Weight Loss Calculator

Weight Loss Calculator: Find Your Calorie Deficit, Rate of Loss, and Timeline to Goal Weight

TL;DR: Losing 0.5 kg per week requires a sustained 500 kcal daily deficit (3,500 kcal per kg of fat). Losing 1 kg per week requires a 1,000 kcal daily deficit, which is the practical upper limit for most people before muscle loss accelerates. The minimum safe calorie floor is 1,200 kcal/day for women and 1,500 kcal/day for men. This calculator takes your current weight, goal weight, and TDEE to return a daily calorie target, weekly rate of loss, and the number of weeks to reach your goal.

---

Table of Contents

• The 3,500 Calorie Rule: What It Gets Right and Where It Breaks Down
• Eight Scenarios Where the Standard Advice Needs Adjusting
• How Weight Loss Timelines Are Calculated
• Five Steps to Your Calorie Target and Goal Date
• Two Weight Loss Plans, Fully Worked
• Where Fat Loss Plans Go Wrong
• FAQ
• Assumptions and Notes
• The Only Number That Actually Matters
• Further Reading

---

The 3,500 Calorie Rule: What It Gets Right and Where It Breaks Down

One pound of fat stores approximately 3,500 kcal of energy. Cut 500 kcal per day, lose one pound per week. The logic is clean and the arithmetic works — in the short term.

The problem is that the body is not a static system. As body weight falls, TDEE falls with it. A person who starts a diet at 90 kg with a 2,400 kcal TDEE and a 500 kcal daily deficit will not lose weight at the same rate in week 20 as in week 1. By week 20, if they have lost 10 kg, their TDEE has dropped by approximately 150–200 kcal. The original 500 kcal deficit has become a 300–350 kcal deficit, producing closer to 0.3 kg per week than the intended 0.5 kg. This is not a plateau caused by metabolic adaptation in some mysterious sense. It is simple arithmetic: lighter bodies require fewer calories to maintain.

The second issue is body composition. Not all weight loss is fat loss. At a 500 kcal daily deficit with adequate protein (1.6 g/kg body weight), approximately 75–85% of weight lost is fat and 15–25% is lean mass. At a 1,000 kcal deficit with insufficient protein, lean mass loss rises to 35–45% of total weight lost. The scale shows the same number in both cases, but the metabolic and physical outcome differs significantly.

Enter your current weight, goal weight, and activity level into the calculator above to get a timeline that accounts for your starting TDEE.

---

Eight Scenarios Where the Standard Advice Needs Adjusting

• You have less than 5 kg to lose and the standard 500 kcal deficit feels aggressive. For people close to their goal weight, a smaller deficit (250–300 kcal) is both safer and more sustainable. At 5 kg to lose, a 500 kcal daily deficit produces a goal date 10 weeks away; a 300 kcal deficit reaches the same target in 17 weeks with substantially less muscle loss and hunger. For the final 5 kg, slower is almost always better for body composition.

• You train with weights 4–5 days per week and want to preserve as much muscle as possible during the cut. Resistance training during a calorie deficit is the most effective intervention for maintaining lean mass. People who combine a 400–500 kcal deficit with 3–4 resistance sessions per week retain 10–15% more lean mass than those doing the same deficit with cardio only. Protein must be set at 1.6–2.0 g per kg of current body weight for this protective effect to apply.

• Your weight has been stuck for 3 or more weeks despite a tracked calorie deficit. True plateaus have two common causes: TDEE has fallen to match the new calorie intake, or tracking accuracy has drifted by 200–300 kcal per day. The first is resolved by recalculating TDEE at the new body weight and reducing intake by a further 100–150 kcal. The second is resolved by re-weighing all food for one week before concluding that a metabolic issue exists.

• You want to lose weight while training for a half marathon or endurance event. Combining a large calorie deficit with high training volume produces injuries, immune suppression, and performance decline. An endurance athlete running 40+ km per week should cap the calorie deficit at 250–300 kcal per day to protect training quality, accepting a slower rate of fat loss (0.25 kg per week) in exchange for sustainable training performance.

• You lost weight quickly before and regained it all. Rapid weight loss above 1 kg per week is associated with significantly higher rates of weight regain within 12 months because it preferentially depletes lean mass and downregulates thyroid output. At a regain rate of approximately 60–80% within 2 years for rapid losers, a 12-month plan achieving 0.5 kg per week produces better sustained outcome than a 3-month plan achieving 1.5 kg per week.

• You are postmenopausal and find that previously effective calorie targets no longer produce the expected results. Oestrogen loss after menopause reduces resting metabolic rate by approximately 50–100 kcal per day above the expected age-related decline. A postmenopausal woman whose TDEE was accurately calculated at 1,900 kcal at age 45 may have a true TDEE closer to 1,750–1,800 kcal at age 55 without any change in activity or weight. Recalculating TDEE using current measurements rather than historical figures is the correct starting point.

• You have been eating at a deficit for more than 12 consecutive weeks. Extended dieting periods produce measurable adaptive thermogenesis: the body reduces NEAT (non-exercise activity thermogenesis) by 100–300 kcal per day through unconscious movement reduction. Taking a 1–2 week diet break at maintenance calories every 10–12 weeks of dieting partially resets NEAT, reducing total adaptive thermogenesis by approximately 30–40% compared to continuous dieting.

• You are using a fitness tracker to estimate calories burned and eating back exercise calories. Consumer fitness trackers overestimate energy expenditure by 15–30% on average depending on activity type. Eating back 100% of tracker-estimated exercise calories while in a deficit frequently produces no net deficit at all. A reliable approach is to eat back 50–60% of tracker-estimated exercise calories, or to include training in the activity multiplier when calculating TDEE and not eat back exercise calories separately.

---

How Weight Loss Timelines Are Calculated

The calculation converts a total fat loss target into weeks using a daily calorie deficit, adjusted for the declining TDEE that occurs as body weight drops.

Step 1: Calculate TDEE (Mifflin-St Jeor)
  Male:   BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5
  Female: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161
  TDEE = BMR × activity multiplier

Step 2: Calculate total fat loss required
  Fat to lose (kg) = current weight − goal weight
  (assumes pure fat loss; actual loss includes some lean mass)

Step 3: Set daily calorie deficit
  Conservative:  250 kcal/day → 0.25 kg/week
  Moderate:      500 kcal/day → 0.5 kg/week
  Aggressive:    750 kcal/day → 0.75 kg/week
  Maximum safe:  1,000 kcal/day → 1.0 kg/week

Step 4: Apply calorie floor check
  Daily intake = TDEE − deficit
  Minimum: 1,200 kcal/day (women), 1,500 kcal/day (men)
  If intake < floor, reduce deficit until floor is met

Step 5: Estimate weeks to goal
  Weeks = (fat to lose in kg × 7,700 kcal) / (daily deficit × 7)

Rate of Loss by Calorie Deficit

Weeks to Goal by Starting Deficit and Fat to Lose

Genetic variation meaningfully affects fat loss rate at identical calorie deficits. Variants in the FTO gene (present in approximately 43% of Europeans) are associated with 1.5–3 kg higher baseline body weight and modestly slower fat loss per unit of deficit, likely through effects on appetite hormone signalling and energy expenditure. MC4R gene variants reduce the anorexigenic response to leptin, making hunger suppression harder to maintain during a calorie deficit. Neither variant changes the physics of the calorie deficit; they affect how difficult it is to sustain one behaviourally. People who find deficit maintenance disproportionately difficult despite accurate tracking may have a genetic component that justifies a smaller, more sustainable deficit rather than a more aggressive one.

The 3,500 kcal per kg figure applies specifically to fat tissue. In practice, weight loss at a deficit always includes some lean tissue, water, and glycogen. In the first week of a deficit, scale weight loss is often 1.5–2.5 kg due to glycogen and water depletion (each gram of stored glycogen carries approximately 3 g of water). This is not fat loss and does not continue at the same rate. From week 2 onward, actual fat loss rate normalises to what the calorie deficit produces.

---

Five Steps to Your Calorie Target and Goal Date

1. Calculate your TDEE accurately before setting any deficit. The TDEE is the total you need to maintain your current weight. Use the Mifflin-St Jeor formula with your current measurements, not values from months or years ago. A 10% error in TDEE produces a 10% error in the resulting deficit, which at 500 kcal becomes a true deficit of 450 or 550 kcal depending on direction.

2. Select your deficit size based on how much you have to lose and your training schedule. More than 15 kg to lose: a 500–750 kcal deficit is appropriate and safe. Under 10 kg to lose: a 300–500 kcal deficit preserves more muscle and is more sustainable. Training 4+ days per week: stay at 400–500 kcal maximum to protect performance and recovery.

3. Check the calorie floor before committing. Subtract your deficit from your TDEE. If the result is below 1,200 kcal for a woman or 1,500 kcal for a man, the deficit is too large for safe sustained dieting. Reduce the deficit until the floor is met, even if this extends the timeline. A 1,200 kcal intake for a woman with a 1,600 kcal TDEE means a 400 kcal deficit, not the 500 kcal originally planned. Accept the adjusted timeline.

4. Set protein first within your calorie budget. At 1.6 g per kg of current body weight, protein calories are calculated and allocated before fats and carbohydrates. This is the most important single decision within the calorie budget for protecting lean mass. On a 1,500 kcal plan for a 70 kg person, protein at 1.6 g/kg is 112 g (448 kcal, 30% of the budget). The remaining 1,052 kcal is distributed between fat and carbohydrate based on preference.

5. Plan for recalculation, not just the starting number. Set a calendar reminder for every 5 kg of confirmed weight loss (not scale fluctuations). At each checkpoint, recalculate TDEE using the new weight and adjust the daily calorie target accordingly. Without this step, the deficit shrinks as body weight drops and progress slows to apparent plateau through arithmetic rather than true metabolic resistance.

Non-obvious insight: The first two weeks of a calorie deficit almost always produce faster scale weight loss than the deficit mathematics would predict, and weeks 3–8 almost always produce slower scale weight loss than expected. This is glycogen and water in the first phase, and normalisation in the second. A person who loses 2.5 kg in week 1 on a 500 kcal deficit has not lost 2.5 kg of fat. They have lost approximately 0.5 kg of fat and 2.0 kg of water. Expecting this pattern prevents premature abandonment of a plan that is actually working.

---

Two Weight Loss Plans, Fully Worked

Example 1: Woman Returning to Fitness Post-Pregnancy, Age 35

Priya is 158 cm, weighs 73 kg, and wants to return to her pre-pregnancy weight of 62 kg. She has an 18-month-old child, walks daily, and does two gym sessions per week. She wants a realistic timeline before committing to a plan.

BMR (Mifflin-St Jeor, female):
  = 10 × 73 + 6.25 × 158 − 5 × 35 − 161
  = 730 + 987.5 − 175 − 161 = 1,381.5 kcal

TDEE (Lightly Active, 1.375):
  = 1,381.5 × 1.375 = 1,900 kcal

Fat to lose: 73 − 62 = 11 kg
Deficit chosen: 450 kcal/day
Daily calorie target: 1,900 − 450 = 1,450 kcal

Floor check: 1,450 > 1,200 kcal minimum (safe).

Protein target (1.6 g/kg): 73 × 1.6 = 117 g/day

Weeks to goal (unadjusted): (11 × 7,700) / (450 × 7) = 27 weeks

Priya's actionable notes: 27 weeks is a realistic planning horizon, approximately 6.5 months from start. She should expect scale weight loss of 1.5–2.5 kg in the first 2 weeks (glycogen and water), then approximately 0.45 kg per week thereafter. At week 14, when she has lost approximately 5 kg, she should recalculate TDEE at 68 kg (new BMR approximately 1,341 kcal, new TDEE approximately 1,844 kcal) and reduce daily intake to approximately 1,394 kcal to maintain the 450 kcal deficit. Without this adjustment, her deficit will have eroded to approximately 356 kcal and her rate of loss will have slowed to 0.35 kg per week.

---

Example 2: Male Retiree Starting His First Structured Diet, Age 67

Bernard is 176 cm, weighs 96 kg, and has been told by his GP that reaching 82 kg would meaningfully reduce his cardiovascular risk. He walks 30 minutes daily and does no structured exercise. His goal is sustainable, not rapid.

BMR (Mifflin-St Jeor, male):
  = 10 × 96 + 6.25 × 176 − 5 × 67 + 5
  = 960 + 1,100 − 335 + 5 = 1,730 kcal

TDEE (Sedentary to Lightly Active, 1.30):
  = 1,730 × 1.30 = 2,249 kcal

Fat to lose: 96 − 82 = 14 kg
Deficit chosen: 500 kcal/day (conservative given age)
Daily calorie target: 2,249 − 500 = 1,749 kcal

Floor check: 1,749 > 1,500 kcal minimum (safe).

Protein target (1.2 g/kg for older adult): 96 × 1.2 = 115 g/day

Weeks to goal (unadjusted): (14 × 7,700) / (500 × 7) = 30.8 weeks

Bernard's protein target uses 1.2 g/kg rather than 1.6 g/kg because he is not resistance training; the higher end of the older adult recommendation (1.0–1.2 g/kg) is appropriate for his activity level. His actionable priority: at 1,749 kcal, the calorie budget is generous enough that dietary quality rather than calorie counting is his most practical tool. Replacing two ultra-processed food items per day with whole food alternatives typically reduces intake by 300–400 kcal without formal tracking. His GP appointment in 3 months gives him a natural recalculation checkpoint.

---

Where Fat Loss Plans Go Wrong

Setting the deficit from a TDEE that has not been updated in months or years. TDEE falls as weight drops. A TDEE calculated at 95 kg is not accurate at 82 kg. For every 10 kg of fat lost, resting metabolic rate drops by approximately 100–160 kcal per day. Using an outdated TDEE understates the true calorie intake relative to the current maintenance level, turning what looks like a 500 kcal deficit into a 340 kcal deficit or less. Recalculate at every 5 kg checkpoint.

Eating too little and triggering accelerated lean mass loss. Below 1,200 kcal (women) or 1,500 kcal (men), the body increasingly turns to protein from muscle tissue to meet energy needs, particularly if dietary protein is also low. At 1,000 kcal with 60 g of protein, lean mass loss can represent 40–50% of total weight lost. At 1,200 kcal with 120 g of protein and resistance training, lean mass loss falls to 10–15%. The calorie floor is not a suggestion; it is the threshold below which body composition outcomes diverge significantly from scale weight outcomes.

Relying on exercise alone without dietary adjustment. Running burns approximately 70 kcal per km for a 70 kg person. A 5 km run burns approximately 350 kcal — meaningful, but equivalent to one medium latte and a small muffin. Exercise is most effective for weight loss as a TDEE booster that expands the calorie budget, not as the primary deficit driver. Studies comparing diet-only to exercise-only weight loss at matched total deficits consistently show diet-only produces 2–3 times faster initial weight loss because food restriction is arithmetically more efficient than exercise addition for creating a given deficit.

Not accounting for the first-week glycogen drop when setting expectations. Losing 2 kg in the first week on a 500 kcal deficit is not 2 kg of fat. A 500 kcal daily deficit produces 0.5 kg of fat loss per week. The remaining 1.5 kg is water released from depleted glycogen stores (approximately 300–400 g glycogen × 3 g water per gram = 900–1,200 g water) plus general scale fluctuation. Setting expectations based on week-1 results leads to discouragement in weeks 3–6 when the rate normalises to the actual fat loss pace.

Changing the plan after two weeks because the scale is not moving. Scale weight fluctuates by 1–3 kg day to day depending on hydration, sodium intake, hormonal cycle phase, and bowel content. Two weeks is not long enough to distinguish a true absence of fat loss from normal weight fluctuation. A reliable minimum evaluation period is 4 weeks, using weekly weigh-in averages rather than daily readings. Most people who abandon a correctly calculated diet plan in week 2 or 3 are abandoning one that is actually working.

Ignoring diet breaks during extended fat loss phases. Continuous dieting beyond 10–12 weeks produces adaptive thermogenesis: unconscious reductions in NEAT of 100–300 kcal per day that partially offset the deficit. A 1–2 week period of eating at maintenance calories every 10–12 weeks reduces this adaptation by approximately 30–40%. Diet breaks do not erase progress; they preserve the metabolic rate needed to make the next dieting block effective.

---

Assumptions and Notes

• Margin of error: The Mifflin-St Jeor formula predicts TDEE within ±10% for approximately 80% of healthy adults. The 3,500 kcal per kg rule is a planning estimate; actual fat loss per kilogram of scale weight varies between individuals based on body composition, protein intake, and training status. Timeline projections should be treated as indicative rather than precise, and evaluated against actual 4-week average weight trends rather than week-to-week fluctuations.
• Professional disclaimer: Weight loss calorie targets from this calculator are based on Mifflin-St Jeor estimates and established dietary guidelines. They are for informational and planning purposes only and do not constitute medical advice. People with diabetes, metabolic disorders, eating disorder history, cardiovascular disease, or those who are pregnant or breastfeeding should consult a physician or registered dietitian before applying any calorie deficit plan from this calculator.

---

The Only Number That Actually Matters

Priya's 27-week timeline replaced the vague hope that she would reach her pre-pregnancy weight "sometime this year" with a specific date. Bernard's 1,749 kcal daily target was generous enough that he did not need to track anything — just identify and replace two ultra-processed food items per day.

Neither plan required perfection. Both required a specific number to start from.

Enter your current weight, goal weight, and activity level above to get your calorie target and goal date now.

---

Further Reading

• Calorie and TDEE Calculator: Calculate the Maintenance Calories Your Weight Loss Deficit Is Built On
• Macro Calculator: Set Your Protein, Fat, and Carbohydrate Gram Targets Within Your Daily Calorie Budget
• Protein Intake Calculator: Confirm Your Daily Protein Minimum Before Cutting Calories