Calorie Deficit Calculator

How the calorie deficit calculator works

1. It estimates your basal metabolic rate (BMR) with the Mifflin-St Jeor equation.
2. It multiplies BMR by an activity factor to estimate your total daily energy expenditure (TDEE) – your maintenance calories.
3. It subtracts your chosen deficit to give a daily calorie target.
4. It projects your weight week by week, re-estimating TDEE as your weight falls, to show a realistic timeline.

Example: a 35-year-old woman, 165 cm, 75 kg, who exercises 3–5 times a week. BMR = 10 × 75 + 6.25 × 165 − 5 × 35 − 161 = 1,445 kcal. TDEE = 1,445 × 1.55 = 2,240 kcal. A 500 kcal deficit gives a target of about 1,740 kcal a day; a 20% deficit gives 1,792 kcal.

How big should your calorie deficit be?

Health organisations such as the CDC suggest aiming to lose about 0.5–1 kg (1–2 lb) a week, which people are more likely to keep off. A deficit of around 10–25% of TDEE, or 300–750 kcal a day, suits most adults. Bigger deficits make it harder to get enough protein and nutrients, increase hunger and fatigue, and can speed up muscle loss.

Why the 3,500-calorie rule overestimates long-term loss

The classic rule says a pound of fat holds about 3,500 kcal (about 7,700 kcal per kg), so a 500 kcal daily deficit loses a pound a week. That works reasonably for the first few weeks, but not forever. As you lose weight, your body needs fewer calories to move and maintain itself, so the same intake gives a smaller deficit. Research led by Kevin Hall at the US National Institutes of Health showed that weight loss slows over time for this reason; the NIDDK Body Weight Planner uses a similar dynamic model. Our timeline recalculates TDEE as your weight drops, so it is longer – and more realistic – than a straight-line estimate.

Calories to reach a goal by a date

If you want to lose 5 kg in 20 weeks (140 days), a simple estimate is 5 × 7,700 ÷ 140 ≈ 275 kcal a day below maintenance. Because loss slows as you go, you may need a slightly larger deficit later or a few extra weeks. If the calculator shows a target below safe limits, extend the date rather than cutting calories further.

Minimum calories and safety

• Many health services advise not going below about 1,200 kcal a day for women or 1,500 kcal for men without medical supervision; the calculator warns you below these levels.
• Very low-calorie diets (around 800 kcal a day or less) should only be followed under medical supervision.
• A calorie deficit is not suitable during pregnancy or breastfeeding, for under-18s, or for anyone with a history of eating disorders – speak to a professional.
• If you take medication for diabetes or blood pressure, check with your doctor before reducing calories.

Keep muscle while you lose fat

Losing weight is not the same as losing fat. To make sure most of the loss is fat, eat enough protein (around 1.6–2.2 g/kg during a deficit – see the protein calculator) and do resistance training two or more times a week. Track your waist or body fat every few weeks as well as your weight.

Plateaus and maintenance

Weight often stalls for a week or two because of water shifts, then drops again. If there is no change for three to four weeks, recalculate with your new weight (your TDEE will be lower), check portion sizes, or increase daily activity. When you reach your goal, raise calories gradually to your new maintenance level – the calculator shows it – and keep the habits that got you there.

This calculator gives estimates for general information and is not medical advice. Individual metabolism varies. Speak to a doctor or registered dietitian before starting a weight-loss plan, especially if you have a medical condition, are pregnant or breastfeeding, are under 18, or have had an eating disorder.

Splitting your calories into macros

Once you have a calorie target, a simple way to plan meals is to set protein first, then fat, then carbohydrate:

1. Protein: about 1.6–2.2 g per kg of body weight during a deficit (4 kcal per gram).
2. Fat: at least about 20–25% of calories for hormones and vitamin absorption (9 kcal per gram).
3. Carbohydrate: the remaining calories (4 kcal per gram), mostly from whole grains, fruit, vegetables and legumes.

Example: 1,740 kcal for a 75 kg woman. Protein 1.6 × 75 = 120 g = 480 kcal. Fat 25% = 435 kcal ≈ 48 g. Carbohydrate gets the rest: 1,740 − 480 − 435 = 825 kcal ≈ 206 g. The keto calculator uses a different split if you prefer a low-carb approach.

Exercise and the deficit

You can create a deficit by eating less, moving more, or both. Exercise adds to your TDEE, so the calculator's activity level should reflect your real routine – overestimating activity is one of the most common reasons for slower-than-expected loss. Walking is an easy way to add 100–300 kcal a day for many people; the walking calorie calculator estimates how much. Strength training helps keep muscle while you lose weight.

Tracking your progress

• Weigh at the same time of day (morning, after the toilet) and compare weekly averages rather than single days.
• Measure your waist every 2–4 weeks – it often changes when the scale stalls.
• Recalculate your target every 4–5 kg (about 10 lb) lost.
• Note sleep, stress and menstrual cycle, all of which affect water weight and hunger.

Calorie deficit examples

The third example shows why one-size-fits-all diets fail: a 500 kcal deficit takes her to about 1,197 kcal, close to the level where many experts recommend professional support. A smaller deficit plus extra daily activity may be easier to sustain.

Related guides

• Calorie Deficit Explained: How to Set a Safe and Realistic Target
• TDEE vs BMR: What's the Difference and Which Number Should You Use?
• Activity Multipliers for TDEE: How to Pick the Right Level
• How Much Protein Do You Need Per Day? How to Work It Out

Sources

1. Mifflin MD et al. (1990), A new predictive equation for resting energy expenditure – PubMed — pubmed.ncbi.nlm.nih.gov/2305711
2. CDC – Steps for losing weight — cdc.gov/healthy-weight-growth/losing-weight/index.html
3. Hall KD et al. (2011) – Quantification of the effect of energy imbalance on bodyweight (The Lancet) – PubMed — pubmed.ncbi.nlm.nih.gov/21872751
4. NIDDK – Body Weight Planner — niddk.nih.gov/bwp
5. NIDDK – Dieting and gallstones (includes very low-calorie diets) — niddk.nih.gov/health-information/digestive-diseases/gallstones/dieting