How do you calculate a calorie deficit?
To calculate a calorie deficit, find your maintenance calories with the Mifflin-St Jeor equation and an activity factor, then take off about 20%. For a sedentary 35-year-old woman who weighs 180 lb (81.6 kg), maintenance is about 1,796 calories. Her deficit is about 359 calories, and her eating target is about 1,440 a day.
A friend says she lost weight by "just eating in a calorie deficit." Sounds easy, until you hit a wall of apps and rules that don't agree. The good news is that the math takes a few minutes. 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.
What are the 3 steps to calculate a calorie deficit?
Find your maintenance calories, take off 20%, and check the safety floor. That's the whole method.
Step 1: Find your maintenance calories
Start with your BMR. Our calculators use the Mifflin-St Jeor equation. It was 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)
- 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. Your eating target is maintenance × 0.8.
Step 3: Check the floor
Our calculators stop at 1,200 calories a day for women or 1,500 for men. If your target lands below that, it gets raised to the floor.
That floor 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.
What does a calorie deficit look like with real numbers?
For a sedentary 35-year-old woman who is 5 ft 4 in and weighs 180 lb, the deficit is about 360 calories a day. Her eating target is about 1,440. In metric she is 162.6 cm and 81.6 kg. She has a desk job with little exercise.
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. Once you see it laid out, it is just a few lines of arithmetic.
Is a 20% cut better than a fixed 500 calories?
Yes, for most people. A percentage scales with your body. A fixed 500 calories is a much bigger bite for a smaller person.
"Just cut 500 calories a day" is a popular rule. But 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.
Why does the 3,500-calorie rule fall short?
It assumes weight loss runs in a straight line, and real bodies don't. As you lose weight, you burn a bit less, so loss slows over time.
The classic rule says 3,500 calories equals 1 pound (0.45 kg) of fat. By that math, the example woman's 359-calorie daily deficit adds up to about 2,514 calories a week. That is roughly 0.7 lb (0.33 kg) a week, forever.
A 2011 study in The Lancet, led by Kevin Hall at the National Institutes of Health, tells a different story. It showed that body weight responds slowly to a change in eating, with half times of about 1 year. So 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.
For a forecast that accounts for this slowdown, the NIH Body Weight Planner is built on Hall's model.
Should a deficit come from food, exercise, or both?
Both count. Your deficit can come from eating less, moving more, or a mix.
Here is a quick example using the 2024 Adult Compendium of Physical Activities. It is 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 in the research, the mix works best for most people: a modest cut in food, plus regular movement you actually enjoy.
How do you protect muscle in a calorie deficit?
Eat enough protein. When you eat less, some of the weight you lose can be muscle, and 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.
What if you're on a GLP-1 medicine?
A deficit often happens on its own, so the focus shifts to making each smaller meal count. Medicines like Ozempic, Wegovy, Zepbound, and Mounjaro often make people less hungry.
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: 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.
How do you adjust your calorie deficit over time?
Treat your number as a first guess and check it after 2 to 3 weeks. 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.
Check with your doctor or a registered dietitian before starting a deficit if you:
- are pregnant
- have diabetes or kidney disease
- take medicines
- have a history of an eating disorder
My take
I like the percentage approach because it is fair. A petite, sedentary woman and a tall, active man get cuts that feel similar. Neither one gets hit much harder.
Knowing that the 3,500 rule overestimates long-term loss helps a lot too. Here's the thing: a slower month is not failure. It is your body adapting, just like the research predicts.
The bottom line
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.