Most people lose weight steadily by eating about 500 fewer calories per day than their body burns, which typically produces a loss of roughly half a pound to one pound per week. But the exact number of calories you need depends on your size, age, sex, and how active you are. There’s no single magic number that works for everyone.
How a Calorie Deficit Works
Your body burns a certain number of calories every day just by existing: keeping your heart beating, your lungs breathing, your cells repairing themselves. On top of that, you burn calories through movement, exercise, and even digesting food. The total is called your total daily energy expenditure, or TDEE. When you consistently eat fewer calories than your TDEE, your body pulls energy from stored fat (and some muscle) to make up the difference. That’s a calorie deficit, and it’s the only mechanism behind every weight loss diet regardless of what foods are included.
You may have heard that 3,500 calories equals one pound of fat, so cutting 500 calories a day should drop exactly one pound per week. That rule has been repeated for decades, but it doesn’t hold up. When researchers tested it against seven closely monitored weight loss studies, participants consistently lost less weight than the formula predicted, and their weight loss slowed as the weeks went on. The rule fails because it treats your body like a static machine. In reality, as you lose even a pound or two, your body’s calorie needs drop slightly, which shrinks the deficit over time.
The same calorie cut also produces different results in different people. Men tend to lose faster than women on an identical deficit. Younger adults lose faster than older adults. Even within those groups, individual variation is significant.
Estimating Your Daily Calorie Needs
To find your deficit, you first need a reasonable estimate of how many calories you burn. Start with your basal metabolic rate (BMR), the calories your body uses at complete rest. The most accurate widely used formula is the Mifflin-St Jeor equation, which predicted resting metabolic rate within 10% of the actual measured value in more people (both at a healthy weight and with obesity) than any competing formula. It uses your weight in kilograms, height in centimeters, age, and sex.
For men: (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
For women: (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161
Once you have your BMR, multiply it by an activity factor to get your TDEE:
- Sedentary (desk job, little exercise): BMR × 1.2
- Lightly active (light exercise 1–3 days per week): BMR × 1.375
- Moderately active (moderate exercise 3–5 days per week): BMR × 1.55
- Very active (hard exercise 6–7 days per week): BMR × 1.725
A concrete example: a 35-year-old woman who is 5’5″ (165 cm), weighs 170 pounds (77 kg), and exercises lightly a few times a week would have a BMR of about 1,430 calories. Multiplied by 1.375, her estimated TDEE is roughly 1,965 calories per day. To create a 500-calorie deficit, she’d aim for about 1,465 calories daily.
How Low Is Too Low
There’s a floor you shouldn’t drop below. Harvard Health recommends that women eat no fewer than 1,200 calories per day and men no fewer than 1,500 calories per day unless supervised by a healthcare professional. Going below these thresholds makes it very difficult to get enough essential vitamins, minerals, and protein from food alone. It also increases the risk of muscle loss, fatigue, and a sharper metabolic slowdown that can stall your progress.
If a 500-calorie deficit would push you below those minimums, a smaller deficit of 250 to 300 calories is a safer starting point. The weight loss will be slower, around a quarter to half a pound per week, but it’s far more sustainable and easier to maintain over months.
Why Weight Loss Slows Down
Nearly everyone experiences a plateau, and understanding why makes it less frustrating. In the first few weeks, weight drops quickly because your body is burning through its glycogen stores (a form of carbohydrate stored in muscle and liver). Glycogen holds onto water, so when it’s used up, you lose a noticeable amount of water weight. This initial drop is real but temporary, and it exaggerates the pace you can expect going forward.
As you lose weight, three things work against you. First, a smaller body burns fewer calories at rest because there’s simply less tissue to maintain. Second, you lose some muscle along with fat, and muscle is more metabolically active than fat tissue. Third, your body undergoes what researchers call metabolic adaptation: hormonal shifts that reduce your resting metabolic rate beyond what the loss of tissue alone would explain. Levels of hormones that regulate hunger and metabolism shift in ways that slow calorie burning and increase appetite. Your nervous system also dials down energy expenditure in subtle ways, like making your movements slightly more efficient.
The practical result is that the same calorie intake that created your deficit in month one may only be at maintenance by month three or four. When the calories you eat equal the calories you burn, weight loss stops. At that point, you either need to eat slightly less, move more, or accept a slower rate of loss.
Protecting Muscle During a Deficit
Losing muscle is one of the biggest downsides of cutting calories, because muscle is what keeps your metabolism higher. Two strategies make the biggest difference: eating enough protein and doing some form of resistance exercise.
Research on athletes cutting weight suggests that 1.6 to 2.4 grams of protein per kilogram of body weight per day provides the best protection against muscle loss during a deficit. For someone weighing 170 pounds (77 kg), that works out to roughly 125 to 185 grams of protein daily. Going above 2.4 grams per kilogram doesn’t appear to offer additional benefit. You don’t need to be an athlete to apply this principle. Spreading protein across meals (rather than loading it all at dinner) helps your body use it more effectively for muscle repair.
Strength training, even two to three sessions per week, sends a strong signal to your body to hold onto muscle tissue. Without it, a greater proportion of the weight you lose will come from lean mass rather than fat.
Tracking Calories Accurately
Calorie counting only works if the numbers are reasonably close to reality, and most people underestimate what they eat. The good news is that tracking tools have gotten significantly better. A recent meta-analysis of 47 trials found that calorie tracking apps using AI-powered photo recognition estimated intake within about 2% of actual values on average. That’s a meaningful improvement over manual text entry, which was off by roughly 7%. On a 2,000-calorie diet, that’s the difference between being off by about 42 calories versus 146 calories per day.
The biggest source of error isn’t the food database itself. It’s portion size. People consistently misjudge how much they’re actually putting on their plate. Apps that use camera-based depth estimation cut portion size errors by more than half compared to selecting a serving size from a dropdown menu. If you’re going to track, using a food scale for the first few weeks can recalibrate your sense of what a portion actually looks like. Most people find that after a few weeks of measuring, they can eyeball portions with reasonable accuracy.
Putting It All Together
A practical starting point: calculate your TDEE using the Mifflin-St Jeor equation and an honest activity multiplier, then subtract 500 calories. Check that the result doesn’t fall below 1,200 (women) or 1,500 (men). Prioritize protein at each meal, aiming for at least 1.6 grams per kilogram of body weight. Add some form of resistance training to preserve muscle.
Expect faster results in the first two to three weeks (mostly water), then a more gradual pace of half a pound to one pound per week. If weight loss stalls for more than two or three weeks, your TDEE has likely decreased enough that your original deficit has shrunk. Recalculating at your new weight, adding a bit more activity, or trimming another 100 to 200 calories can get things moving again. The people who succeed long-term treat this as an ongoing adjustment, not a one-time calculation.