BMR Calculator with Activity Level: How It Works

A BMR calculator with activity level estimates your Basal Metabolic Rate (BMR) and then multiplies it by an activity factor to give your Total Daily Energy Expenditure (TDEE) – the number of calories you burn per day. You enter your age, sex, height, weight, and typical activity level, and you get back your BMR and your maintenance calories.

Formula

The calculator uses the Mifflin-St Jeor equation to estimate BMR, then multiplies by an activity factor. For men: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) + 5. For women: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) − 161. The result is in kilocalories per day. Then TDEE = BMR × activity factor, where the factor ranges from 1.2 (sedentary) to 1.9 (very active).

Worked Example

Take a 30-year-old man, 175 cm tall, weighing 70 kg, with moderate activity (factor 1.55).

• BMR = 10×70 + 6.25×175 − 5×30 + 5 = 700 + 1093.75 − 150 + 5 = 1648.75 kcal/day
• TDEE = 1648.75 × 1.55 = 2555.56 kcal/day

So this person needs about 2,556 calories per day to maintain his weight.

What BMR and TDEE Mean

BMR is the energy your body burns at complete rest – breathing, circulating blood, and keeping organs working. It accounts for about 60–70% of total daily expenditure. TDEE adds physical activity and the energy used to digest food (thermic effect). Knowing your TDEE helps set calorie targets: eat below it to lose weight, above it to gain, or around it to maintain.

How to Choose Your Activity Level

Be honest – most people overestimate. Use these descriptions:

• Sedentary (1.2): desk job, no exercise
• Light (1.375): light exercise 1–3 days/week
• Moderate (1.55): moderate exercise 3–5 days/week
• Active (1.725): hard exercise 6–7 days/week
• Very Active (1.9): physical job + intense daily training

If you’re unsure, choose the lower option – you can adjust later based on real weight changes.

Common Mistakes and Limitations

The biggest error is mixing units – always use kilograms and centimeters. Also, the Mifflin-St Jeor equation is an estimate; individual variation can be ±10% due to muscle mass, genetics, and hormones. The activity factors are rough averages – a 30-minute walk is not the same as a 30-minute sprint. Use the result as a starting point, not a precise prescription.

Alternatives to Mifflin-St Jeor

Other equations include Harris-Benedict (older, tends to overestimate) and Katch-McArdle (uses lean body mass, more accurate for athletes but requires body fat measurement). For most people, Mifflin-St Jeor is considered the most reliable general formula. You can also use a wearable device, but those often overestimate calories burned.

Frequently Asked Questions

Is BMR the same as resting metabolic rate (RMR)?

They’re close but not identical. RMR is measured under less strict conditions and is slightly higher than BMR. For practical purposes, many calculators treat them as the same.

Should I eat exactly my TDEE to maintain weight?

Yes, but only as a starting point. Your actual maintenance may be 100–200 calories higher or lower. Track your weight for 2–4 weeks and adjust by 100–200 calories if needed.

Does age affect BMR?

Yes – the formula subtracts 5 per year for both sexes. BMR naturally declines with age due to muscle loss and hormonal changes, which is why strength training is recommended as you get older.

Can I use this calculator if I’m pregnant or breastfeeding?

No – the formula doesn’t account for the extra energy needs. Pregnant women typically need an additional 300–500 calories per day in the second and third trimesters, and breastfeeding adds about 500. Consult a healthcare provider.

Why does my fitness tracker give a different number?

Trackers use algorithms that combine your stats with heart rate and movement data. They can be off by 20–30%. The calculator gives a consistent baseline; the tracker gives a real-time estimate that varies with your activity.

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in age | Age | years | 30 | 15
in sex | Sex | Male=5, Female=-161 | 5 | 0
in weight | Weight | kg | 70 | 30
in height | Height | cm | 175 | 120
sel activity | Activity Level | Sedentary=1.2, Light=1.375, Moderate=1.55, Active=1.725, Very Active=1.9 | 1.55 | 0
out bmr | BMR | kcal/day | 10 * weight + 6.25 * height - 5 * age + sex | 0
out tdee | TDEE | kcal/day | bmr * activity | 0