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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.
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.
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 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.