Week {label}
{rows.map(([key, color, lbl]) => { const entry = payload.find(p => p.dataKey === key); if (!entry) return null; return ({lbl}: {fmt(entry.value)}
); })}{hint}
)} {warning && (Honest weight loss forecasting — a range of realistic outcomes, not a fake straight line.
Your details
Sex
Goal weight must be less than current weight
)}Weight over time — 5,000 simulations (faint lines = individual journeys)
What this means: Losing {totalLossDisplay} at a{" "} {params.dailyDeficit} kcal/day deficit with {params.adherence}% adherence — half of simulated people reach their goal by{" "} week {result.weeks.median}, and 80% get there by{" "} week {result.weeks.slow}.{" "} Try lowering adherence to 60% to see how much uncertainty grows.
Statistical estimates only. Not medical advice. Consult a healthcare professional before starting any weight loss programme.
) : (Most weight loss calculators give you one date and pretend it's certain. HonestSlim shows the honest spread — a fan of 5,000 simulated journeys so you can see best case, most likely, and realistic worst case all at once.
{/* How it works */}Every weight loss calculator you've ever used gives you one date: "At a 500 kcal/day deficit you'll reach your goal in 14 weeks." That date is almost always wrong — not because the maths is bad, but because it assumes perfect consistency every day, a metabolism that never adapts, and a scale that moves in a straight line. None of those things are true.
HonestSlim uses a Monte Carlo simulation — the same technique used by financial analysts and climate modellers — to treat weight loss as the noisy, variable process it actually is. It runs 5,000 independent simulated journeys, each with its own random week-to-week adherence swings, water retention noise, and metabolic slowdown. The result is a fan chart: a spread of outcomes from best case to realistic worst case.
The fan chart shows five percentile lines. The median (50%) is your most likely timeline — half of all simulated journeys reach goal faster, half take longer. The best-case (10%) line shows how fast things can go with a consistent run. The slow (90%) line is the realistic outer limit: only 1 in 10 journeys takes longer than this. The 30 faint individual lines cutting through the fan are actual simulated journeys — you can see real plateaus, rebounds and zigzags, not a theoretical smooth curve.
The Adherence slider is the most important input on the page. Set it to 95% and the fan stays narrow — you'll be close to your plan nearly every week. Drop it to 60% and watch the fan widen dramatically. No other weight loss calculator asks this question. It's also the most honest one: most people aren't perfectly consistent, and this slider lets you model that upfront rather than discovering it six months later.
The calorie model uses the Mifflin-St Jeor equation — the most widely validated BMR formula in clinical use — recalculated every single week as your weight changes. This is what causes the fan to visibly flatten and slow in the second half: metabolic adaptation is real, and the model captures it.{" "} Source: Mifflin MD, St Jeor ST, et al. (1990). "A new predictive equation for resting energy expenditure in healthy individuals." American Journal of Clinical Nutrition.
{/* FAQ */}{a}