Metabolic age is an estimate of how old your body “acts” based on how many calories it burns at rest, compared to average values for different age groups. There’s no single standardized formula for it. Instead, the concept works by calculating your basal metabolic rate (BMR), then matching that number against population averages to find which age group your metabolism most closely resembles. If your BMR matches the average for someone younger than you, your metabolic age is lower than your actual age, and vice versa.
What Metabolic Age Actually Measures
Your BMR is the number of calories your body needs just to keep you alive: breathing, circulating blood, maintaining body temperature, and running basic cellular processes. It accounts for roughly 60 to 75 percent of the calories you burn each day. BMR naturally declines with age, mostly because people tend to lose muscle mass and gain fat as they get older. Muscle tissue burns more calories at rest than fat tissue does, so body composition is a major driver of your metabolic rate.
Metabolic age takes that BMR number and reframes it. Instead of telling you “your body burns 1,450 calories at rest,” it tells you “your resting calorie burn is typical of a 38-year-old.” The idea is to give you an intuitive snapshot of your metabolic health relative to your chronological age.
The Two Main BMR Equations
Since metabolic age starts with estimating your BMR, the equation you use matters. Two formulas dominate.
The Harris-Benedict equation is the most widely cited:
- Males: BMR = 88.362 + (13.397 × weight in kg) + (4.799 × height in cm) − (5.677 × age in years)
- Females: BMR = 447.593 + (9.247 × weight in kg) + (3.098 × height in cm) − (4.330 × age in years)
The Mifflin-St Jeor equation is considered more accurate for overweight and obese individuals, according to the Academy of Nutrition and Dietetics:
- Men: BMR = (9.99 × weight in kg) + (6.25 × height in cm) − (4.92 × age) + 5
- Women: BMR = (9.99 × weight in kg) + (6.25 × height in cm) − (4.92 × age) − 161
Both equations use weight in kilograms and height in centimeters. To convert, divide your weight in pounds by 2.205 and multiply your height in inches by 2.54.
How to Estimate Your Metabolic Age Step by Step
Here’s the practical process. Pick one of the equations above and plug in your actual numbers. Let’s say you’re a 45-year-old woman who weighs 68 kg (about 150 pounds) and stands 165 cm (about 5’5″). Using Mifflin-St Jeor, your BMR would be: (9.99 × 68) + (6.25 × 165) − (4.92 × 45) − 161 = roughly 1,350 calories per day.
Now comes the comparison step. You’d recalculate the same equation but swap in different ages while keeping your height and weight constant. If your BMR of 1,350 matches what the formula predicts for, say, a 40-year-old with your same height and weight, your metabolic age would be approximately 40. That five-year gap below your actual age suggests your metabolism is performing better than average for your age group.
The math works in the other direction too. A higher body weight or lower muscle-to-fat ratio pushes BMR down relative to what’s expected, which can produce a metabolic age that’s older than your real age.
What Smart Scales Do Differently
Many consumer body composition scales display a metabolic age reading, and they use a different approach than the pen-and-paper equations above. These devices rely on bioelectrical impedance analysis, which sends a small electrical current through your body to estimate how much of your weight is fat, muscle, water, and bone. The scale then factors in your body mass index, these composition readings, and other metabolic variables to generate the metabolic age number.
This method has one advantage over the standard BMR equations: it accounts for body composition directly, rather than inferring it from weight alone. Two people with identical height, weight, and age can have very different ratios of muscle to fat, and the scale picks up on that difference. The equations above cannot.
That said, bioelectrical impedance readings fluctuate based on hydration, recent meals, and even the time of day. If you use a smart scale, measure yourself under consistent conditions (same time of morning, before eating) to get comparable readings over time.
How Reliable Is Metabolic Age?
Metabolic age is a useful motivational tool, but it’s not a clinical measurement. No standardized definition exists across the medical field, and different devices and formulas can produce different results for the same person. Research published in The Lancet’s eBioMedicine has noted that current metabolic age models suffer from limited prediction accuracy, partly because they capture only a narrow slice of the complex factors that drive aging.
The concept works best as a general directional signal. A metabolic age consistently higher than your actual age suggests your body composition, fitness level, or both could use attention. But treating the number as a precise diagnostic marker overstates what it can tell you. Think of it the way you’d think of BMI: a rough screening tool, not a definitive verdict on your health.
How to Lower Your Metabolic Age
Because metabolic age is driven primarily by your BMR, and BMR is driven primarily by how much muscle you carry, the most effective strategy is building or preserving lean muscle mass. A meta-analysis in Medicine & Science in Sports & Exercise reviewed 49 studies of men ages 50 to 83 who did progressive resistance training and found subjects gained an average of 2.4 pounds of lean body mass. That added muscle raises resting calorie burn, which in turn pushes metabolic age downward.
A practical resistance training program looks like 8 to 10 exercises targeting all major muscle groups, performed for sets of 12 to 15 reps at moderate effort, two or three times per week. You don’t need to train to exhaustion. Aiming for about a 5 to 7 on a 10-point difficulty scale is enough to stimulate muscle growth.
Protein intake matters too. For adults who resistance train, research suggests aiming for 1 to 1.3 grams of protein per kilogram of body weight each day. For a 155-pound person, that’s roughly 70 to 90 grams of protein daily. Timing also plays a role: consuming a meal or shake with a carbohydrate-to-protein ratio of about three-to-one or four-to-one within 30 minutes after a workout can improve muscle recovery and growth.
Beyond strength training, reducing excess body fat through any sustainable combination of diet and activity will also shift the number. Carrying less fat and more muscle at the same body weight produces a meaningfully higher BMR, which is the core input that determines where your metabolic age lands.