Written and maintained by Christin Bowles, founder of TidyCalcs.
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Your Basal Metabolic Rate (BMR)
At complete rest, your body burns - calories per day just to maintain basic functions.
What is TDEE?
Total Daily Energy Expenditure is the total calories your body burns in a day, including exercise and daily activities.
What is BMR?
Basal Metabolic Rate is the calories your body needs at complete rest — just to breathe, circulate blood, and stay alive.
Losing weight
A deficit of 500 calories per day typically leads to about 1 lb (0.45 kg) of weight loss per week — a healthy, sustainable rate.
Building muscle
A surplus of 250-500 calories per day combined with strength training helps build lean muscle mass over time.
Already know your calorie target?
Use the Macro Calculator to split it into protein, carb, and fat grams for your diet style.
The two-step math behind your number — BMR and TDEE
Your daily calorie number isn't one calculation — it's two stacked together. First, your body calculates BMR (Basal Metabolic Rate): the calories you'd burn lying in bed all day doing nothing but breathing and keeping your heart beating. Then that number gets multiplied by an activity factor to get TDEE (Total Daily Energy Expenditure) — what you actually burn living your real life.
This calculator uses the Mifflin-St Jeor equation, published in 1990 and validated as the most accurate BMR formula for the general population in multiple clinical studies since:
Women: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) − 161
For example, a 35-year-old woman who is 165cm (5'5") and weighs 68kg (150 lbs) has a BMR of roughly 1,401 calories. If she's moderately active, her TDEE multiplier of 1.55 brings her maintenance calories to approximately 2,172 per day.
How activity level changes everything
The biggest variable in your final number isn't your weight or height — it's how honestly you assess your activity level. Here's the same person's calorie needs across every activity tier:
| Activity level | Multiplier | TDEE |
|---|---|---|
| Sedentary (little/no exercise) | 1.2 | 1,681 cal |
| Lightly active (1-3 days/week) | 1.375 | 1,926 cal |
| Moderately active (3-5 days/week) | 1.55 | 2,172 cal |
| Very active (6-7 days/week) | 1.725 | 2,417 cal |
| Extra active (physical job + training) | 1.9 | 2,662 cal |
Most people overestimate their activity level by one full tier. Someone who exercises 3 days a week but sits at a desk the other 11 waking hours is usually "lightly active," not "moderately active." Overestimating by one tier can add 200-250 calories to your daily target — enough to stall weight loss completely while believing you're in a deficit.
How many calories a day for women and men, by age
If you just want a ballpark before using the calculator, these are the daily calorie estimates from the U.S. Dietary Guidelines for Americans. They assume an average height and a healthy weight, so your own number from the calculator above will be more precise.
| Age | Sedentary | Moderately active | Active |
|---|---|---|---|
| 19-30 | 1,800-2,000 | 2,000-2,200 | 2,400 |
| 31-50 | 1,800 | 2,000 | 2,200 |
| 51-60 | 1,600 | 1,800 | 2,200 |
| 61+ | 1,600 | 1,800 | 2,000 |
| Age | Sedentary | Moderately active | Active |
|---|---|---|---|
| 19-30 | 2,400-2,600 | 2,600-2,800 | 3,000 |
| 31-50 | 2,200-2,400 | 2,400-2,600 | 2,800-3,000 |
| 51-60 | 2,200 | 2,400 | 2,800 |
| 61+ | 2,000 | 2,200 | 2,400-2,600 |
To lose about 1 pound a week, subtract roughly 500 calories from the number that fits you. For example, a moderately active woman aged 31-50 would aim for about 1,500 calories a day, and a moderately active man the same age for about 1,900-2,100.
Without a doctor's supervision, the NIH suggests women generally shouldn't eat fewer than about 1,200 calories a day and men fewer than about 1,500. If subtracting 500 puts you below that, aim for a smaller deficit and a slower pace.
Why a 500-calorie deficit became the standard
One pound of body fat contains approximately 3,500 calories of stored energy. Divide that across 7 days and you get exactly 500 calories per day — the deficit needed to lose roughly one pound per week. This is where the "500 calorie rule" comes from, and it has held up well in nutrition research for decades.
| Daily deficit | Weekly loss | Sustainability |
|---|---|---|
| 250 calories | ~0.5 lb/week | Very sustainable, slow |
| 500 calories | ~1 lb/week | Standard recommendation |
| 750 calories | ~1.5 lb/week | Requires more discipline |
| 1,000 calories | ~2 lb/week | Upper safe limit for most |
Deficits beyond 1,000 calories per day are generally not recommended without medical supervision — they often lead to muscle loss, metabolic slowdown, and are difficult to sustain long enough to matter.
Where this calculator can be wrong — and why
The Mifflin-St Jeor equation is accurate within about 10% for most people, but several factors can shift your real number:
Muscle mass
Muscle burns more calories than fat at rest. Two people with identical height and weight but different body composition can have meaningfully different real BMR.
Metabolic adaptation
Extended dieting can lower your metabolism below what the formula predicts, a phenomenon sometimes called "starvation mode" or adaptive thermogenesis.
Hormonal factors
Thyroid conditions, menopause, and certain medications can shift energy expenditure by 5-15% in either direction.
NEAT variability
Non-Exercise Activity Thermogenesis — fidgeting, walking, standing — varies hugely between individuals and isn't fully captured by activity level alone.
The practical fix: use this calculator as a starting point, track your actual weight change over 2-3 weeks, and adjust your calorie target up or down by 100-150 calories based on real results rather than the formula alone.
Common questions
How many calories should I eat per day?
It depends on your age, sex, weight, height, and activity level. Most adults need between 1,600 and 3,000 calories per day. Use the calculator above to get your personalized number.
How many calories should a woman eat a day?
Most adult women need about 1,600 to 2,400 calories a day, depending on age and activity. A moderately active woman aged 31-50 needs about 2,000. To lose about 1 pound a week, most women would aim for around 1,500, and generally not below 1,200 without medical supervision.
How many calories should a man eat a day?
Most adult men need about 2,000 to 3,000 calories a day, depending on age and activity. A moderately active man aged 31-50 needs about 2,400-2,600. To lose about 1 pound a week, most men would aim for roughly 1,900-2,100, and generally not below 1,500 without medical supervision.
How many calories do I need to lose weight?
To lose weight, you need to eat fewer calories than you burn. A deficit of 500 calories per day leads to roughly 1 pound of weight loss per week. Our calculator shows your personalized weight loss calorie target.
What is the Mifflin-St Jeor equation?
This calculator uses the Mifflin-St Jeor equation, which is considered the most accurate formula for estimating BMR. It factors in your age, sex, weight, and height, then multiplies by your activity level to get your TDEE.
How many calories should I eat to gain muscle?
To gain muscle, eat 250-500 calories above your maintenance level and combine with strength training. Our calculator shows your gain target automatically.
How accurate is this calorie calculator?
This calculator uses the Mifflin-St Jeor equation, which research shows is accurate within 10% for most people. Individual results may vary based on genetics, hormones, and other factors. Consult a dietitian for a personalized plan.
Is eating below my BMR safe?
Eating below your BMR, not your TDEE, for extended periods isn't recommended. It can slow your metabolism, cause muscle loss, and is hard to sustain. The deficit target above already accounts for this by cutting from your TDEE rather than your BMR.
Should I eat back the calories I burn from exercise?
If your activity level above already reflects your normal exercise routine, no, it's already factored in. Only add back calories for exercise well beyond your selected activity level, and even then, add back conservatively since most trackers overestimate calories burned.
Why does this calculator give a different number than my fitness tracker?
Fitness trackers estimate calories burned using heart rate and movement, which can vary widely in accuracy, while this calculator uses the validated Mifflin-St Jeor equation based on your body stats. Treat both as estimates, and use your actual weight change over 2-3 weeks to fine-tune your number.
Sources and further reading
For independent, authoritative information on this topic, these are good places to start:
- Dietary Guidelines for Americans — the federal government's nutrition recommendations.
- National Institutes of Health (NIDDK): Body Weight Planner — a research-based tool for planning calorie targets over time.
- USDA National Agricultural Library: DRI Calculator — estimates daily nutrient needs from the Dietary Reference Intakes.
Results are estimates for general information and are not medical advice. A doctor or registered dietitian can give guidance specific to you.