Calorie Calculator

How Many Calories Do You Need?

The number of calories you need each day depends on several factors: your age, sex, weight, height, and how physically active you are. There is no single answer that works for everyone — a 25-year-old male athlete needs significantly more calories than a 60-year-old sedentary woman.

Your body uses calories for three main purposes: basal metabolism (keeping organs, brain, and cells functioning at rest), the thermic effect of food (digesting what you eat), and physical activity (everything from walking to intense exercise). Of these, basal metabolism accounts for the largest share — typically 60–75% of your total daily burn.

The Mifflin-St Jeor Equation

Published in 1990, the Mifflin-St Jeor equation is widely regarded as the most accurate formula for estimating Basal Metabolic Rate (BMR) in healthy adults. It replaced the older Harris-Benedict equation (1919) and is recommended by the Academy of Nutrition and Dietetics.

Once you know your BMR, multiply it by an activity factor to get your TDEE (Total Daily Energy Expenditure) — the number of calories you actually burn in a day including all movement and exercise.

What the Calculator Does With Your Four Numbers

Three arithmetic steps sit between the form above and the figure it returns. Each one adds a different kind of uncertainty, and it helps to know which step is doing what.

• 1. Resting burn, from the Mifflin-St Jeor equation. Sex, weight, height and age go in; a BMR comes out. This is the only step with published accuracy data attached: Frankenfield and colleagues (2005) found the equation within 10% of measured values for 82% of the adults they reviewed.
• 2. Activity multiplier. The calculator multiplies that BMR by a factor between 1.2 and 1.9. Nobody measured your factor. You picked it from a dropdown, and the gap between two neighbouring options is larger than the error in step one.
• 3. Goal adjustment. It then adds or subtracts a flat 250 or 500 kcal. A floor of 1,200 kcal for women and 1,500 kcal for men catches the answer, so a steep deficit on a small frame returns that floor instead of the arithmetic.

Step two carries most of the risk. Skip a tier and the answer moves by more than 250 kcal for an average adult, which is larger than the difference between any two respected BMR equations. How many calories you need per day covers how to place yourself honestly on that scale.

Worked Example: One Person, Start to Finish

Hannah is 34, stands 168 cm, weighs 74 kg, walks a fair bit and lifts twice a week, and wants to lose weight. Every term substituted:

Subtract 500 and her target is 1,506 kcal a day. The calculator splits that 30/40/30 by default, which gives 113 g of protein, 151 g of carbohydrate and 50 g of fat. Check the protein against her body weight and it comes to 1.53 g/kg, just under the 1.6–2.2 g/kg band that shows up in the muscle-retention literature. Raising her to 1.8 g/kg means 133 g, which pulls 80 kcal out of carbohydrate and fat, and the macro calculator will do that split for you. See how much protein you need per day for where that band comes from.

Now change one input and watch what happens. Hannah's activity level is the only field she guessed at, and it is the field the answer is most sensitive to.

Why Another Site Gives You a Different Number

Type the same four values into three calculators and you will get three answers. Two things cause the spread, and neither is a bug.

That is a spread of about 4% at the BMR stage, and the activity multiplier magnifies it: at 1.375 the Harris-Benedict maintenance figure lands 64 kcal above the Mifflin-St Jeor one. Harris and Benedict built their coefficients on 239 subjects measured before 1918; Mifflin and St Jeor drew their 1990 sample from a population that looks more like today's. The American Dietetic Association, now the Academy of Nutrition and Dietetics, recommended Mifflin-St Jeor for non-critically-ill adults in the same 2005 evidence review, which is why this page runs it.

The second cause is the activity tiers themselves. Some sites use 1.2 to 1.9, some use 1.2 to 2.4, and a few add a separate line for logged workouts on top of the multiplier, which double-counts exercise you already declared. Compare two calculators and check what the tier labels mean before deciding one of them is wrong. Our TDEE calculator uses the same 1.2–1.9 range as the dropdown above, so the two agree by construction.

How Big Should the Deficit or Surplus Be?

A deficit is worth judging as a share of maintenance rather than as a flat number. Hannah's 500 kcal is 25% of her 2,006 kcal maintenance; the same 500 kcal on a 3,000 kcal maintenance is 17%. The larger share is harder to hold and takes more lean tissue with it.

• Fat loss: 10–20% below maintenance. For Hannah that is 200–400 kcal, giving a target between 1,606 and 1,806 rather than 1,506. Slower on paper, and far easier to eat at for the twelve weeks it usually takes.
• Muscle gain: 5–15% above maintenance. Roughly 100–300 kcal for Hannah, paired with progressive resistance training. Larger surpluses add fat faster without adding muscle faster, because muscle grows at its own pace however much food arrives.
• Maintenance: hold, then adjust in 100 kcal steps. Weigh weekly, average the readings, and move the target only when the average has drifted for two weeks running.

The dropdown labels above translate 500 kcal a day into 0.5 kg a week, using the old rule of 3,500 kcal per pound of fat. It is a rounding: 0.5 kg of body fat is closer to 3,850 kcal, so 500 a day is nearer 0.45 kg a week. The rule also assumes the deficit stays constant, and it doesn't — as body mass falls, so does the energy needed to move and maintain it. Hall and colleagues (The Lancet, 2011) modelled that decay and showed the linear rule overshoots long-run weight loss substantially. Expect the first weeks to go faster than the last. How a calorie deficit actually works goes further into it.

Calorie Floors and When to Hand Off

The calculator refuses to return a target below 1,200 kcal for women or 1,500 kcal for men. If your inputs push it there, the number you are reading is the floor, not the arithmetic. That happens most often to shorter women choosing a 500 kcal deficit on a sedentary multiplier: a 158 cm, 58 kg, 50-year-old woman has a sedentary maintenance of 1,388 kcal, and a 500 kcal cut would put her 312 kcal below the floor.

Very low intakes make it hard to reach protein, calcium, iron and B-vitamin targets from food, and they get harder to sustain the longer they run. Take a target under the floor to a doctor or a registered dietitian rather than working around it. Clinicians do run lower intakes, under supervision, and the supervision is the point. Whether 1,200 calories is enough covers what the number does to adherence.

The Number Is a Hypothesis, Not a Verdict

Every step above is a prediction about a body nobody measured. Two to four weeks of your own data beats all of it, and the test is simple.

• Eat the target for 14 days without changing it. Adjusting mid-test destroys the signal you are trying to read.
• Weigh every morning, same conditions, and average by week. Daily weight moves a kilogram or more on water, sodium and gut contents; the weekly mean is the number that means something.
• Compare the trend to the prediction. At Hannah's 500 kcal deficit, expect roughly 0.45 kg a week. Half that, and her real maintenance is below 2,006. Nothing at all, and it is below 1,506.
• Correct in 150–200 kcal steps, then run another two weeks. One adjustment per test, or you won't know which change did the work.

Two things can wreck the test before it starts. Under-reporting is the big one: Lichtman and colleagues (New England Journal of Medicine, 1992) measured intake in people who described themselves as diet-resistant and found they under-reported food by 47% and over-reported activity by 51%. And unconscious movement varies far more than people expect — Levine, Eberhardt and Jensen (Science, 1999) overfed 16 volunteers by 1,000 kcal a day for eight weeks and found their non-exercise activity changing by anywhere from −98 to +692 kcal a day. That range alone is wider than the gap between two activity tiers. The logging errors that break this test is worth reading before you start counting.

Calorie Questions People Actually Ask

• How many calories should I eat a day?
  Whatever your maintenance is, minus 10–20% if you want to lose fat. For Hannah at 168 cm, 74 kg, 34 and lightly active, maintenance is 2,006 kcal, so the sensible loss range is 1,606 to 1,806. Population averages of 1,600–3,000 kcal from the Dietary Guidelines for Americans are too wide to eat by.
• Why does another calculator give me a different number?
  Different equation, different activity tiers, or both. Harris-Benedict returns 1,515 kcal for the same woman Mifflin-St Jeor puts at 1,459, a gap of 56 kcal that the activity multiplier stretches to 64 kcal at maintenance. Neither site measured you.
• Is a 500 calorie deficit right for everyone?
  No, because 500 kcal means different things at different sizes. It is 25% of Hannah's 2,006 kcal maintenance and 17% of a 3,000 kcal one. Judge the deficit as a percentage and keep it between 10% and 20% unless a clinician says otherwise.
• Should I eat back the calories I burn exercising?
  Not if you declared that training in the activity dropdown. The 1.375 and 1.55 multipliers already contain it, so logging a workout on top counts the same session twice. Either use a low multiplier and add sessions, or use an honest multiplier and add nothing.
• How long before I know the number is right?
  Two to four weeks. Hold the target for 14 days, weigh every morning, average by week, and compare the trend to the prediction. At a 500 kcal deficit expect about 0.45 kg a week; half of that means your real maintenance sits below the calculated one.
• What macro split should I use?
  Set protein first and let carbohydrate and fat take the rest. The 30/40/30 default gives Hannah 113 g of protein, or 1.53 g/kg, which is under the 1.6–2.2 g/kg band; 1.8 g/kg would be 133 g. Beyond protein, the split matters far less than hitting the calorie total.
• Is 1,200 calories safe?
  It is the floor this calculator clamps to for women, not a recommendation. Reaching protein, calcium, iron and B-vitamin targets from food gets difficult at that intake. If your inputs return the floor rather than a calculated figure, get the target from a doctor or registered dietitian.
• Do I have to recalculate as I lose weight?
  Every 5 kg or so. Five kilograms off lowers Hannah's BMR by 50 kcal and her maintenance by 69 kcal, so an unchanged target is a quietly smaller deficit than the one you chose.