Starting a calorie deficit means consistently eating fewer calories than your body burns, and the most reliable way to do it is to first estimate how many calories you actually need, then reduce that number by a moderate, sustainable amount. A deficit of 500 calories per day produces roughly one pound of weight loss per week, which falls within the CDC’s recommended pace of one to two pounds weekly. The process is straightforward, but the details matter if you want to lose fat rather than muscle and avoid the rebound that comes from cutting too hard.
Estimate How Many Calories You Burn
Your body burns calories through three channels: your resting metabolism (the energy it takes just to keep you alive), the energy used digesting food, and all physical activity, from workouts to fidgeting. Resting metabolism alone accounts for the largest share, which is why your size, age, and sex matter so much in any calculation.
The most widely recommended formula for estimating resting metabolism is the Mifflin-St Jeor equation. For men, it’s (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5. For women, the same formula applies but you subtract 161 instead of adding 5. To convert pounds to kilograms, divide by 2.2. To convert inches to centimeters, multiply by 2.54.
That number only covers resting metabolism. To get your total daily energy expenditure, you multiply by an activity factor: 1.2 if you’re mostly sedentary, 1.375 for light exercise a few days a week, 1.55 for moderate exercise most days, and 1.725 for intense daily training. The result is a rough estimate of the calories you burn in a full day. It won’t be perfect, but it gives you a starting point to adjust from.
Choose the Right Size Deficit
A 500-calorie daily deficit is the standard starting point for most people, producing about one pound of fat loss per week. If you have more weight to lose, a 750-calorie deficit can work, but going much beyond that raises the risk of muscle loss, hormonal disruption, and the kind of hunger that makes people quit after two weeks.
Research on low energy availability shows that cutting too aggressively can suppress testosterone in as little as five days, reduce thyroid hormone levels, weaken immune function, and increase injury risk. The clinical threshold where these problems tend to appear is below 30 calories per kilogram of fat-free mass per day. You don’t need to calculate that precisely, but it’s a useful reminder that your body has a floor below which it starts shutting down non-essential functions to conserve energy.
For most people, subtracting 500 from your estimated total daily expenditure and eating at that level for two to three weeks is enough to see whether the math is working. If you’re losing weight at roughly the expected rate, your estimate was close. If nothing is changing, your actual expenditure is likely lower than you calculated, and you need to adjust.
Account for Tracking Errors
One of the biggest reasons calorie deficits fail is that people underestimate how much they eat. A large study from the Women’s Health Initiative found that participants underreported their calorie intake by about 25% on average, with some groups underreporting by as much as 32%. That means someone who logs 1,800 calories may actually be eating closer to 2,250.
The most common sources of error are cooking oils, sauces, drinks, and portion sizes that creep upward over time. Weighing food with a kitchen scale, at least for the first few weeks, closes most of that gap. You don’t need to do it forever. The goal is to calibrate your eye so that when you eventually stop weighing, your estimates are closer to reality. Logging everything in an app, including the handful of nuts you grabbed while cooking, forces honesty in a way that mental tracking never does.
Prioritize Protein to Protect Muscle
When you eat fewer calories than you burn, your body doesn’t exclusively pull from fat stores. Research shows that 20 to 30% of weight lost through dieting alone comes from fat-free mass, which includes muscle tissue. Losing muscle slows your metabolism, makes you weaker, and changes your body composition in ways that make future weight loss harder.
The single most effective dietary lever for protecting muscle during a deficit is protein intake. A systematic review published in 2024 found that eating above 1.3 grams of protein per kilogram of body weight per day was associated with increased muscle mass, while intake below 1.0 gram per kilogram raised the risk of muscle loss. The practical range supported by the evidence is 1.2 to 1.6 grams per kilogram daily. For a 180-pound person (about 82 kg), that translates to roughly 100 to 130 grams of protein per day.
Protein also happens to be the most filling macronutrient, so eating more of it tends to make the deficit easier to sustain. Lean meats, fish, eggs, Greek yogurt, cottage cheese, and legumes are all dense sources. If you’re building meals around a protein source first and filling in carbs and fats around it, you’ll naturally hit a more sustainable ratio.
Use Fiber to Manage Hunger
Hunger is the main reason people abandon a calorie deficit, and fiber is one of the most practical tools for managing it. High-fiber foods are bulkier, which means they physically fill your stomach and trigger satiation signals earlier in a meal. Certain types of soluble fiber also form a gel-like consistency during digestion that slows the absorption of nutrients, keeping you feeling full longer after eating.
Most adults should aim for 25 to 30 grams of fiber per day, but the average intake in the U.S. is closer to 15. Vegetables, beans, lentils, whole grains, berries, and oats are all high-fiber foods that also happen to be relatively low in calories. Swapping refined grains for whole grains and adding a serving of vegetables to meals you already eat are two changes that meaningfully increase fiber without requiring a complete diet overhaul.
Add Resistance Training
Resistance training is the most effective method for preserving muscle during a calorie deficit. A meta-analysis in BMJ Open Sport & Exercise Medicine found that people who lifted weights while dieting lost the same total weight as those who only dieted, but a significantly larger proportion of what they lost was fat rather than muscle. The strength training group also got measurably stronger, even while eating at a deficit.
You don’t need an elaborate program. Two to four sessions per week covering major muscle groups (legs, back, chest, shoulders, arms) with progressively heavier loads is enough to send the signal that your body needs to hold onto its muscle tissue. If you’re new to lifting, even bodyweight exercises like squats, push-ups, and rows provide that stimulus initially.
One important finding from the same meta-analysis: the muscle-preserving benefit was strongest in interventions lasting less than five months, likely because adherence tends to drop off over longer periods. Consistency matters more than the perfect routine. Pick something you’ll actually do three times a week for the next several months.
Expect and Manage Metabolic Adaptation
As you lose weight, your body burns fewer calories than it did at your starting weight. Part of this is simple physics: a smaller body requires less energy to move and maintain. But your body also actively reduces energy expenditure beyond what the weight loss alone would predict, a process researchers call adaptive thermogenesis. Hormones related to thyroid function, insulin, and appetite all shift to conserve energy during sustained restriction.
This means a deficit that produced steady weight loss in month one may produce noticeably slower loss by month three. It doesn’t mean something is broken. It means you need to either slightly reduce calories, slightly increase activity, or accept a slower rate of loss. Making small adjustments of 100 to 200 calories every few weeks is more sustainable than making one large cut.
Diet breaks, where you eat at maintenance calories for one to two weeks, are another strategy some people use to partially reset these adaptive signals. The evidence on their effectiveness is mixed, but they can be psychologically helpful during a long cut by giving you a stretch where you aren’t actively restricting.
Practical Steps to Get Started This Week
If you want to put all of this into action, here’s a concrete sequence:
- Calculate your estimated daily expenditure using the Mifflin-St Jeor equation and an activity multiplier.
- Subtract 500 calories from that number to set your initial daily target.
- Set a protein floor of at least 1.3 grams per kilogram of your body weight per day.
- Track your food with a scale and an app for at least two to three weeks to calibrate your sense of portions.
- Weigh yourself at the same time each morning and look at the weekly average, not daily fluctuations.
- Start lifting weights two to three times per week, focusing on compound movements.
- Adjust after three weeks based on whether the scale is moving at roughly the rate you expected.
The first week often shows a large drop on the scale, sometimes three to five pounds. Most of that is water and gut contents, not fat. The real signal starts in weeks two and three, when the rate of loss stabilizes. If you’re losing one to two pounds per week at that point, your numbers are dialed in. If you’re losing faster than that, consider eating slightly more to preserve muscle and energy levels.