Losing weight feels unpredictable when you only step on the scale. A diet starts strong, the scale stalls for two weeks, and suddenly you wonder whether anything is working. This weight loss calculator turns the guesswork into a simple timeline: tell it your current weight, the goal weight you are working toward, and the daily calorie deficit you plan to hold, and it tells you roughly how many weeks stand between you and that goal.
The math is not magic. It is built on the well-known rule of thumb that about 3,500 calories of deficit equals roughly one pound of stored body fat. That means a steady 500-calorie-per-day gap — achieved through eating a little less, moving a little more, or both — tends to produce about one pound of loss per week. Adjust the sliders below and the timeline updates instantly, so you can see how a more aggressive or more patient plan changes the finish date.
How to Use
- Enter your Current Weight in pounds. Use a recent morning reading after using the bathroom, before eating, and with similar clothing on — consistency matters more than any single number.
- Type your Goal Weight. Pick a target that is realistic for your height, frame, and lifestyle rather than a number pulled from a celebrity photo. A modest, maintainable target beats an aggressive one you will rebound from.
- Set your Planned Daily Calorie Deficit. The classic starting point is 500 kcal/day, which lands near 1 lb per week. If you are already quite active and have a large amount to lose, 750–1,000 kcal/day may be reasonable; if you are close to your goal, 250 kcal/day is gentler.
- Read the result card: the total pounds to lose, the expected weekly pace, the weeks to reach your goal, and the equivalent in months.
- Check the Pace Check line. The National Institutes of Health suggests 0.5–2 lb per week as a sustainable range. If your chosen deficit pushes you above 2 lb/week, the calculator flags the plan as aggressive — faster loss usually comes from water, muscle, and unsustainable restriction rather than fat.
- Re-run the numbers as you go. If your real weekly loss runs slower than projected, you have either overestimated activity, underestimated intake, or your body has adapted; tweak the deficit and the timeline updates.
The Math Behind the Number
The engine uses the Wishnofsky rule, named after the 1958 study that first quantified the energy cost of a pound of body tissue:
weeks to goal = (current weight − goal weight) × 3,500 ÷ (daily deficit × 7)
Where:
- The numerator is the total pounds you still have to lose.
- 3,500 is the approximate calories stored in one pound of body fat.
- The denominator is your weekly deficit — daily deficit multiplied by seven.
Worked example: you weigh 200 lb and want to reach 180 lb. That is 20 lb to lose. At a 500 kcal/day deficit, the weekly deficit is 3,500 kcal, which equals one pound per week. Twenty pounds divided by one pound per week is 20 weeks, or about 4.6 months. That is the single most common result people see on this page, and it lines up with the CDC's recommendation that 1–2 lb per week is a safe, sustainable rate.
If you doubled the deficit to 1,000 kcal/day, the pace doubles to 2 lb/week and the timeline halves to 10 weeks — but the calculator flags that rate as aggressive, because very few people can sustain a 1,000 kcal/day deficit without extreme hunger, muscle loss, or rebound. The 500 kcal/day path is slower on paper but far more likely to still be running a year from now.
Why the Scale Lies (and Why This Calculator Is a Planning Tool, Not a Prediction)
Real weight loss is not a straight line. Water retention, sodium intake, menstrual cycles, sleep, stress, and new-exercise inflammation can hide fat loss for a week or two at a time. A 2023 review in the American Journal of Clinical Nutrition found that the 3,500 kcal-per-pound rule systematically overestimates long-term loss, because metabolism slows as body mass drops. That is why the timeline here should be treated as a planning estimate rather than a contract: use it to set expectations, then re-baseline every four to six weeks by averaging weekly weigh-ins.
Two practical rules keep the projection honest:
- Protein and resistance training. A deficit without strength work loses muscle along with fat, which lowers your resting metabolism and slows future progress. Aim for 0.7–1.0 g of protein per pound of target body weight and two full-body lifting sessions per week.
- Re-feeding days. A periodic higher-carb day every 7–10 days helps replenish glycogen and leptin, and reduces the biological drive to quit. It will not ruin a multi-week plan.
Examples
Example 1 — The classic 20-lb goal. A 34-year-old woman weighs 200 lb and wants to reach 180 lb. She sets a 500 kcal/day deficit through a smaller lunch plate plus a 30-minute walk. The calculator shows 20 lb to lose, 1.0 lb/week, and 20 weeks (about 4.6 months) to her goal. The Pace Check reads "sustainable," so she stays the course and re-weighs every Sunday.
Example 2 — A more aggressive plan. A 28-year-old man weighs 240 lb and wants to reach 200 lb (40 lb to lose). He sets a 1,000 kcal/day deficit. The calculator projects 2.0 lb/week and 20 weeks, but flags the pace as aggressive — at the edge of the NIH range. He trims back to 750 kcal/day, which yields 1.5 lb/week and 27 weeks, a timeline he can actually keep.
Example 3 — A gentle maintenance-phase cut. Someone at 185 lb wants to reach 175 lb (10 lb to lose) and chooses a modest 250 kcal/day deficit — roughly a soda and a half less per day, plus a short walk. The calculator shows 0.5 lb/week and 20 weeks. The pace is slow, but the plan requires almost no willpower and is the kind of change that stays in place after the goal is hit.
FAQ
How long does it take to lose 20 pounds?
At the standard 500 kcal/day deficit, which equals about 1 lb per week, losing 20 pounds takes roughly 20 weeks, or about 4.5–5 months. A more aggressive 1,000 kcal/day plan cuts that to about 10 weeks but is harder to sustain and is flagged as aggressive by this calculator.
Is 500 calories a day enough to lose weight?
A 500-calorie daily deficit (eating 500 calories below your maintenance, not eating only 500 calories total) is the most widely recommended starting point. Very-low-calorie diets of 500–800 calories total per day should only be used under medical supervision, because they carry risks of muscle loss, gallstones, and nutrient deficiencies.
How many calories do I need to cut to lose 1 lb a week?
About 3,500 calories of deficit per week, which works out to 500 calories per day. That can come from reducing food intake by 300 calories and burning an extra 200 through walking, lifting, cardio, or daily movement.
Why is my weight not dropping even though I am in a deficit?
Short-term stalls are normal. Water retention from a higher-sodium meal, a new exercise routine, poor sleep, or hormonal cycles can hide fat loss for one to three weeks. Weigh yourself once a day under the same conditions and look at the 7-day average rather than any single reading. If the average is flat for more than three weeks, re-check your intake tracking and activity.
What is a safe rate of weight loss?
The CDC and NIH recommend 1–2 lb per week. Slower loss of 0.5 lb per week is perfectly safe and often more sustainable, especially for people close to their goal. Losing faster than 2 lb per week without medical supervision usually means losing water and muscle, not just fat.
Should I eat back the calories I burn from exercise?
Usually only partially. If your deficit is built on food intake alone, adding exercise on top without refueling is fine. If you set a deficit assuming a hard workout that you then skip, your real deficit shrinks. Track both sides — intake and activity — for two weeks, and adjust the deficit number on this calculator to match what you actually do.
When will I see visible results?
Most people notice changes in how clothes fit within 2–4 weeks, and other people start to notice around 8–12 weeks, which corresponds to roughly 8–24 lb lost. Visible facial changes tend to appear later, once about 10% of body weight has been shed.
Disclaimer
This weight loss calculator is provided for informational purposes only and is not medical advice, diagnosis, or treatment. The 3,500 kcal-per-pound rule is a planning estimate, not a precise biological law, and actual results vary with age, sex, genetics, body composition, medication, sleep, stress, and adherence. Do not use this tool to design a very-low-calorie diet, to guide treatment of an eating disorder, or to set weight goals during pregnancy. If you have diabetes, high blood pressure, heart disease, kidney disease, are pregnant or breastfeeding, or plan to lose more than 50 lb, please consult a registered dietitian or your physician before starting.