Weight Loss Calculator
Plan a calorie deficit, see the week-by-week curve, and find out when you will reach your goal.
Your plan
Light exercise 1–3 days a week
Safe limit at your weight: 2 lb a week
This model recalculates your metabolism as you get lighter, which is why the curve below flattens. That flattening is real, and it is why a diet that worked for two months stops working in the third, not a failure of willpower.
This works out the calorie deficit needed to reach a goal weight, and roughly how long it will take.
It uses the standard 3,500-calorie-per-pound model and explains where that model stops being accurate.
How weight loss actually works
Weight loss comes down to a calorie deficit: eating less than you burn. A pound of body fat is conventionally treated as 3,500 calories, so a 500-calorie daily deficit gives roughly a pound a week.
That model is a useful approximation for the first weeks and it overestimates loss over months. The reason is that [TDEE](/health/tdee-calculator) falls as you do. A lighter body burns less at rest and less in motion, so a fixed intake produces a shrinking deficit.
This is why plateaus arrive without anything going wrong. The deficit that produced a pound a week at the start produces less at 15 lb down, and the calculation has to be redone rather than the effort increased.
The timeline stretches as you get lighter, because the same intake represents a smaller deficit against a falling expenditure.
What to enter
- Current and goal weight
- The difference sets the total. A goal of 1-2 lb a week is sustainable; faster costs more muscle.
- TDEE
- Your maintenance calories. Everything is measured against this, so an inaccurate TDEE makes the whole plan wrong.
- Daily deficit
- 500 gives about a pound a week. Larger deficits are harder to hold and cost more lean mass.
- Activity
- Exercise contributes to the deficit, though usually less than people expect. Diet moves it far more efficiently.
What this assumes
The 3,500-calorie rule assumes fat loss only. Early loss includes water and glycogen, which is why the first week is often dramatic and misleading.
It assumes TDEE stays constant, which it does not. Recalculate every 10-15 lb.
How to calculate your weight loss timeline
Find your maintenance, subtract a deficit, and treat the timeline as an estimate that will need revising.
- 3,500
- Calories conventionally attributed to a pound of fat
- daily deficit
- TDEE minus intake
Work out your TDEE. Choose the activity level conservatively. Overstating it is the most common reason a deficit does not produce results.
Subtract a sustainable deficit. 500 a day for about a pound a week. Aggressive deficits work faster and cost more muscle and adherence.
Estimate the timeline. 20 lb at 500 a day is about 20 weeks on the simple model. Expect it to run longer.
Recalculate every 10-15 lb. TDEE falls with body weight. Without adjusting, the deficit quietly shrinks to nothing.
See a worked example: why the timeline stretches
- Goal
- Lose 20 lb
- Deficit
- 500 calories a day
20 lb × 3,500 = 70,000 calories to lose.
At 500 a day, that is 140 days, or 20 weeks.
But TDEE falls as you lose. By 15 lb down, the same intake is no longer a 500-calorie deficit, so the real timeline runs longer.
The first week often shows 3-5 lb. Most of that is water and glycogen, not fat, and it does not continue.
About 20 weeks, and longer in practice
Frequently asked questions
1-2 lb a week, or roughly 0.5-1% of body weight, is the usual guidance. It preserves muscle and is sustainable.
Faster loss costs more lean tissue and is harder to maintain. Two things protect muscle in a deficit: adequate protein and continued resistance training.
Most often because TDEE has fallen with your weight. The intake that created a 500-calorie deficit at the start creates less now.
Adaptive thermogenesis adds a little more, reducing expenditure slightly beyond what weight loss alone predicts. Recalculate every 10-15 lb and adjust intake or activity.
It is a reasonable approximation short-term and it overestimates over months, because it assumes a fixed expenditure.
Use it for the first weeks and expect the real timeline to be longer. It is a planning tool, not a physical law.
Water and glycogen, mostly. Each gram of stored glycogen holds several grams of water, and a reduced-carbohydrate intake depletes both quickly.
It is not fat and it does not continue. It also reverses when you eat normally again, which is why the scale can jump after a single large meal.
Diet moves a deficit far more efficiently. A 30-minute run burns around 360 calories, which is roughly one large muffin.
Exercise still matters a great deal: it preserves muscle during the deficit, improves health independently of weight, and helps regulate appetite. It is just not the efficient lever for the calorie gap.
For total weight loss, the deficit is what matters. For what you lose, composition matters a lot.
Higher protein preserves muscle so that more of the loss is fat, and it is more satiating, which makes the deficit easier to hold. See the macro page for splitting the rest.
Problems people actually run into
Setting an aggressive deficit and abandoning it
A 1,000-calorie deficit loses two pounds a week on paper and is difficult to sustain, tends to trigger rebound eating, and costs more muscle.
A 500-calorie deficit held for six months beats a 1,000-calorie deficit held for three weeks. Choose the one you will still be doing in month four.
Not recalculating as you lose
Someone 20 lb lighter has a lower TDEE, so the original intake no longer creates the original deficit. Progress slows, and it looks like the plan stopped working.
It did not stop working; it succeeded and the numbers moved. Recalculate every 10-15 lb.
Results are estimates for general information only and are not professional financial, medical, or legal advice. Read our full disclaimer.
Last updated: September 4, 2026