Metabolic age, explained
Metabolic age is an estimate of how old your metabolism behaves compared with your calendar age. There is no single official formula. Many tools simply compare your resting energy use (BMR) with an average for your age. This calculator works it out from markers of metabolic health instead: your waist-to-height ratio, your BMI and how active you are. A metabolic age at or below your real age is the better result.
BMR equation: Mifflin et al., Am J Clin Nutr 1990. Waist-to-height ratio: Ashwell & Gibson, BMJ Open 2016.
How to calculate your metabolic age
Enter your sex, age, height, weight, waist and activity level. The calculator then does two separate things:
- It estimates your BMR (basal metabolic rate, the energy you burn at rest) with the Mifflin-St Jeor equation: 10 × weight in kg + 6.25 × height in cm − 5 × age, then +5 for men or −161 for women. A systematic review found this equation predicted resting metabolic rate within 10% of the measured value in more people than the other common equations (Frankenfield et al., 2005).
- It sets your metabolic age by starting at your calendar age and adding or removing the years in the table below.
Why not use BMR alone? A larger body burns more energy at rest, so a BMR-only method can make a heavier, less healthy body look metabolically “younger”. Waist size and activity track metabolic risk much better, so they drive the result here.
| Input | Your answer → years |
|---|---|
| Waist-to-height ratio | (ratio − 0.5) × 50 · so 0.40 → −5 · 0.45 → −2.5 · 0.50 → 0 · 0.55 → +2.5 · 0.60 → +5 · 0.65 → +7.5 |
| BMI | Under 18.5: +1 · 18.5–24.9: 0 · 25–29.9: +1.5 · 30 or more: +3 |
| Physical activity | Sedentary +2.5 · Some activity 0 · Active, 30 min most days −2.5 · Very active −4 |
Worked example. A 45-year-old man, 175 cm tall, 78 kg, with an 88 cm waist and some activity: waist-to-height ratio 0.50 (about 0 years), BMI 25.5 (+1.5 years), activity 0. His metabolic age is 47, two years older than his calendar age. His BMR is about 1,654 kcal a day.
What is a good metabolic age?
A metabolic age the same as or lower than your real age is a good result. The calculator reads your result like this:
| Your result vs calendar age | What it suggests |
|---|---|
| 2 or more years younger | A lean waist for your height and an active routine. Your metabolic markers are in the lower-risk range. |
| Up to 1 year younger, or the same | About what is expected for your age. |
| 1–4 years older | Usually a waist at or above half your height, a BMI over 25 or little activity. Small changes can bring it back. |
| More than 4 years older | Several markers are in the higher-risk range. Waist size and activity are the quickest levers. |
The single most useful number on this page is your waist-to-height ratio. NICE, the health guidance body for England, advises adults to keep their waist to less than half their height. In an analysis of English health survey data, a ratio of 0.5 or more flagged early cardiometabolic risk better than BMI did (Ashwell & Gibson, 2016).
| Your height | Keep your waist under (0.5) | High-risk waist (0.6) |
|---|---|---|
| 155 cm (5 ft 1 in) | 78 cm (30.5 in) | 93 cm (36.6 in) |
| 160 cm (5 ft 3 in) | 80 cm (31.5 in) | 96 cm (37.8 in) |
| 165 cm (5 ft 5 in) | 83 cm (32.5 in) | 99 cm (39.0 in) |
| 170 cm (5 ft 7 in) | 85 cm (33.5 in) | 102 cm (40.2 in) |
| 175 cm (5 ft 9 in) | 88 cm (34.4 in) | 105 cm (41.3 in) |
| 180 cm (5 ft 11 in) | 90 cm (35.4 in) | 108 cm (42.5 in) |
| 185 cm (6 ft 1 in) | 93 cm (36.4 in) | 111 cm (43.7 in) |
| 190 cm (6 ft 3 in) | 95 cm (37.4 in) | 114 cm (44.9 in) |
Measure your waist halfway between your lowest rib and the top of your hip bone, after breathing out normally.
Body shape index (ABSI): is your waist large for your size?
The ABSI card under your result uses the same height, weight and waist, but asks a different question from the waist-to-height ratio: is your waist larger or smaller than usual for someone of your height and BMI? A body shape index was built so that it is almost unrelated to BMI (correlation 0.019 in the original study). It picks up where fat sits, which BMI can’t.
- ABSI = waist ÷ (BMI2/3 × height1/2), with waist and height in metres. The raw values are small (around 0.08), so the card shows ABSI × 1000.
- z-score = (your ABSI − the average for your age and sex) ÷ the standard deviation. The averages come from about 14,000 US adults in the National Health and Nutrition Examination Survey (NHANES) 1999–2004. Ages 85 and over share one row.
- Band: your z-score is placed in one of five equal-sized groups (fifths) of that population, and the card shows how death rates in your fifth compared with the middle fifth.
| ABSI z-score | Hazard ratio (95% CI) |
|---|---|
| Below −0.868 (lowest fifth) | 0.97 (0.69–1.37), not significantly different |
| −0.868 to −0.272 | 0.93 (0.64–1.35), not significantly different |
| −0.272 to +0.229 (middle fifth) | 1 (reference) |
| +0.229 to +0.798 | 1.46 (1.08–1.99) |
| Above +0.798 (top fifth) | 1.93 (1.39–2.68) |
Worked example. The default 45-year-old man (175 cm, 78 kg, 88 cm waist) has a BMI of 25.5 and an ABSI of 76.8. The average for his age is 81.65 with a standard deviation of 3.68, so his z-score is −1.31: the lowest fifth. With the same height and weight but a 98 cm waist, his BMI is still 25.5, yet his ABSI is 85.6 and his z-score +1.07: the top fifth. For his height and weight, an average shape is a waist of about 93.5 cm. A 70-year-old man of 180 cm and 95 kg with a 110 cm waist has a BMI of 29.3 but a z-score of +0.09, in the middle fifth.
How well does it predict? In the original study (14,105 adults, 828 deaths over about 5 years), each 1 standard deviation of ABSI z-score went with 1.33 times the death rate (95% CI 1.20–1.48), and 1.30 after allowing for smoking, diabetes, blood pressure and cholesterol. In about 353,000 European adults followed for 16 years (EPIC), the top quarter of ABSI held 18–39% of people in every BMI group and had a 22–55% higher risk of death than the lower groups (Christakoudi 2020). A meta-analysis of 9 cohorts found 1.15 times the death rate for every 0.005 increase in ABSI (Jayedi 2020).
Limits. ABSI shows a link with death rates in large groups; it doesn’t prove that waist size causes deaths, and it can’t forecast your own lifespan. The reference table is from US adults in 1999–2004. In the original study it did not significantly predict death in Mexican participants (1.11, 0.95–1.29), and the follow-up was short. A below-average ABSI was not linked to significantly lower death rates. NHANES measured the waist just above the top of the hip bone after a normal breath out, so a waist measured higher up will give a slightly different ABSI. Keep using the waist-to-height ratio as the simple screening rule, and ask your doctor about your own risk.
How to lower your metabolic age
- Bring your waist under half your height. On this calculator every 0.01 drop in your ratio (about 1.75 cm of waist at 175 cm tall) takes half a year off.
- Move more. Going from sedentary to active (30 minutes most days) moves the result by 5 years here. Adults should aim for at least 150 minutes of moderate activity a week plus muscle-strengthening on 2 days (CDC).
- Keep muscle. Strength training helps protect muscle as you age, and muscle is a large part of the energy you burn at rest. Our muscle-loss test checks your risk.
- Check the wider picture. Metabolic health also shows up in blood pressure and cholesterol, which our heart age calculator uses, and in your overall biological age.
Metabolic age FAQ
What is metabolic age?
Metabolic age is an estimate of how old your metabolism behaves compared with your calendar age. It is not a medical measurement. This calculator estimates it from your waist-to-height ratio, BMI and activity level.
What is a good metabolic age?
A metabolic age the same as or lower than your real age. On this calculator, a result 2 or more years younger usually means a waist under half your height and regular activity.
How is metabolic age calculated?
There is no single standard formula. Many tools compare your BMR with an average for your age. This one starts at your calendar age and adds or removes years for waist-to-height ratio, BMI and activity, and shows your BMR from the Mifflin-St Jeor equation separately.
Is metabolic age the same as biological age?
No. Metabolic age looks only at body shape, weight and activity. Biological age is a broader estimate that also covers smoking, sleep, diet, alcohol and stress. You can check both on this site.
Will my metabolic age go down if I lose weight?
Usually, yes, if the weight comes off your waist. A smaller waist lowers your waist-to-height ratio, which has the biggest effect on this calculator. Staying active while you lose weight also helps.
Are smart-scale metabolic ages accurate?
Treat them as rough guides. Scales use their own formulas, often based on body-fat estimates from a small electric current, and different brands give different numbers. Track the trend on one device rather than comparing devices.
How much does obesity shorten life expectancy?
On average by a few years, and more at higher BMI. In a Lancet analysis of about 900,000 adults in 57 studies, median survival was 2–4 years shorter at a BMI of 30–35 and 8–10 years shorter at 40–45 than in the lowest-risk range of about 22.5–25 (Prospective Studies Collaboration, 2009). A later analysis of 3.95 million never-smokers without chronic disease found death rates about 1.45 times as high at BMI 30–35, 1.94 times at 35–40 and 2.76 times at 40 or more, compared with BMI 22.5–25 (Global BMI Mortality Collaboration, 2016). These are population averages, not a forecast for you. BMI also can’t show where fat sits, which is why this calculator uses your waist too. Ask your doctor about your own risk.
What is a body shape index (ABSI)?
ABSI is your waist size adjusted for your height and BMI, so it shows whether your waist is large or small for your size. It was developed from US national survey data (NHANES 1999–2004, 14,105 adults). In that study, people in the top fifth of ABSI had about 1.9 times the death rate of the middle fifth over about 5 years (Krakauer & Krakauer, 2012). This calculator shows your ABSI, your z-score for your age and sex, and which fifth you fall in. It is an educational estimate, not medical advice.
Is ABSI better than BMI or waist-to-height ratio?
It answers a different question. BMI measures weight for height, and waist-to-height ratio is a simple screening rule (keep your waist under half your height). ABSI asks whether your waist is large for your height and weight, so two people with the same BMI can get very different results. In about 353,000 European adults, ABSI split people into higher and lower risk within every BMI group (Christakoudi 2020). A meta-analysis found too few ABSI studies to call it the single best measure (Jayedi 2020), so use it alongside the waist-to-height ratio, not instead of it.
Last reviewed: · Sources · Educational estimate, not medical advice.
Sources
- Mifflin MD, St Jeor ST, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr, 1990;51:241–247.
- Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. J Am Diet Assoc, 2005.
- Ashwell M, Gibson S. Waist-to-height ratio as an indicator of early health risk. BMJ Open, 2016.
- NICE. Keep the size of your waist to less than half of your height, NICE recommends, 2022.
- CDC. Adult Activity: An Overview.
- Prospective Studies Collaboration. Body-mass index and cause-specific mortality in 900 000 adults: collaborative analyses of 57 prospective studies. Lancet, 2009;373:1083–1096.
- Global BMI Mortality Collaboration. Body-mass index and all-cause mortality: individual-participant-data meta-analysis of 239 prospective studies in four continents. Lancet, 2016;388:776–786.
- Krakauer NY, Krakauer JC. A new body shape index predicts mortality hazard independently of body mass index. PLoS One, 2012;7(7):e39504. Reference table: Table S1 (CC BY 4.0).
- Christakoudi S, et al. A Body Shape Index (ABSI) achieves better mortality risk stratification than alternative indices of abdominal obesity: results from a large European cohort. Sci Rep, 2020;10:14541.
- Jayedi A, et al. Central fatness and risk of all cause mortality: systematic review and dose-response meta-analysis of 72 prospective cohort studies. BMJ, 2020;370:m3324.