How Many Calories Should I Eat Per Day? (By Age, Weight & Goal)

"How many calories should I eat?" is one of the most searched nutrition questions on the internet — and the honest answer is: it depends. Your age, weight, height, sex, and how active you are all push the number up or down significantly. This guide walks you through the science behind calorie needs so you can arrive at a number that actually fits your life and your goals.

Step 1 — Understand Your Basal Metabolic Rate (BMR)

Your Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest — just to keep you alive. It accounts for breathing, circulation, cell repair, and temperature regulation. BMR typically represents 60–70% of total daily calorie expenditure for sedentary individuals.

The most widely validated formula for estimating BMR is the Mifflin-St Jeor equation, published in 1990 and recommended by most registered dietitians:

• For men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
• For women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161

The older Harris-Benedict equation (revised 1984) is also commonly used and gives similar results. Both formulas are estimates — individual metabolism can vary by ±10% even between people with identical stats. What matters is using one formula consistently and adjusting based on real-world results over 2–4 weeks.

Example BMR Calculation

A 30-year-old woman weighing 68 kg (150 lbs) and 165 cm (5'5") tall:

BMR = (10 × 68) + (6.25 × 165) − (5 × 30) − 161 = 680 + 1,031 − 150 − 161 = 1,400 calories/day

This means her body needs 1,400 calories just to survive with zero movement. Add activity on top of that.

Step 2 — Calculate Your Total Daily Energy Expenditure (TDEE)

Your TDEE is BMR multiplied by an activity factor. This is the number that actually matters for setting a calorie target. Most people underestimate their activity level, which leads to setting calories too low and burning out.

Continuing the example above: our 30-year-old woman with a BMR of 1,400 who works out 3 days a week (moderately active) has a TDEE of 1,400 × 1.55 = 2,170 calories. This is her maintenance intake — the level that keeps her weight stable.

Step 3 — Adjust for Your Goal

For Weight Loss

Subtract 300–500 calories from your TDEE to create a sustainable deficit. A 500 cal/day deficit produces roughly 0.5 kg (1 lb) of fat loss per week — the rate recommended by most health organizations. Never eat below your BMR without medical supervision, as this risks muscle loss, nutrient deficiencies, and metabolic adaptation.

• Slow cut (recommended): TDEE minus 300–400 cal/day → ~0.3 kg/week
• Standard cut: TDEE minus 500 cal/day → ~0.5 kg/week
• Aggressive cut (short-term only): TDEE minus 750 cal/day → ~0.7 kg/week

For Muscle Gain (Bulk)

Add 200–400 calories above TDEE. A smaller surplus (200–250 cal) is a "lean bulk" — slower muscle gain but minimal fat accumulation. A larger surplus speeds up progress but adds more body fat alongside muscle. For most natural lifters, a 200–300 cal surplus is the sweet spot.

For Maintenance

Eat at your TDEE. This is also a useful starting point — eat at maintenance for 2–4 weeks, track your weight, and see if the number is accurate. If you're gaining weight, your true TDEE is lower. If you're losing weight, it's higher. Real-world calibration beats any formula.

Reference Calorie Needs by Age, Gender & Activity

The table below uses the Mifflin-St Jeor equation with a moderate body weight (70 kg / 154 lbs for men, 63 kg / 139 lbs for women) at average height (175 cm / 5'9" for men, 163 cm / 5'4" for women). Use it as a rough benchmark — your actual needs will differ.

Why Calorie Needs Decline with Age

As you can see in the table, calorie needs decrease about 100–150 calories per decade after age 30. The main reasons:

• Muscle loss (sarcopenia): After 30, adults lose roughly 3–5% of muscle mass per decade without resistance training. Muscle is metabolically expensive — less muscle means a lower BMR.
• Hormonal changes: Declining testosterone and estrogen reduce both muscle maintenance and overall metabolic rate.
• Reduced spontaneous activity: People tend to move less overall as they age, even if they maintain formal exercise routines.

The practical implication: if you've been eating the same amount for years and recently started gaining weight, your maintenance needs may have shifted downward. This is normal — and it's not a character flaw. Adjusting your intake by 100–200 calories is often all that's needed.

Minimum Calorie Thresholds

Regardless of goal, most experts recommend never going below these minimums without medical supervision:

• Men: 1,500 calories/day
• Women: 1,200 calories/day

Very low calorie diets (VLCDs) below these thresholds risk muscle loss, nutrient deficiencies, gallstone formation, and metabolic adaptation — where your metabolism slows to compensate for the deficit, making weight loss harder over time.

The Accuracy Problem: Why Most People Get This Wrong

Studies consistently show that people underestimate food intake by 20–40%. Even nutritionists tend to underreport by ~10–15%. The main errors:

1. Eyeballing portions — A "tablespoon" of peanut butter poured by hand is usually 1.5–2 tablespoons (roughly double the calories).
2. Forgetting cooking oils — A pan sprayed with oil adds 30–120 calories depending on how long the spray is held.
3. Ignoring bites, tastes, and licks — The handful of chips while cooking, the spoonful of batter, the nibble of cheese. These add up to 200+ calories on some days.
4. Using database entries for restaurant food — A restaurant's Caesar salad or pasta can be 30–60% higher in calories than the generic database entry because of extra oil, cheese, or portion size.

Using MyBiteIQ's AI photo tracking eliminates most of these errors — you snap a photo of your actual plate and the AI estimates portion size and calories directly, without you needing to look up each ingredient separately.

How to Use Your Calorie Target Effectively

Once you have a target, the key is consistency over precision. Research on calorie tracking shows that people who track at least 5 days out of 7 see dramatically better results than those who track only on weekdays. A few practical principles:

• Think in weekly averages — Being 300 calories over on Saturday and 300 under on Sunday nets to zero. You don't need to be perfect every single day.
• Reassess every 4 weeks — As you lose or gain weight, your TDEE changes. Recalculate your target every 4–6 weeks based on your new weight.
• Track weight trend, not daily number — Daily weight fluctuates 0.5–2 kg due to water retention, food volume, and hormonal changes. Look at the 7-day or 14-day average to see your real trend.
• Prioritize protein within your calorie target — Getting 1.6–2.2g of protein per kg of body weight helps preserve muscle during a deficit and increases satiety.

The Bottom Line

Most adults need between 1,600 and 3,000 calories per day depending on their size, age, sex, and activity level. Calculate your TDEE, set a target based on your goal, track consistently, and adjust based on 4-week results. The formula is the starting point — your real-world data is the refinement.