How many calories do I need? A simple, honest guide

Calorie needs are the first question most people ask when they start a diet. The honest answer is that no calculator knows your exact number. What a good formula gives you is a sensible starting point that you then adjust using real results over two to four weeks.

Step 1: estimate your resting energy (BMR)

Your basal metabolic rate (BMR) is the energy your body uses at rest to keep you alive. The Mifflin-St Jeor equation is one of the most widely used and tested formulas for healthy adults:

• Men: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) + 5
• Women: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) − 161

If you use pounds and inches, convert first: 1 kg = 2.2 lb and 1 inch = 2.54 cm.

Step 2: add your daily activity (TDEE)

Your total daily energy expenditure (TDEE) is your BMR multiplied by an activity factor. Be conservative here; most people overestimate how active they are.

A worked example

A 35-year-old woman who is 165 cm tall, weighs 70 kg and walks or trains lightly three days a week:

• BMR = 10 × 70 + 6.25 × 165 − 5 × 35 − 161 = about 1,395 kcal
• TDEE = 1,395 × 1.375 = about 1,920 kcal a day

That is roughly the amount that would keep her weight stable.

Step 3: choose a deficit you can live with

To lose fat you need to eat a little less than your TDEE. A daily deficit of about 300 to 500 kcal is a common, sustainable choice. In the example above, that means eating roughly 1,420 to 1,620 kcal a day.

A useful safety rule is to aim for a loss of about 0.5% to 1% of your body weight per week. Faster than that usually means more muscle loss, more hunger and a higher chance of giving up.

Why the scale won't follow the math exactly

The old rule that "3,500 kcal equals one pound of fat" is a rough shortcut. Research on energy balance shows weight loss slows over time, because a smaller body burns less energy and the body adapts. Day-to-day weight also swings by one to two kilograms with water, salt, carbohydrates and digestion.

That is why you should judge progress by the weekly average of your weigh-ins, not by a single morning.

Step 4: adjust after two to four weeks

1. Log your food as honestly as you can, including oils, sauces and drinks.
2. Weigh yourself under the same conditions three or more times a week.
3. Compare the weekly averages after two to four weeks.
4. If your weight is not changing and you are logging accurately, reduce your target by about 100 to 200 kcal, or add more daily movement.
5. If you are losing faster than 1% a week, feel exhausted or your training suffers, eat a little more.

Calories are not the whole story

Two diets with the same calories can feel very different. To make your target easier to stick to:

• Build meals around protein and vegetables, which keep you full.
• Choose mostly whole foods: legumes, whole grains, fruit, dairy or alternatives, eggs, fish and lean meat.
• Keep liquid calories (juice, sweetened tea and coffee drinks, soda) low.
• Leave room for foods you enjoy, so the plan feels livable.

How Fitlyn helps

When you set your goal in Fitlyn, the app suggests a daily calorie and macro target from your details. You can log food by barcode or search, see your weekly weight trend and adjust the target as you learn how your body responds.

Sources

• Mifflin MD, St Jeor ST, et al. A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition, 1990.
• Hall KD, et al. Quantification of the effect of energy imbalance on bodyweight. The Lancet, 2011.
• World Health Organization. Healthy diet fact sheet.