Weight Loss / Gain Timeline Calculator

Project when you'll reach your goal weight using TDEE and a calorie deficit or surplus. Includes Mifflin-St Jeor BMR, activity multiplier, and a weight projection chart.

The weight loss and gain timeline calculator projects when you will reach your goal weight by combining your Total Daily Energy Expenditure (TDEE) with a chosen calorie deficit or surplus. TDEE is calculated from your Basal Metabolic Rate (BMR) using the Mifflin-St Jeor equation — the most validated formula for estimating resting energy needs — multiplied by an activity factor. The 3,500 kcal per pound rule (7,700 kcal per kg) is used to convert the daily deficit or surplus into weekly weight change. BMI Calculator and Daily Calories Calculator are companion tools for a complete picture of caloric health.

This calculator models weight change under a constant calorie deficit, which is a simplification: as body weight decreases, BMR decreases (smaller bodies need fewer calories), so the rate of loss slows over time. The projection chart accounts for this adaptive metabolic change by recalculating TDEE at each projected milestone. This makes the timeline more realistic than static calculators that assume the same rate of loss throughout the journey.

Enter your current weight, height, age, and sex — these are used to calculate your BMR using Mifflin-St Jeor.
Select your activity level — choose the option that best describes your typical week, not your aspirational level.
Enter your goal weight — the calculator works for both weight loss (deficit) and weight gain (surplus).
Enter your daily calorie target — or use the suggested deficit/surplus based on a 0.5kg/week rate of change.
Read your TDEE, daily calorie target, weekly projected loss/gain, and estimated date to reach your goal weight.

The weight loss timeline formula explained

BMR (Mifflin-St Jeor) = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5 (male) or −161 (female)

TDEE = BMR × activity multiplier (1.2 sedentary, 1.375 lightly active, 1.55 moderately active, 1.725 very active, 1.9 extremely active). Weekly weight change = (TDEE − Daily calorie intake) × 7 ÷ 7,700 kg (using 7,700 kcal = 1 kg body mass). The timeline recalculates BMR at each projected weight milestone to model the slowing metabolism effect.

Worked example: 85kg male, 175cm, 35 years, moderately active. BMR = (10×85) + (6.25×175) − (5×35) + 5 = 850 + 1,093.75 − 175 + 5 = 1,774 kcal. TDEE = 1,774 × 1.55 = 2,749 kcal. Goal: 75kg. Deficit: 500 kcal/day → intake 2,249 kcal. Weekly loss ≈ 0.45 kg → 10kg lost in approximately 22 weeks (5.5 months), accounting for metabolic slowdown.

Reading your weight loss timeline

Safe rates of weight loss and metabolic adaptation

The NHS and NICE recommend a rate of 0.5–1 kg (1–2 lb) per week as sustainable and safe. A 500 kcal/day deficit produces approximately 0.45–0.5 kg/week; a 1,000 kcal/day deficit produces approximately 0.9–1 kg/week. Deficits greater than 1,000 kcal/day are generally not recommended without medical supervision, as they significantly increase muscle loss, nutrient deficiency risk, and metabolic adaptation (the body reduces TDEE below what activity alone would predict). Very low calorie diets (<800 kcal/day) are a medical intervention, not a general weight loss strategy.

For weight gain (muscle building), a 250–500 kcal surplus per day produces approximately 0.25–0.5 kg/week — most of which will be fat unless accompanied by structured resistance training. The Protein Intake Calculator provides context on macronutrient targets to support muscle growth during a calorie surplus.

Health tips and best practices

Weigh yourself at the same time each morning after using the bathroom — daily weight fluctuates by 1–3 kg due to water and food mass; use a weekly average for trend tracking.
Expect a larger initial drop in the first 1–2 weeks (water weight loss from glycogen depletion and reduced inflammation) followed by a slower, steadier rate — don't revise your plan based on early results.
As you lose weight, recalculate your TDEE every 5–10 kg — your calorie needs will decrease and the deficit must be recalibrated to maintain the same rate of loss.
Protein intake of 1.6–2.2g per kg of goal body weight per day minimises muscle loss during a calorie deficit and increases satiety.
Diet breaks — eating at maintenance for 1–2 weeks every 8–12 weeks — can partially reverse metabolic adaptation and improve long-term adherence (MATADOR study, 2017).
Sleep deprivation increases ghrelin (hunger hormone) and decreases leptin (satiety hormone) — poor sleep directly undermines calorie control.

The Mifflin-St Jeor equation predicts BMR within ±10% for 82% of adults (Frankenfield et al., Journal of the American Dietetic Association, 2005).
Metabolic adaptation typically reduces TDEE by 5–15% beyond what weight loss alone predicts — this is why weight loss slows even when adherence is maintained.
Approximately 80% of people who lose weight regain it within 5 years; structured maintenance plans (weekly weigh-ins, continued tracking) significantly reduce regain risk.
A 500 kcal/day deficit over 52 weeks would theoretically produce ~24 kg of loss; real-world average is 8–12 kg due to adherence variability and metabolic adaptation.

Common mistakes to avoid

Over-estimating activity level — most people are sedentary or lightly active even if they exercise 3× per week; exercise burns fewer calories than most estimates suggest.
Eating back all exercise calories — calorie burn from exercise is often overestimated by 50–100%; consuming the full estimated burn negates the deficit.
Expecting linear progress — weight loss is not linear due to water retention, hormonal cycles, and glycogen fluctuations; judge progress by 2–4 week averages.
Stopping when weight plateaus rather than recalculating TDEE — plateaus usually indicate the body has adapted to a lower weight and the original deficit is no longer sufficient.

This calculator uses the Mifflin-St Jeor BMR equation (1990) and standard activity multipliers from the DRI (Dietary Reference Intakes) framework. The 7,700 kcal/kg conversion is an approximation — actual energy density of lost tissue varies with the ratio of fat to lean mass loss. This tool provides projections for general wellness planning only and is not medical advice. A calorie deficit should not be taken to an extreme without medical supervision. If you have a history of disordered eating, are pregnant or breastfeeding, have type 1 or type 2 diabetes, or are managing any chronic condition, consult a registered dietitian or your GP before making significant dietary changes.