The rule everyone learned
The classic rule says a pound of fat holds about 3,500 calories, so cutting 500 a day loses a pound a week. The energy math for a pound of fat is roughly right. The problem is assuming the loss continues in a straight line.
Why weight loss curves flatten
As you lose weight, your body needs fewer calories. A smaller body burns less at rest and less during movement. So a fixed calorie cut becomes a smaller and smaller deficit over time.
NIH researcher Kevin Hall and colleagues modeled this in a 2011 Lancet paper. Their rule of thumb for adults: each 10-calorie-a-day change in intake eventually changes weight by about 1 pound, with about half of that change in the first year and nearly all of it within about 3 years.
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- Your body burns energy even at rest. That's your BMR: about 1,528 kcal a day for a 35-year-old woman who is 5 ft 6 in and 180 lb.
- Daily movement multiplies it. Light activity is × 1.375, so 1,528 becomes about 2,101 kcal. That's your TDEE.
- Eat about 500 kcal less than your TDEE and you're in a deficit: around 1,601 kcal a day.
- That's roughly 1 lb a week at first. Loss slows as your body adapts, so treat the number as a starting point and adjust.
Two rules, one example
| A 500-calorie daily cut | 3,500-calorie rule | Dynamic model (approx.) |
|---|---|---|
| After 1 year | 52 lb | About 25 lb |
| After 3 years | 156 lb | About 50 lb, then levels off |
These are rough adult averages. The NIH Body Weight Planner runs the full model for your numbers.
Setting a deficit you can keep
- Estimate your needs with the TDEE calculator.
- Pick a modest cut. Many clinicians suggest aiming for about 0.5% to 1% of body weight a week.
- Keep protein high and do strength work to protect muscle.
- Track your trend for 2 to 4 weeks, then adjust.
- Re-run your numbers every 10 to 15 pounds.
The calorie deficit calculator shows a target and an estimated timeline.
Small swaps that add up
A deficit doesn't have to come from one big sacrifice. These approximate swaps each save around 100 to 250 calories:
| Instead of | Try | Saves (approx.) |
|---|---|---|
| 12 oz regular soda | Sparkling or still water | 150 |
| Large flavored latte | Small latte with low-fat milk | 150 to 250 |
| 2 tbsp creamy dressing | 1 tbsp vinaigrette | 100 |
| Fries with a meal | Side salad or vegetables | 200 to 300 |
| Second helping at dinner | Saving it for lunch tomorrow | 200 or more |
Two or three swaps a day can create a meaningful deficit without feeling like a diet. Values vary by brand and portion, so treat them as rough guides.
How low is too low
Very low-calorie diets, under about 800 calories a day, are normally used only with medical supervision. Eating far below your needs can cost muscle, energy and nutrients and raise the risk of gallstones.
On a GLP-1, appetite can drop so much that this happens by accident. The GLP-1 TDEE calculator shows a sensible floor.
Where GLP-1 drugs fit
GLP-1 drugs mostly work by making a deficit easier to keep. Trial average weight loss also leveled off after about a year, consistent with the same energy math. See your expected curve in the weight-loss projection.
Run the numbers
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Common questions
Is a 3,500-calorie deficit really a pound?
Roughly, for a pound of fat. But because calorie needs fall as you lose weight, a fixed daily cut produces less loss over time than the simple rule predicts.
How big should my calorie deficit be?
Many clinicians suggest a deficit that loses about 0.5% to 1% of body weight a week. Your doctor or dietitian can tailor it.
Why did my weight loss slow down?
Your calorie needs drop as you get smaller, so the same eating pattern creates a smaller deficit. Re-check your numbers every 10 to 15 pounds.