How many calories should I eat?
Key points
- Your daily energy need (TDEE) is your resting burn (BMR) plus movement and the energy used to digest food.
- The Mifflin-St Jeor equation is the most commonly recommended way to estimate BMR, but it is an average, not a measurement of you.
- Activity multipliers are rough conventions. Pick the lower one if you are unsure.
- Treat the result as a starting point for two to three weeks, then adjust using your weight trend.
Most calorie targets begin with a formula. That is a reasonable place to start, as long as you know what the formula can and cannot tell you. This article explains where the number comes from, works through two examples, and shows why even a good estimate drifts over time.
Step 1: BMR, your resting burn
Basal metabolic rate (BMR) is the energy your body uses at rest to keep you alive: breathing, circulation, temperature, and running your organs. In most adults it is the largest single part of daily energy use. Strictly, BMR is measured under tightly controlled conditions; the related figure measured in less strict conditions is called resting metabolic rate (RMR). Calculators use the two terms loosely, and so will we.
In 1990, Mifflin and colleagues published an equation fitted to measurements in healthy adults. A 2005 systematic review by the Academy of Nutrition and Dietetics team compared the four equations most used in practice and found that Mifflin-St Jeor predicted resting metabolic rate within 10 percent of the measured value in more people than any of the others, with the narrowest error range (Frankenfield et al.). That is why most modern calculators, including ours, use it.
| Sex | Mifflin-St Jeor equation (kcal per day) |
|---|---|
| Men | (10 x weight in kg) + (6.25 x height in cm) - (5 x age in years) + 5 |
| Women | (10 x weight in kg) + (6.25 x height in cm) - (5 x age in years) - 161 |
Two cautions. First, "within 10 percent for more people" still means a meaningful share of individuals land outside that range. Second, the equation takes sex as a two-way input because it was fitted that way; it does not capture muscle mass, which varies a great deal between people of the same weight. If you have a lot of muscle your true resting burn is likely higher than predicted, and if you have little it may be lower.
Step 2: from BMR to TDEE
Total daily energy expenditure (TDEE) adds the rest of the day to your resting burn:
- Everyday movement: walking, chores, fidgeting, standing. This varies more between people than most expect.
- Exercise: deliberate training sessions.
- The thermic effect of food: a smaller share of energy used to digest and absorb what you eat.
Calculators bundle these into one multiplier applied to BMR. The values below are widely used conventions, and they are the ones in the SnapMacro TDEE calculator. They are not laboratory measurements.
| Activity level | Multiplier | What it roughly means |
|---|---|---|
| Sedentary | 1.2 | Desk job, little deliberate exercise |
| Light | 1.375 | Light exercise one to three days a week |
| Moderate | 1.55 | Moderate exercise three to five days a week |
| High | 1.725 | Hard exercise most days, or an active job |
| Very high | 1.9 | Very hard training or physical labour daily |
Most people overestimate how active they are, because a single workout feels large compared with the other 23 hours. If you sit between two categories, choose the lower one and let your data correct you upward if needed.
Two worked examples
Example A: a 35-year-old woman, 70 kg, 165 cm
BMR = (10 x 70) + (6.25 x 165) - (5 x 35) - 161 = 700 + 1,031 - 175 - 161 = about 1,395 kcal.
- Light activity (1.375): about 1,920 kcal per day.
- Moderate activity (1.55): about 2,160 kcal per day.
Example B: a 40-year-old man, 85 kg, 178 cm
BMR = 850 + 1,113 - 200 + 5 = about 1,768 kcal.
- Sedentary (1.2): about 2,120 kcal per day.
- Moderate activity (1.55): about 2,740 kcal per day.
Notice the spread. Choosing between two neighbouring activity levels moves the answer by 200 to 600 kcal. That is larger than the typical daily deficit people aim for, which is why the activity guess matters more than the choice of equation.
You can run your own numbers in the TDEE calculator.
Step 3: turn maintenance into a target
The TDEE figure is an estimate of your maintenance calories, the intake at which weight would stay roughly stable. From there:
- To lose weight, eat somewhat below maintenance. A smaller gap is easier to sustain and is less likely to cost muscle. See how a calorie deficit works for realistic expectations.
- To gain muscle, eat slightly above maintenance and train with resistance exercise.
- To maintain, aim for maintenance and judge it by your trend, not by one day.
If you have a medical condition, are pregnant or breastfeeding, are under 18, or have a history of disordered eating, do not use a formula alone. A clinician or registered dietitian can set targets that fit your situation.
Why every estimate drifts
A formula describes an average person with your numbers. Your real expenditure differs, and it also changes.
- Your body changes. When weight falls, there is less of you to maintain and move, so energy needs fall too. A US National Institutes of Health model of this process found that the body's weight response to a change in intake is slow, with half-times of around a year, and that the energy gap you started with shrinks as you lose weight (Hall et al., Lancet 2011). The NIDDK offers a free Body Weight Planner built on that research.
- Everyday movement shifts. You may fidget less and walk less on a diet without noticing.
- Logging is imperfect. If your food records are 10 to 20 percent off, your sense of "maintenance" will be off by about as much. Our own benchmark of photo estimates is on the accuracy page.
- The scale is noisy. Water, salt, and digestive contents move weight by a kilogram or more in a day, so a few days of data say little.
The practical fix is to treat the formula as a hypothesis and test it. Eat near the estimate for two to three weeks, log honestly, and compare your average weight at the start and the end. If weight is flat, the estimate is close to your true maintenance. If it moves faster or slower than you intended, change intake by a modest amount and repeat. We describe this feedback loop in why your calorie target should change over time.
A simple checklist
- Calculate BMR with Mifflin-St Jeor.
- Multiply by the lower of your two plausible activity levels.
- Choose a goal (deficit, maintenance, or small surplus).
- Log for two to three weeks and watch the weekly average weight, not single days.
- Adjust by a modest amount if the trend does not match your goal.
Protein matters as much as total calories for most goals. Once you have a calorie figure, see how much protein per day and the protein calculator.
How SnapMacro helps
SnapMacro starts from the same standard formulas and then uses your logged intake and your real weight trend to adjust your targets over time, rather than leaving a one-time number in place. Logging is quick with a photo, a barcode scan, text or voice, and every estimate is editable. Photo estimates are approximate, so the weekly view is more reliable than any single meal. You can try the formulas first in the free TDEE calculator.
This article is general information, not medical advice. SnapMacro is not a medical device. Talk to a clinician or registered dietitian about your own needs.
Sources
- Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241-247. PubMed
- 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;105(5):775-789. PubMed
- Hall KD, Sacks G, Chandramohan D, et al. Quantification of the effect of energy imbalance on bodyweight. Lancet. 2011;378(9793):826-837. PubMed
- National Academies of Sciences, Engineering, and Medicine. Dietary Reference Intakes for Energy. 2023. nap.nationalacademies.org
- National Institute of Diabetes and Digestive and Kidney Diseases. Body Weight Planner. niddk.nih.gov/bwp
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