The math behind a calorie deficit
This is the backbone of almost every weight-loss calculation you'll see, including the one behind ToolPico's own Weight Loss Calculator. It's not a law of physics carved in stone — real bodies also shift water weight, glycogen stores, and muscle mass — but it's a solid planning estimate that's used across nutrition science and clinical guidance alike.
The practical upshot: if you want to lose 1 lb a week, you need to eat about 500 kcal/day less than you burn. Want to go slower and gentler? A 250 kcal/day deficit gets you roughly 0.5 lb a week, which is easier to sustain and less likely to cost you muscle.
Why BMR and TDEE come first
BMR (basal metabolic rate) is the energy your body needs just to keep the lights on — breathing, circulation, cell repair — with zero movement. TDEE (total daily energy expenditure) multiplies BMR by an activity factor, from about ×1.2 for a sedentary desk job up to ×1.9 for people doing hard physical labor or training twice a day.
The Weight Loss Calculator estimates BMR with the Mifflin-St Jeor equation from your age, gender, height, and weight — and if you know your body fat percentage, it can switch to the more precise Katch-McArdle formula, which is based on lean body mass rather than total weight.
Once you know your TDEE, the deficit math is simple subtraction: eat below TDEE to lose, at TDEE to maintain, above it to gain.
Estimating your timeline
Here's a worked, hypothetical example — not a real case, just numbers to illustrate the formula. Say someone currently weighs 187 lb and wants to reach 165 lb, a difference of 22 lb. At a pace of 1 lb/week, that's roughly 22 weeks, or about 5 months. At a gentler 0.5 lb/week, it stretches to roughly 44 weeks, or about 10 months.
| Weekly pace | Daily deficit needed | Estimated weeks |
|---|---|---|
| 0.5 lb/week | ~250 kcal | ~44 weeks |
| 1.0 lb/week | ~500 kcal | ~22 weeks |
| 1.5 lb/week | ~750 kcal | ~15 weeks |
| 2.0 lb/week | ~1,000 kcal | ~11 weeks |
In practice, the real timeline is often a bit longer than the simple math suggests. As you lose weight, your TDEE drops too (a smaller body burns fewer calories), so the same calorie target produces a slightly smaller deficit over time. Plan for a few extra weeks of buffer rather than a hard deadline.
For people using GLP-1 medications like semaglutide (Ozempic/Wegovy) or tirzepatide (Mounjaro/Zepbound), the appetite-suppressing effect changes the equation somewhat — the calculator includes a dose-and-timeline reference table for that scenario, but any medication plan should be guided by a doctor, not a spreadsheet.
BMI, ideal weight, and macros: rounding out the picture
BMI (BMI = 703 × weight in lb ÷ height in inches²) is a quick, widely used screening number. A BMI of 18.5–24.9 is generally considered the normal range, but BMI can't tell the difference between muscle and fat, so it works best alongside an ideal-weight estimate — the calculator averages four classic formulas (Devine, Robinson, Miller, and Hamwi) for a more rounded picture.
Once you have a daily calorie target, splitting it into protein, carbs, and fat (macros) helps with fullness and preserving muscle while losing weight. A common starting split is roughly 30% protein / 40% carbs / 30% fat, though higher-protein or lower-carb splits are common alternatives depending on personal preference and goals.
Plug in your own numbers — BMR/TDEE, deficit and timeline, BMI, and macros — all in pounds, instantly.
Try the free Weight Loss Calculator →