How Many Calories Do You Need?
A calorie is a unit of energy. What food labels call a Calorie is technically a kilocalorie — the energy needed to raise one kilogram of water by one degree Celsius. Your body spends this energy continuously: keeping you warm, pumping blood, repairing tissue, digesting food and moving you around.
The number you need depends on age, sex, height, weight, body composition and above all how much you move. As a rough guide, adult men generally need 2,000–3,000 Calories per day to maintain weight and adult women 1,600–2,400, according to U.S. Department of Health figures. But a physically active 25-year-old man of 6 feet and a sedentary 70-year-old woman of 5 feet sit at opposite ends of that range, which is why an individual estimate is more useful than a population average.
The Three BMR Equations
Basal metabolic rate is the energy your body uses at complete rest. Three equations are in common use, and the calculator above offers all three.
Mifflin-St Jeor Equation (default)
Introduced in 1990 and now considered the most accurate general-purpose equation.
Men: BMR = 10W + 6.25H − 5A + 5
Women: BMR = 10W + 6.25H − 5A − 161
Revised Harris-Benedict Equation
The original dates from 1919 and was revised in 1984. It was the standard until Mifflin-St Jeor replaced it.
Men: BMR = 13.397W + 4.799H − 5.677A + 88.362
Women: BMR = 9.247W + 3.098H − 4.330A + 447.593
Katch-McArdle Formula
This one calculates resting daily energy expenditure from lean body mass rather than total weight, so it needs a body fat percentage. For lean people who know their body fat, it is the most accurate of the three; for everyone else it is the least, because the body fat figure is usually a guess.
BMR = 370 + 21.6 × (1 − F) × W
In all three equations W is body weight in kilograms, H is height in centimetres, A is age in years and F is body fat as a decimal fraction.
Activity Multipliers
BMR covers rest only. Multiplying it by an activity factor gives total daily energy expenditure (TDEE) — the number you actually eat against.
| Level | Description | Multiplier |
|---|---|---|
| Sedentary | Desk job, little or no exercise | 1.2 |
| Light | Light exercise 1–3 times per week | 1.375 |
| Moderate | Exercise 4–5 times per week | 1.465 |
| Active | Daily exercise, or intense exercise 3–4 times per week | 1.55 |
| Very Active | Intense exercise 6–7 times per week | 1.725 |
| Extra Active | Very intense exercise daily, or a physical job | 1.9 |
Most people overestimate their activity level. If your weight is not moving as the calculator predicts after three or four weeks, step down one category rather than adjusting the calorie target directly.
Calories and Weight Change
Approximately 3,500 Calories equal one pound of body weight, or 7,700 Calories per kilogram. So a deficit of 500 Calories per day is theoretically one pound per week, and 1,000 per day is two pounds.
In practice the relationship is not perfectly linear. As you lose weight, BMR falls because there is less body to maintain, so an unchanged deficit produces progressively slower loss. Adaptive thermogenesis reduces energy expenditure further. This is why plateaus are normal rather than a sign of failure.
Losing more than about two pounds a week is generally inadvisable. Aggressive deficits cost muscle as well as fat, and losing muscle lowers BMR, which makes maintaining the loss harder afterwards. Harvard Health Publications recommends that women get at least 1,200 Calories and men at least 1,500 unless supervised by a doctor.
Zigzag Calorie Cycling
Zigzag cycling alternates higher and lower calorie days while keeping the weekly total at the target. The reasoning is that a constant intake gives the body a stable environment to adapt to, whereas a varying one does not.
It also solves a practical problem. A fixed daily number is unforgiving of dinners out and family occasions; a weekly budget lets you plan a high day around them and compensate on the others without a sense of failure. There is no established optimal pattern — a spread of roughly 200–300 Calories either side of the target is typical, wider for very active people. The calculator produces two schedules: one with two high days and five low, and one that ramps up and down gradually.
Calories in Common Foods
| Food | Serving | Calories | kJ |
|---|---|---|---|
| Fruit | |||
| Apple | 1 (4 oz) | 59 | 247 |
| Banana | 1 (6 oz) | 151 | 632 |
| Grapes | 1 cup | 100 | 419 |
| Orange | 1 (4 oz) | 53 | 222 |
| Strawberry | 1 cup | 53 | 222 |
| Watermelon | 1 cup | 50 | 209 |
| Vegetables | |||
| Broccoli | 1 cup | 45 | 188 |
| Carrots | 1 cup | 50 | 209 |
| Lettuce | 1 cup | 5 | 21 |
| Tomato | 1 cup | 22 | 92 |
| Protein | |||
| Beef, cooked | 2 oz | 142 | 595 |
| Chicken, cooked | 2 oz | 136 | 569 |
| Egg | 1 large | 78 | 327 |
| Tofu | 4 oz | 86 | 360 |
| Shrimp, cooked | 2 oz | 56 | 234 |
| Meals and snacks | |||
| Bread, white | 1 slice | 75 | 314 |
| Butter | 1 tbsp | 102 | 427 |
| Cheeseburger | 1 | 285 | 1,193 |
| Pizza | 1 slice (14") | 285 | 1,193 |
| Rice | 1 cup cooked | 206 | 862 |
| Caesar salad | 3 cups | 481 | 2,014 |
| Drinks and dairy | |||
| Beer | 1 can | 154 | 645 |
| Cola | 1 can | 150 | 628 |
| Diet cola | 1 can | 0 | 0 |
| Milk (whole) | 1 cup | 146 | 611 |
| Orange juice | 1 cup | 111 | 465 |
Calories Burned by Common Exercise
| Activity (1 hour) | 125 lb | 155 lb | 185 lb |
|---|---|---|---|
| Walking (3.5 mph) | 215 | 267 | 319 |
| Golf (using cart) | 198 | 246 | 294 |
| Kayaking | 283 | 352 | 420 |
| Basketball | 340 | 422 | 503 |
| Swimming (moderate) | 397 | 492 | 587 |
| Tennis | 397 | 492 | 587 |
| Soccer | 397 | 492 | 587 |
| Bicycling (12–14 mph) | 454 | 562 | 671 |
| Running (9 min/mile) | 624 | 773 | 923 |
Note the scale. An hour of brisk walking burns roughly what is in a large muffin. Exercise has real health benefits, but for weight change, intake is the larger and more controllable lever.
Energy in the Food Components
| Component | Calories per gram | kJ per gram |
|---|---|---|
| Fat | 8.8 | 37 |
| Ethanol (alcohol) | 6.9 | 29 |
| Protein | 4.1 | 17 |
| Carbohydrate | 4.1 | 17 |
| Organic acids | 3.1 | 13 |
| Polyols (sugar alcohols) | 2.4 | 10 |
| Fibre | 1.9 | 8 |
Are All Calories Equal?
For the purposes of weight change, the energy balance holds: eat more than you spend and you gain, eat less and you lose. But the number on the label and the number your body retains are not identical, and the source of the calories affects hunger, satiety and adherence.
Foods requiring more chewing and digestion — vegetables, whole grains, lean meats — consume more energy in processing, an effect called the thermic effect of food. They also produce satiety for longer, which makes the same calorie target much easier to keep to. There is a well-known case of a nutrition professor who lost 27 pounds over two months eating mostly snack cakes while counting calories: proof that the arithmetic works, and not a diet anyone should copy, since it ignores everything except weight.
Drinks deserve particular attention. They account for roughly 21% of the average diet's calories and produce almost no satiety. Juices and flavoured milks carry as much sugar as many soft drinks. Water, unsweetened tea and black coffee are the cheapest calorie savings available to most people.
Frequently Asked Questions
How accurate is this estimate?
BMR equations are population averages and individual metabolic rates vary by roughly ±10% around them. Treat the result as a starting point: track your weight for three to four weeks and adjust the target by 100–200 Calories if the trend does not match the prediction.
Why has my weight loss stalled?
Usually because your maintenance level fell with your weight, so the old target is now close to maintenance. Recalculate at your current weight. Water retention also masks fat loss for weeks at a time — weigh weekly under identical conditions rather than daily.
Should I eat back the calories I burn exercising?
The activity multiplier already accounts for regular exercise, so eating back tracked exercise calories double-counts them. Fitness tracker estimates are also known to overstate energy burned, often substantially.
What is the lowest safe intake?
About 1,500 Calories per day for men and 1,200 for women without medical supervision. Below that it becomes difficult to meet micronutrient needs, and muscle loss accelerates.