Weight Loss Calculator

How Weight Loss Works

Weight loss comes down to one principle: energy balance. When you consume fewer calories than your body burns, it taps into stored energy (primarily body fat) to make up the difference. This is called a calorie deficit.

It takes a deficit of roughly 7,700 kcal to lose 1 kg of body fat (or 3,500 kcal per pound). So a daily deficit of 500 kcal produces about 0.45 kg (1 lb) of fat loss per week — and a 1,000 kcal deficit doubles that to ~0.9 kg (2 lbs) per week.

Your body burns calories through three channels: Basal Metabolic Rate (BMR) — the energy needed to keep organs, brain, and cells running at rest (60–75% of total burn); the Thermic Effect of Food (TEF) — energy used digesting meals (~10%); and Physical Activity — everything from walking to intense exercise (15–30%). Together, these make up your Total Daily Energy Expenditure (TDEE).

Safe Weight Loss Pace

Not all deficit sizes are equally sustainable. A smaller deficit is easier to maintain, preserves more muscle mass, and is less likely to trigger metabolic adaptation. Here's a breakdown of common weight loss paces:

Medical guidelines generally recommend losing no more than 0.5–1 kg per week. Going faster increases the risk of muscle loss, nutrient deficiencies, gallstones, and metabolic slowdown. Never eat below 1,200 kcal/day (women) or 1,500 kcal/day (men) without medical supervision.

Why Weight Loss Slows Down

Adaptive thermogenesis is your body's survival mechanism. As you lose weight, your body becomes more efficient — burning fewer calories at rest and during activity. Studies show metabolic rate can drop by 10–15% beyond what weight loss alone would predict. This is why the last few kilograms feel so much harder than the first.

Water weight fluctuations are the biggest source of day-to-day confusion. Starting a diet often causes a dramatic initial drop (2–3 kg in the first week) as your body depletes glycogen stores, each gram of which holds 3–4 grams of water. Conversely, a high-sodium meal or carb-heavy day can add 1–2 kg overnight — none of which is fat.

The plateau effect: As you get lighter, your TDEE decreases because there's less body mass to fuel. A deficit that worked at 90 kg might be maintenance at 75 kg. This is why recalculating your calorie target every 4–6 weeks is essential for continued progress.

Worked Example: 88 kg to 72 kg

Marta is 38, stands 168 cm, weighs 88 kg and wants 72 kg. She trains three to five times a week and eats 1,700 kcal. Here is every step the calculator takes, with nothing rounded away.

Twenty-four weeks from today, and the tool prints a date. That date is wrong, and the reason has nothing to do with willpower. Line 2 holds Marta's TDEE at 2,447 kcal for the whole run, but 2,447 was the maintenance of an 88 kg woman. She stops being one in week two.

Feed each week's new weight back into the same Mifflin-St Jeor equation and the deficit shrinks on its own. At 80 kg her maintenance is 2,323 kcal, so the same 1,700 kcal plate buys a 623 kcal deficit instead of 747. At 72 kg it buys 499 kcal, and her weekly loss has fallen from 0.68 kg to 0.45 kg without her eating a single extra bite.

Through week 12 the three columns sit within 600 g of each other, which is less than a normal Tuesday-to-Wednesday swing. Past week 12 they separate fast. The third column applies an 8% haircut to expenditure from week 13 onward, the sort of gap Leibel, Rosenbaum and Hirsch (NEJM, 1995) measured when energy expenditure after a 10% weight loss ran below what the lighter body predicted. That haircut alone pushes Marta's finish from week 29 to week 37 — two more months for the same 16 kg.

Why Week One Beats the Maths

Marta's arithmetic says 0.68 kg in week one. Her scale will probably say 2 kg, and she'll believe the diet is working twice as well as it is.

Glycogen explains most of the gap. The liver and muscles hold roughly 400–500 g of it in a well-fed adult, and every gram holds 3–4 g of water with it. Cut carbohydrate and calories at once, drain that store, and 1.5–2 kg leaves the body before any real fat has moved. Lower insulin also nudges the kidneys to shed sodium and the water travelling with it.

None of it is fake — the scale genuinely reads lower — but it's a one-off withdrawal from an account you can only empty once. Week three arrives, the glycogen deficit stabilises, and the loss drops to the rate the equation predicted all along. Plenty of people quit right there, convinced something broke. Nothing broke. The first two weeks just paid out early. What water weight does to the scale covers the same effect running in reverse after a salty meal.

How Fast Should You Actually Lose?

Absolute targets like "half a kilo a week" ignore how big the person is. A percentage of body weight travels better. The usual working range is 0.5–1% of body weight per week, and Marta's 0.68 kg sits at 0.77% of her 88 kg — comfortably inside it. The same 0.68 kg on a 60 kg frame would be 1.1%, past the top of the band.

The band tightens as you get lighter, which is the part most people miss. Marta's 0.68 kg is fine at 88 kg and becomes 0.94% of body weight by the time she reaches 72 kg — the same absolute loss, now near the ceiling. Someone with a lot to lose can run the top of the range for months; someone chasing the last five kilos cannot.

Speed costs lean tissue. Garthe and colleagues (International Journal of Sport Nutrition and Exercise Metabolism, 2011) put athletes on either 0.7% or 1.4% of body weight per week, both lifting weights throughout. Both groups reached similar leanness; the slow group added lean mass over the study while the fast group did not. Same destination, different body arriving. If you're down to the final stretch, losing the last 5 kg explains why the percentages force a slower pace there.

Plateaus, and What Actually Breaks Them

A plateau is four weeks of flat average weight while you believe you're still in a deficit. Three weeks isn't one — that's noise. Once you're past four, one of four things is happening, and they need different fixes.

• The deficit closed by itself. Marta's dropped 33% between 88 kg and 72 kg with her intake unchanged. Fix: recalculate at every 4–5 kg lost, not when progress stops.
• Intake crept up. Portions drift, weekends stop making it into the log, oil goes in the pan unmeasured. Self-reported intake runs well under actual intake in metabolic-ward studies, and the gap widens the longer you've been dieting. Fix: weigh food for one honest week before changing anything else.
• Movement fell. Non-exercise activity — fidgeting, walking, standing — drops during energy restriction without any decision on your part. Fix: a step floor, checked daily. It's the one component you can see.
• Water is masking fat loss. New training, high sodium, poor sleep and menstrual-cycle timing each add 1–2 kg of retained fluid that can hide a fortnight of genuine progress. Fix: wait, and use the weekly average.

What rarely works: cutting another 300 kcal on top of a deficit you've already been running for months. That's the reflex, and it deepens whichever problem is already there. Recalculating your target from your current weight fixes the first cause, and honest logging fixes the second. Breaking a weight loss plateau goes further into diagnosing which one you've got.

The Part the Timeline Can't Show You

Every projection on this page counts kilograms. None of them say which tissue leaves. Marta reaching 72 kg having kept her muscle and one reaching 72 kg having lost 4 kg of it are the same number on the scale and two different outcomes — different shape, different strength, and a different maintenance calorie budget to hold afterwards.

Two things decide the split, and neither is the size of the deficit.

• Protein at 1.6–2.2 g per kg of body weight. For Marta at 88 kg that's 141–194 g a day. Morton and colleagues (British Journal of Sports Medicine, 2018) pooled 49 trials and found gains from resistance training plateauing around 1.6 g/kg/day in people eating enough. In a deficit the requirement sits at the upper end, because protein is defending tissue rather than just building it. Run your own figure through the protein calculator.
• Resistance training two to four times a week. Lifting tells the body it still needs that muscle. Longland and colleagues (American Journal of Clinical Nutrition, 2016) ran young men through four weeks of steep restriction with hard training; the group on 2.4 g/kg protein gained lean mass while the 1.2 g/kg group lost it. Cardio alone doesn't send that signal.

This matters more the faster you're moving. Appetite suppression from GLP-1 medication makes hitting a protein target harder precisely when protein is doing the most work — muscle loss on GLP-1 medication covers what the trial data shows about lean-mass loss, and how much protein you need per day works through the daily numbers.

Calorie Floors, and When to Stop Doing This Alone

The calculator flags intakes under 1,200 kcal for women and 1,500 kcal for men. Those aren't magic thresholds — they're the point below which hitting protein, fibre, iron and calcium from food becomes difficult in practice, and the point where the common advice stops applying to you.

There's a second trap the floor doesn't catch. A goal weight that demands a deficit larger than about 1,000 kcal a day is telling you the goal is too far away for the timeline you picked, not that you need to eat less. Marta wanting 72 kg by week 12 rather than week 29 would need to eat around 900 kcal, which no floor should let her do.

Talk to a doctor or a registered dietitian before continuing if any of these apply:

• The target intake this calculator produces falls below 1,200 kcal (women) or 1,500 kcal (men).
• You're pregnant, breastfeeding, under 18, or over 65.
• You take medication for diabetes, thyroid conditions, or blood pressure — a shrinking body changes what those doses do.
• You have a history of disordered eating, or tracking numbers makes you anxious.
• Weight is moving against a genuine deficit held for six weeks with weighed food.

Tips for Sustainable Weight Loss

• Prioritize protein: Aim for 1.6–2.2 g per kg of bodyweight daily. Protein preserves muscle during a deficit, increases satiety, and has a higher thermic effect than carbs or fat.
• Track consistently: You don't have to track forever, but logging meals for a few weeks builds awareness of portion sizes and calorie-dense foods that cause overshoot.
• Weigh yourself daily, judge weekly: Daily weight fluctuates due to water, sodium, and digestion. Use the weekly average to assess real progress.
• Strength train: Resistance training is the single best tool for preserving muscle mass during weight loss. Even 2–3 sessions per week makes a significant difference.
• Sleep 7–9 hours: Poor sleep increases ghrelin (hunger hormone) by up to 28% and decreases leptin (satiety hormone), making your deficit feel much harder than it needs to be.
• Take diet breaks: Eating at maintenance for 1–2 weeks every 8–12 weeks of dieting can restore leptin levels, reduce fatigue, and improve long-term adherence.
• Don't drink your calories: Liquid calories (juice, soda, alcohol) don't trigger satiety signals the way solid food does. Swapping to water, coffee, or tea can save 200–500 kcal/day effortlessly.

Weight Loss Timeline Questions People Actually Ask

• How long does it take to lose 10 kg?
  At 0.5 kg a week, 20 weeks. At 0.75 kg a week, about 14. Both assume the deficit holds, and it won't: 10 kg off an 88 kg body drops maintenance by roughly 120 kcal a day, so the real answer runs 15–25% longer than the straight-line one.
• Why is the calculator's date optimistic?
  It projects one weekly loss rate forward forever. Marta's rate falls from 0.68 kg to 0.45 kg between 88 kg and 72 kg because a lighter body burns less, so her true finish is week 29 rather than week 24, and week 37 once adaptive thermogenesis enters the arithmetic.
• Is losing 1 kg a week realistic?
  Only above about 100 kg, where 1 kg is 1% of body weight. At 70 kg the same kilogram is 1.4% per week, which needs a deficit near 1,100 kcal a day and costs lean tissue. Set the target as a percentage of what you weigh now.
• I lost 2.5 kg in week one, then almost nothing. What happened?
  Glycogen. A well-fed adult stores 400–500 g of it with 3–4 g of water attached to each gram, and the first week of a deficit drains most of that. Weeks three and four show the actual fat-loss rate, which is why the average of a full week beats any single morning.
• What breaks a plateau?
  Recalculating your target from your current weight fixes the most common cause, which is a deficit that quietly closed as you got lighter. Weighing food for one honest week fixes the second. Cutting another 300 kcal on top of an old target usually fixes neither.
• How low can I set my calories?
  Below 1,200 kcal for women or 1,500 kcal for men, hitting protein, fibre and micronutrients from food gets hard, and this calculator flags it. A goal that demands more than a 1,000 kcal daily deficit means the deadline is too tight, not that the intake should drop.
• Will I lose muscle along with fat?
  Some, unless you defend it. Protein at 1.6–2.2 g per kg of body weight plus two to four resistance sessions a week is what shifts the split toward fat. Longland and colleagues (2016) had the high-protein group gain lean mass during a steep deficit while the lower-protein group lost it.
• How often should I recalculate?
  Every 4–5 kg lost, or every 4–6 weeks, whichever comes first. Marta's deficit shrinks by about 62 kcal for every 4 kg she drops, so a target left alone for three months is no longer the target she set.