A friend tells you she lost weight by "just eating in a calorie deficit." Sounds easy. Then you try to figure out your own number and hit a wall of apps, charts, and rules that don't agree. Here's the good news: a calorie deficit is one of the simplest ideas in nutrition. Once you see the math, you can work out your own number in a few minutes.
Let me show you exactly how, with a real example. Or jump straight to our calorie deficit calculator and let it do the work.
What is a calorie deficit?
A calorie deficit is the gap between the calories you eat and the calories your body burns. When you eat less than you burn, your body makes up the difference from stored energy, including body fat. Over time, that is what leads to weight loss.
Your daily burn has two big parts:
- BMR (basal metabolic rate): the energy you burn at rest, just to keep your body running.
- Activity: everything else, from walking to the car to a gym class.
Add them up and you get your maintenance calories, also called TDEE (total daily energy expenditure). Your deficit is measured from that number.
How to calculate your calorie deficit in 3 steps
This part is actually really easy. Three steps, one calculator, and you are done.
Step 1: Find your maintenance calories
Start with your BMR. Our calculators use the Mifflin-St Jeor equation, published in 1990 from lab measurements of 498 healthy adults.
- Women: (10 × weight in kg) + (6.25 × height in cm) - (5 × age) - 161
- Men: (10 × weight in kg) + (6.25 × height in cm) - (5 × age) + 5
Then multiply by your activity factor: 1.2 (sedentary), 1.375 (lightly active), 1.55 (moderately active), 1.725 (very active), or 1.9 (extremely active).
Step 2: Choose your deficit size
Our calculators take 20% off maintenance for weight loss. In math terms, your deficit is maintenance × 0.2, and your eating target is maintenance × 0.8.
Step 3: Check the floor
If your target lands below 1,200 calories a day for women or 1,500 for men, our calculators stop at that floor. That is a safety line, not a goal. If you land there, adding more daily movement is a gentler way to widen the gap than eating less.
A worked example, start to finish
When I ran the numbers for a 35-year-old woman who is 5 ft 4 in (162.6 cm), weighs 180 lb (81.6 kg), and has a desk job with little exercise (sedentary), here is what the formula gave:
Step 1, BMR:
- 10 × 81.6 = 816
- 6.25 × 162.6 = 1,016.25
- 5 × 35 = 175
- BMR = 816 + 1,016.25 - 175 - 161 = about 1,496 calories
Step 1, maintenance: 1,496.25 × 1.2 = about 1,796 calories a day
Step 2, deficit: 1,795.5 × 0.2 = about 359 calories a day
Step 2, target: 1,795.5 - 359.1 = about 1,436 calories a day (our calculator rounds to 1,440)
Step 3, floor check: 1,440 is above 1,200, so she is good to go.
That's it. Her calorie deficit is about 360 calories a day, and her eating target is about 1,440.
Why a percentage beats a fixed 500 calories
You have probably heard "just cut 500 calories a day." It is a popular rule. But here is something most people don't notice: 500 calories means very different things for different bodies.
| Person | Maintenance | 500-calorie cut is | 20% cut is |
|---|---|---|---|
| Woman in the example above | about 1,796 | 28% of her intake | about 359 calories |
| 45-year-old man, 5 ft 10 in (177.8 cm), 200 lb (90.7 kg), moderately active | about 2,787 | 18% of his intake | about 557 calories |
For her, 500 would be a steep cut. For him, it is fairly gentle. A percentage-based deficit adjusts to your size, so both people get a fair, similar-feeling cut. That is why our calculators use 20% instead of a fixed number.
The 3,500-calorie rule, and why it falls short
The classic rule says 3,500 calories equals 1 pound (0.45 kg) of fat. By that math, our example woman's 359-calorie daily deficit adds up to about 2,514 calories a week, or roughly 0.7 lb (0.33 kg) a week, forever.
This is where it gets interesting. Real bodies don't work in a straight line.
A 2011 study in The Lancet, led by Kevin Hall at the National Institutes of Health, showed that body weight responds slowly to a change in eating, with half times of about 1 year. As you lose weight, you also burn a bit less. So weight loss naturally slows over time, and the 3,500 rule overestimates how much you will lose in the long run.
None of this means a deficit stops working. It just means patience is part of the plan. If you want a forecast that accounts for this slowdown, the NIH Body Weight Planner is built on Hall's model.
Food, exercise, or both?
Your deficit can come from eating less, moving more, or a mix. Both sides count.
Here is a quick example using the 2024 Adult Compendium of Physical Activities, a research list of energy costs for over 1,000 activities. Brisk walking at 3.5 to 3.9 mph (5.6 to 6.3 km/h) has a MET value of 4.8. The math is MET × weight in kg × hours:
- 4.8 × 81.6 kg × 0.5 hours = about 196 calories for a 30-minute brisk walk
That number includes the calories she would have burned anyway just sitting, so the true extra burn is a bit smaller. Still, a daily walk adds up. Our guide on how many calories walking burns goes deeper.
Based on what I've seen, the mix works best for most people: a modest cut in food, plus regular movement you actually enjoy.
Protect your muscle while you're in a deficit
When you eat less, some of the weight you lose can be muscle. Protein helps you keep more of it.
The International Society of Sports Nutrition says 1.4 to 2.0 g of protein per kg of body weight a day is enough for most people who exercise. It also notes that strength-trained people may need even more during a calorie deficit to hold on to lean mass.
Our protein calculator gives you a daily target that fits your goal.
If you're on a GLP-1 medicine
Medicines like Ozempic, Wegovy, Zepbound, and Mounjaro often make people less hungry, so a calorie deficit can happen on its own. Sometimes the deficit gets bigger than planned.
A 2025 joint advisory from four US nutrition and obesity groups lists nutrient gaps from eating less, plus loss of muscle and bone, among the challenges of GLP-1 treatment. The takeaway for your plate is simple: make each smaller meal count, and put protein first. Our guide to protein on Ozempic covers how.
This guide is about food only. For your calorie target on a GLP-1 medicine, talk to your doctor or a registered dietitian.
What this means for you
Your calculated calorie deficit is a smart first guess, not a promise. Here is how to turn it into real progress:
- Eat at your target for 2 to 3 weeks.
- Weigh yourself a few mornings a week and watch the weekly average.
- The CDC notes that people who lose about 1 to 2 pounds (0.5 to 0.9 kg) a week are more likely to keep it off. If you are well below that, trim your target a little or add movement. If you are well above it, eat a bit more.
If you are pregnant, have diabetes or kidney disease, take medicines, or have a history of an eating disorder, check with your doctor or a registered dietitian before starting a deficit.
My take
I like the percentage approach because it is fair. It gives a petite, sedentary woman and a tall, active man cuts that feel similar, instead of hitting one of them much harder. And knowing the 3,500 rule overestimates long-term loss helps a lot. A slower month is not failure. It is your body adapting, just like the research predicts.
The bottom line
To calculate a calorie deficit, find your maintenance calories, take off about 20%, and check that you stay above the floor. Then give it a few weeks and adjust. You don't need a perfect number, just a good starting point. Get yours in seconds with our calorie deficit calculator, or read our guide on how many calories you should eat to lose weight for more examples.