How Many Calories Should You Eat to Lose Weight?

Skinny Scanning team · first published 2026-08-30 · updated 2026-10-24

To lose weight, most people need to eat somewhat less energy than their body uses in a day: a calorie deficit. You can estimate your maintenance calories with the Mifflin-St Jeor formula and an activity factor. Below are the formula, a worked example and a chart of deficit sizes. And, because the number is only half the story, a look at why what you eat changes how much you end up eating.

This is general nutrition information, not a diagnosis or a personalised medical recommendation — see the disclaimer at the end.

What is the Mifflin-St Jeor formula?

The Mifflin-St Jeor equation estimates your basal metabolic rate (BMR), roughly the calories your body burns at rest, from weight, height, age and sex. It is one of the more accurate resting-energy equations and is widely used in nutrition research and practice.

• 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

BMR is your body at rest, not what you should eat. To estimate your daily maintenance calories (TDEE, total daily energy expenditure), multiply BMR by an activity factor.

How do I find my activity multiplier?

These are estimates, not a lab measurement. Most people land near "lightly" or "moderately active"; adjust after a few weeks if real-world results don't match the maths.

Worked example

A 35-year-old woman, 70 kg (about 154 lb), 165 cm (about 5'5"), moderately active:

1. BMR = 10 × 70 + 6.25 × 165 − 5 × 35 − 161 = 700 + 1,031.25 − 175 − 161 = 1,395 kcal/day
2. TDEE (maintenance) = 1,395 × 1.55 = ≈ 2,163 kcal/day
3. Deficit for about 0.5 kg (1 lb) a week: the common rule of thumb of roughly 7,700 kcal per kg of body fat ÷ 7 days ≈ 550 kcal/day
4. Starting target = 2,163 − 550 = ≈ 1,610 kcal/day

The 7,700 kcal rule is a simplification: as you lose weight your body uses less energy, so progress usually slows over time even if you keep eating the same amount. Treat the result as a starting point, not a promise.

How big a deficit?

The number is half the story: what you eat changes how much you eat

They told you losing weight was all about willpower. The research is less simple. In a tightly controlled NIH trial, 20 adults lived on an inpatient ward and could eat as much as they wanted from two diets matched for calories on offer, sugar, fat, fibre and salt. On the ultra-processed diet they ate about 500 kcal a day more and gained weight; on the unprocessed diet they lost it. The deficit you planned can quietly disappear depending on what the food is made of.

That is why it helps to know what is in your food, not only how many calories it has: added sugars, sugar-sweetened drinks, ultra-processed products and the other ingredients research links to weight gain.

Checklist — setting your own starting goal:

• Calculate BMR with the formula above
• Pick the activity multiplier that best matches your typical week
• Multiply BMR × multiplier to get your maintenance calories (TDEE)
• Choose a moderate weekly rate (0.25–1% of body weight per week is a commonly used range)
• Subtract the deficit from TDEE to get a starting daily target
• Recalculate every 4–6 weeks, since TDEE drops as body weight drops
• Look at what your calories are made of, not only how many there are

Where Skinny Scanning fits in

Skinny Scanning does not count calories and has no diary to fill in. It answers the other half of the question: what is in this product or this meal, and how strongly does research link it to weight gain? Scan a barcode or a plate and you get a Wellness Score from 0 to 100, traffic lights for sugar, saturated fat and salt, every flagged ingredient with the study behind it, and two healthier swaps. Use the formula above for the number, and the scanner for what goes into it.

FAQs

Is it safe to eat under 1,200 calories a day? Very low intakes can be hard to sustain and may not provide enough nutrition. If a calculation puts your target very low, or you have any health condition, check with a doctor or registered dietitian first.

How often should I recalculate? Every 4–6 weeks, or whenever your weight has changed meaningfully.

Does the Mifflin-St Jeor formula work for everyone? It is a population-level estimate. It can be less precise for people with very high muscle mass and is not intended for children or during pregnancy or breastfeeding.

Do I need to eat back the calories I burn exercising? The activity multiplier already includes your usual activity, so adding workout calories on top usually double-counts exercise.

Does Skinny Scanning calculate my calorie goal? No. Skinny Scanning focuses on food quality: the ingredients research links to weight gain, the evidence behind each one and healthier swaps. It never rewards eating less or a lower weight.

References. Mifflin MD, St Jeor ST, Hill LA, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr 1990;51(2):241–247. doi:10.1093/ajcn/51.2.241 · Hall KD, Ayuketah A, Brychta R, et al. Ultra-processed diets cause excess calorie intake and weight gain: an inpatient randomized controlled trial of ad libitum food intake. Cell Metabolism 2019. doi:10.1016/j.cmet.2019.05.008 · Garthe I, Raastad T, Refsnes PE, et al. Effect of two different weight-loss rates on body composition and strength and power-related performance in elite athletes. Int J Sport Nutr Exerc Metab 2011;21(2):97–104. doi:10.1123/ijsnem.21.2.97

Disclaimer: This article is educational information about a common nutrition formula, not medical or dietary advice, and it is not a diagnosis or treatment plan for any condition. Energy needs vary by individual. Talk to a qualified doctor or registered dietitian before changing your diet — especially if you are pregnant or breastfeeding, have a history of disordered eating, or have a medical condition.