A calorie deficit means consuming fewer calories than your body burns in a day. That’s the only requirement for losing body fat, regardless of which diet you follow. The practical challenge is figuring out how large your deficit should be, how to create it without constant hunger, and how to sustain it long enough to reach your goal.
Calculate Your Starting Number
Before you can eat below your maintenance calories, you need a reasonable estimate of what maintenance looks like. The most widely used formula, the Mifflin-St Jeor equation, works like this:
- Men: (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) + 5
- Women: (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) – 161
This gives you your basal metabolic rate, the calories your body burns at complete rest just to keep your organs functioning, your blood circulating, and your cells alive. To get your total daily expenditure, multiply that number by an activity factor: 1.2 if you’re mostly sedentary, 1.375 if you exercise lightly a few days a week, 1.55 for moderate exercise most days, and 1.725 if you’re highly active. The result is your estimated maintenance calories.
For a 35-year-old woman who weighs 70 kg (154 lbs), stands 165 cm (5’5″), and exercises moderately, the math works out to roughly 2,050 calories per day. That’s her starting point, not a precise measurement. These formulas estimate within about 10% for most people, so treat the number as a baseline you’ll adjust over time based on what actually happens on the scale.
Choose a Deficit Size That Lasts
A deficit of 300 to 500 calories per day works for most people. That’s enough to produce visible results within a few weeks without leaving you miserable or triggering the kind of fatigue and irritability that makes people quit. You can create this gap by eating less, moving more, or a combination of both.
You may have heard that cutting 500 calories a day equals one pound of fat loss per week, based on the old rule that a pound of fat contains 3,500 calories. The American Institute for Cancer Research has flagged this as a myth. When researchers tested the rule against data from seven closely monitored weight loss studies, most participants lost significantly less weight than the formula predicted. The reason: your body isn’t a static machine. As you lose weight, your calorie needs drop, and your metabolism adapts. A 500-calorie daily cut might produce close to a pound per week initially, but the rate slows over time. The NIH offers a free online Body Weight Simulator that accounts for these changes and gives more realistic projections.
Track What You’re Actually Eating
Most people significantly underestimate their calorie intake. Research from Penn State found that women underestimated their portion sizes by about 21% when using visual estimation alone. That’s enough to erase a moderate deficit entirely. If you think you’re eating 1,800 calories but you’re actually eating 2,200, you’re not in a deficit at all.
A food scale removes the guesswork. Weigh your cooking oil, your rice before cooking, your peanut butter, and your snacks for at least the first few weeks. You don’t need to do this forever. Most people develop a reliable sense of portion sizes after a month or two of weighing, and can then shift to eyeballing with much better accuracy. Logging your food in an app helps you spot patterns: the mid-afternoon handful of nuts that adds 300 calories, the cooking oil that doubles the calorie count of a vegetable stir-fry, the “small” pour of wine that’s actually two servings.
Eat Foods That Keep You Full
The biggest threat to a calorie deficit is hunger, and your food choices determine how hungry you’ll be at any given calorie level. A landmark study published in the European Journal of Clinical Nutrition ranked 38 common foods by how full they kept people over two hours. The results were striking: boiled potatoes scored seven times higher than croissants for satiety, despite similar calorie counts in the portions tested.
Three properties predicted how filling a food was: water content (the strongest predictor), fiber content, and protein content. Fat content actually had a negative relationship with fullness. In practical terms, this means a large bowl of soup, a baked potato, a plate of grilled chicken with vegetables, or a bowl of oatmeal will keep you satisfied far longer than calorie-equivalent portions of pastries, chips, or cheese.
This doesn’t mean you need to avoid all calorie-dense foods. It means building your meals around high-volume, high-protein, high-fiber options and fitting smaller portions of richer foods around them. A plate that’s three-quarters vegetables, lean protein, and whole grains with a quarter reserved for something more indulgent is a practical framework that works at almost any calorie target.
Prioritize Protein
Protein does more than keep you full. It’s the key nutrient for preserving muscle mass while you lose fat. When you’re in a calorie deficit, your body doesn’t only burn fat for energy. It also breaks down some muscle tissue, especially if your protein intake is low. Losing muscle slows your metabolism, changes your body composition in unfavorable ways, and leaves you looking “soft” even at a lower weight.
Research published in the International Journal of Sport Nutrition and Exercise Metabolism recommends 1.6 to 2.4 grams of protein per kilogram of body weight per day during a calorie deficit. For someone weighing 70 kg (154 lbs), that’s 112 to 168 grams of protein daily. If you’re resistance training regularly, aiming toward the higher end of that range is worthwhile. For most people, hitting even the lower end requires deliberate effort: adding a protein source to every meal, choosing Greek yogurt over regular, keeping hard-boiled eggs or jerky on hand for snacks.
Use Exercise to Widen the Gap
You can create a calorie deficit through diet alone, but adding exercise gives you more flexibility. A 30-minute brisk walk burns roughly 150 to 200 calories for most people. That means you can eat 200 more calories and still maintain the same deficit, or keep your food the same and lose fat slightly faster.
Resistance training deserves special emphasis. It doesn’t burn as many calories per session as cardio, but it sends a signal to your body to hold onto muscle. Combined with adequate protein, lifting weights two to four times a week during a deficit is the most effective strategy for ensuring that the weight you lose comes primarily from fat rather than muscle. Cardio and walking are useful for burning extra calories and improving cardiovascular health, but they’re not substitutes for strength training when body composition matters to you.
Expect Your Body to Push Back
After several weeks in a deficit, your body starts adapting. This process, called adaptive thermogenesis, involves a cluster of hormonal shifts in insulin signaling, thyroid hormones, and appetite-regulating hormones like leptin. The net effect is that your body burns fewer calories than predicted for your new, lower weight. You also tend to move less without realizing it: fidgeting less, taking fewer steps, choosing the elevator over stairs.
This is normal biology, not a broken metabolism. It does mean that the deficit you calculated at the start will shrink over time, and weight loss will slow or stall. When that happens, you have two options: reduce your calorie intake slightly (by 100 to 200 calories) or increase your activity. Some people also benefit from periodic “diet breaks,” eating at maintenance for one to two weeks before returning to the deficit, which can help manage fatigue and restore some metabolic output.
Respect the Calorie Floor
Cutting calories aggressively might seem like a shortcut, but going too low creates real problems. Harvard Health Publishing recommends that women not drop below 1,200 calories per day and men not below 1,500 without medical supervision. Below those thresholds, it becomes very difficult to get adequate vitamins, minerals, and fiber. You’re also more likely to lose significant muscle mass, feel chronically fatigued, and eventually binge in response to the restriction.
A slower, more moderate deficit preserved over months will always outperform an aggressive crash that lasts three weeks before you rebound. If your calculated deficit puts you below those floors, the better strategy is to increase your activity level rather than cut food further.
Adjust Based on Real Results
No formula can perfectly predict your calorie needs. The most reliable approach is to pick a calorie target based on the math, follow it consistently for two to three weeks, and then evaluate. Weigh yourself daily at the same time (first thing in the morning) and look at the weekly average, since daily weight fluctuates by one to three pounds based on water retention, sodium intake, and digestion.
If your weekly average is dropping by 0.5 to 1% of your body weight, you’re in a good range. If it’s not moving, you’re likely eating more than you think or your maintenance estimate was too low. Reduce by 100 to 200 calories and reassess after another two weeks. If you’re losing faster than 1% per week consistently, you may be cutting too aggressively, and it’s worth adding some calories back to protect muscle mass and energy levels.