caloric Need?

If I am a 155# (70.31 kg) woman, who is 5’8” (172.72cm) tall and “moderatey active”, based on the Mifflin-St Jeor equation, here’s what you’d get:

• BMR (Basal Metabolic Rate): ~1,447 calories/day
  → That’s what your body burns just to stay alive (no activity at all).
• TDEE (Total Daily Energy Expenditure): ~2,242 calories/day
  → That’s your estimated daily need based on being moderately active (exercise 3–5 days/week).

So, to maintain your weight with your current activity level, you’d aim for around 2,240 calories/day.

🔍 Want To Calculate Yours?

Okey-Dokey!!  Now that you know HOW to get the numbers, you can just fill in the info into this handy-dandy calculator below, and the results will pop out!!  You're welcome :) 

✨ Now That You Understand Calories, Let’s Fuel With Intention!

Calculating calories doesn’t have to be confusing—or restrictive. It’s just data, and data is power. Now that you’ve got a handle on how to estimate your daily caloric needs, you are one step closer to fueling your body with intention, for the rest of your life...and that is empowering in a way that many of us, and our parents, didn't have access to, growing up.  To be honest, it might be empowering and educational in a way that many of your primary health providers have not had access to. And that's awesome.  It's not to bash on them, but to celebrate how far science and education has come.  

In upcoming posts, we’ll get into how to turn these caloric numbers into smart food choices, and how these caloric needs shift in pregnancy, postpartum, or if you're recovering from surgery, or going through perimenopause or menopause.  I'll probably recommend books and nutritionists that I personally love.  

The more you grow to understand food as fuel, power and even medicine, the less anxiety-producing it becomes.  

I am NOT a nutritionist.  But I am a Physical Therapist, and a working mom of two is is going through perimenopause herself, so I have a few boxes to speak from, and I am happy to share whatever insights from my appropriate lanes that might be helpful to you.  This post has been from my Doctor of Physical Therapy perspective.  

From my "working-mom" perspective, I think the biggest thing about nutrition and calories is that, it should never be about "cutting out", or seeing foods as "good or bad".  The more we make room for the good things, the less we have to worry about "eliminating the bad", because, we start to realize there isn't any "bad"...because we choose to only fill our body with things that fulfill and serve us.  It's kind of amazing, I think.  And your kids SEE YOU.  They hear what you say, but WAY more than that, they SEE YOUR CHOICES.  They SEE HOW YOU NOURISH your body, and look at your body.  If you won't honor yourself by treating your body kindly with enough calories, honor your children by treating your own body with enough calories, please :)   

Thanks for being here to learn about this.  I'm so excited to be here to teach!  Comment below (or better yet on my socials (IG @therealpelvicfloored because that's where I'm most active), with what you liked, what questions you have, and what other topics you'd like to see. And PLEASE, if this helped you, PLEASE share it with others, so we can share GOOD INFO.  I want to save the world, one pelvis at a time, and I need your help to do it. Thanks friend.  I'm glad you're here with me.  

XOXO

Dr. Kelly

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For Science!!!  

You Can, (& SHOULD), Look it Up, and share this information with others.  

Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990 Feb;51(2):241-7. doi: 10.1093/ajcn/51.2.241. PMID: 2305711.

Osilla EV, Safadi AO, Sharma S. Calories. [Updated 2022 Sep 12]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK499909/#

Pavlidou E, Papadopoulou SK, Seroglou K, Giaginis C. Revised Harris-Benedict Equation: New Human Resting Metabolic Rate Equation. Metabolites. 2023 Jan 28;13(2):189. doi: 10.3390/metabo13020189. PMID: 36837808; PMCID: PMC9967803.