Metabolic Age Calculator

Plus Nutritionist’s Tips to Lower Your Metabolic Age

By: Jeremy Fox, CNC, CPTPublished: April 22, 2021

Have you ever wondered how your metabolic rate compares with other people your size and age? If so, then the number you’re looking for is called metabolic age.

With this metabolic age calculator, you can see how old you are based on your metabolism. And find out if your metabolism is faster or slower than average.

metabolic age calculator

What Is Metabolic Age?

Metabolic age is how your basal metabolic rate (BMR) compares to other people in your biological age group. BMR is simply the number of calories your body burns at rest. Also called resting metabolic rate (RMR).

A lower BMR means you burn fewer calories and have a higher metabolic age. While a higher BMR means you burn more calories and have a lower metabolic age. 

Keep in mind, metabolic age is not necessarily correlated with good health or life expectancy. But it can be an indicator that you need to make adjustments for a healthier lifestyle.

How Metabolism Changes With Age

One of the most common ways to calculate BMR is a formula that uses your height, weight, and age. This is called the Harris-Benedict equation.

It’s based on the fact that, in general, your metabolism slows down as you get older. In the charts below, you can see how metabolism gradually declines at a steady rate once you pass the age of 20.

Metabolic Age Calculator - BMR By Age Men
Metabolic Age Calculator - BMR By Age Women

Using this trend, the equation can estimate your BMR according to your age and general body size. However, since it assumes everyone has a similar amount of muscle and fat, it’s not always accurate.

How Metabolism Changes With Body Composition

In reality, body composition varies greatly even among people of the same height, weight, and age. That’s why there’s a second formula for calculating BMR using lean body mass. It’s called the Katch-Mcardle equation.

This formula is based on the fact that the more muscle (lean mass) you have, the higher your metabolic rate. In fact, for every 10 lbs of muscle, your body burns about 100 more calories per day.

Metabolic Age Calculator - BMR By Lean Mass

How Body Composition Affects Metabolic Age

As an example, let’s say two women are both 5’5″, 150 lbs, and 40 years old. The Harris-Benedict equation says the average BMR for these women 1,416 calories. But one of them has 40% body fat and the other has 20% body fat. 

Using the Katch-Mcardle formula, the woman with 40% body fat has a BMR of about 1,254 calories. Whereas the woman with 20% body fat has a BMR of 1,548. That makes their metabolic ages 78 and 18 respectively.

Since the first woman has more body fat and less lean mass, her metabolic age is older than her actual age. While the second woman has less body fat and more lean mass, so her metabolic age is younger than her actual age.

In the image below, it’s easy to see how metabolic age changes with weight and body composition.

Metabolic Age Calculator For Men and Women

Another way to visualize the effect of body composition on metabolism is by plotting body weight and body fat percentage. At the intersection of these two numbers is the resulting metabolic age.

With this data, you can see how metabolic age changes with body composition for a person of a given height. Again, more muscle and less fat give you a healthy metabolic age.

Metabolic Age Chart Men and Women

These charts also show that your metabolic age isn’t fixed like your chronological age. By changing your body composition, you can actually lower your metabolic age!

How To Lower Metabolic Age

In order to lower metabolic age, you need to decrease body fat, increase lean mass, or both. If you’re over 24% body fat for women or 18% body fat for men, you should usually start with fat loss.

Decrease Body Fat

At this point, you may be expecting a list of fat loss supplements you should take to lower your metabolic age. Well, I hate to break it to you, but the best way to burn fat is good old-fashioned diet and exercise.

I’m going to share some best practices to take your nutrition and workouts to the next level.

1. Do Cardio

One of the best ways to burn body fat is by increasing the number of calories you burn. And cardiovascular exercise burns a high percentage of calories from stored body fat.

High-intensity interval training (HIIT) is popular due to the fact that it burns more calories in less time. But low intensity steady-state (LISS) actually burns more calories from fat per minute.

How To Lower Metabolic Age - Cardio

Cardio is a good way to burn more calories. But you still have to ensure you eat fewer calories than you burn. This is called a calorie deficit.

2. Create A Calorie Deficit

In order to create an appropriate calorie deficit, you need to know your BMR as well as the number of calories you burn through your daily activity and exercise. This is called your total daily energy expenditure (TDEE).

To lose fat at a sustainable rate, you should target a calorie intake less than your TDEE but more than your BMR. This decreases the likelihood that your metabolism will slow down in response to calorie restriction.

3. Reduce Carb Intake

Another way to burn fat is to reduce the number of carbohydrates in your diet. By eating fewer carbs, your body shifts to using more stored body fat for energy.

A good target for fat loss is to get anywhere from 5 to 30% of your calories from carbs depending on your activity level. The more active you are the more carbs you can have.

Adjusting your carb intake to your activity level is called carb cycling.

How To Lower Metabolic Age - Carb Cycling

4. Eat Healthy Foods

Of course, you should also get most of your carbs and calories from nutritious whole foods. This improves your overall health while you reduce body fat and lower your metabolic age.

Increase Lean Mass

In addition to decreasing body fat, you can also lower your metabolic age by increasing lean mass. Remember, the more muscle you have, the faster your metabolism and the lower your metabolic age.

5. Lift Weights

The best way to add lean muscle is by incorporating resistance training workouts into your exercise plan. Lifting weights gives your body a reason to build muscles bigger and stronger.

How To Lower Metabolic Age - Lean Bulk

6. Eat Enough Calories

In order to build muscle after your workouts, you need to eat at least as many calories as you burn. Or even be in a slight calorie surplus. That extra energy is what your body uses to create new muscle.

Of course, you can’t be in a deficit and a surplus at the same time. This is why it’s a good idea to focus on fat loss and muscle gain separately.

Once you lose the fat, gradually bump up your calories to gain muscle while maintaining lower body fat. This is called a lean bulk.

7. Increase Protein Intake

Many studies have shown that a higher protein intake results in greater lean muscle gains1. That’s because protein provides the building blocks your body needs to build new muscle.

The best way to calculate your protein intake is to target about 30% of your calories from protein.

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How To Calculate Metabolic Age

If you still haven’t found your metabolic age, the easiest way to do it is by using my free metabolic age calculator. But if you’re curious about how to calculate metabolic age, I’ll show you the math.

The calculation for metabolic age involves a combination of both the Harris-Benedict formula and the Katch-Mcardle formula.

Harris-Benedict Formula

Men: BMR = 88.362 + ( 13.397 * Weight in kg ) + ( 4.799 * Height in cm ) – ( 5.677 * Age )

Women: BMR = 447.593 + ( 9.247 * Weight in kg ) + ( 3.098 * Height in cm ) – ( 4.33 * Age )

Because these equations don’t account for body composition, they are only accurate if you have “normal” body fat and muscle mass for your weight and age. For this reason, we’ll call this your “predicted BMR”.

Katch-Mcardle Formula

Men & Women: BMR = 370 + ( 21.6 * Lean Mass in kg )

Most of the time, the Katch-Mcardle equation is more accurate since it takes your body composition into account. We’ll call this your “actual BMR”.

So if we plug your actual BMR back into the Harris-Benedict equation and solve for “Age” the result is your true metabolic age. With some simple algebra, the resulting equations for metabolic age look like this:

Metabolic Age Formula

Men: Metabolic Age = ( 88.362 + ( 13.397 * Weight in kg ) + ( 4.799 * Height in cm ) – Actual BMR ) / 5.677

Women: Metabolic Age = ( 447.593 + ( 9.247 * Weight in kg ) + ( 3.098 * Height in cm ) – Actual BMR ) / 4.33

Basically, this new equation compares the results of your actual BMR to your predicted BMR. The further you get from “normal” body composition, the bigger the difference between your actual age and metabolic age.

With my metabolic age calculator, I perform these calculations in the background including converting from pounds and inches to kilograms and centimeters. So all you have to do is enter your information to find your metabolic age.


When all is said and done, a high or low metabolic age doesn’t have much to do with your overall health or life expectancy. So it’s really just a tool to visualize the effect of body composition on your metabolism.

That being said, the metabolic age calculator shows how inaccurate the Harris-Benedict equation can be for finding your BMR and your ideal calorie intake. And this is the reason I use lean mass when creating your personalized meal plan.

One key takeaway is that you should always account for body composition when calculating your daily caloric needs. Especially when your lean mass or body fat is higher or lower than average.

If you found this calculator useful, make sure you check out some of my other great tools and content below!

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