A calorie deficit calculator for fat loss estimates your maintenance calories, called TDEE (total daily energy expenditure), from your age, sex, height, weight and activity level, then subtracts a deficit. A moderate deficit is usually 10 to 25 percent below maintenance, which for many adults is about 300 to 500 calories a day. Treat the result as a starting estimate, not a fact. The resting-burn formula is reasonably close for most adults, but the activity setting can swing the answer by a couple of hundred calories. Eat the number for two to three weeks, keep protein high, keep lifting, and adjust from your weight trend.

Last updated: October 2026

What the calculator is really doing

Almost every calorie deficit calculator runs the same two steps, whatever the website looks like.

Step one: estimate your resting burn. That's your BMR (basal metabolic rate), the calories your body uses just to keep you alive at rest. Most calculators use the Mifflin-St Jeor equation:

  • Men: (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
  • Women: (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161

Step two: multiply by an activity factor to get TDEE. The common multipliers are 1.2 for sedentary, 1.375 for lightly active, 1.55 for moderately active and 1.725 for very active.

Then it subtracts your deficit.

A worked example: a 35-year-old woman, 5'6" (168 cm) and 165 pounds (75 kg). Her BMR comes out to about 1,464 calories. At "lightly active," her TDEE is about 2,013. A 20 percent deficit puts her target near 1,610 calories a day.

That's the whole machine. Nothing in it knows anything about you beyond those five inputs.

The part almost nobody questions: the activity multiplier

The resting-burn formula is the more trustworthy half. A 2005 review in the Journal of the American Dietetic Association found Mifflin-St Jeor was the most reliable of the common equations, landing within 10 percent of measured resting burn for most, though not all, adults.

The multiplier is where the guessing happens, and most people don't realize how much of the answer rides on it.

Run the same woman through "moderately active" instead of "lightly active" and her TDEE jumps to about 2,269. That's roughly 256 extra calories a day, which is about half of a typical deficit, created by one dropdown. The categories are vague ("exercise 3–5 days a week" covers a 20-minute walk and a 90-minute training session), and they're tempting to read generously.

If a calculator ever gave you a number that didn't seem to work, no wonder. There's a second trap. Some people pick a higher activity level to account for their workouts, then also eat back the "calories burned" their watch or app reports for those same workouts. That counts the exercise twice.

My view: when you're torn between two activity levels, pick the lower one. If you're wrong, you'll find out in a few weeks because you're losing faster than planned, and adding calories back is a much nicer correction than discovering a month of eating at maintenance.

Fat loss and weight loss aren't the same number

A calculator tells you how to lose weight. It doesn't tell you what that weight will be made of. Two levers decide how much of the loss comes from fat rather than muscle, and neither appears in the formula.

Protein. Keeping protein high during a deficit helps you hold on to lean mass. The International Society of Sports Nutrition's 2017 position stand notes that higher protein intakes can help preserve lean mass while you're eating below maintenance. A common working range is about 0.7 to 1 gram per pound of body weight.

Resistance training. Lifting gives your body a reason to keep the muscle it has. Without it, a bigger share of the loss can come from muscle.

Deficit size matters here too. A bigger cut isn't more disciplined, it's just harder to sustain, and very aggressive deficits make it harder to train well and hold on to muscle. If you want to turn your calorie target into protein, fat and carb numbers, how to count macros for weight loss walks through the order.

Reading the result once you've used it

The calculator's number is a hypothesis. Your body supplies the test results.

Eat the target consistently for two to three weeks, weigh yourself a few times a week, and watch the weekly average rather than any single morning. Day-to-day weight moves with water, salt and digestion far more than with fat. Your maintenance also drifts as you lose, and tdee calculator for cutting and bulking covers how to re-find it from your own log.

Then adjust one thing at a time:

  • Average not moving after three honest weeks? Trim 100 to 200 calories, or add some activity, but not both at once.
  • Dropping faster than about 1 percent of your body weight a week? Add 100 to 200 calories back.
  • Moving at a pace you can live with on the way to your goal? Leave it alone.

Expect the target to drift over time. As your body gets smaller it uses a little less energy, so a deficit that worked early can turn into maintenance later. That isn't failure, it's the math catching up, and it's also why the old "3,500 calories equals one pound" rule overstates how much you'll lose over months. For the step-by-step on picking a number and steering it, see calories to eat to lose weight.

When a calculator isn't the right tool

Fair warning: these formulas were built for average, healthy adults. They're a poor fit if you're pregnant or breastfeeding, under 18, managing a medical condition or medication that affects weight, or have a history of disordered eating. In those cases, a doctor or registered dietitian should set your numbers, not a web form. And regardless of the formula, very low intakes (often cited as below about 1,200 calories a day for women or 1,500 for men) make it hard to get enough nutrients without medical supervision.

Where AllStrong fits

AllStrong works out your TDEE for you from your stats and activity level, using the same Mifflin-St Jeor calculation with an activity multiplier described above. It turns that into calorie and macro targets, and a progress bar fills in as you log through the day, so you can see where you stand before dinner instead of after it. The TDEE estimate and the live targets are on the free plan.

Logging is where most deficits succeed or quietly fall apart, so it helps when it's quick. You can type or say what you ate, like "turkey sandwich and an apple," and AllStrong breaks it into ingredients and estimates the calories and macros using USDA FoodData Central data. That's free too. Barcode scanning and photo meal logging are Premium features, and Premium is included free when your gym runs AllStrong. If you're still choosing a tracker, best calorie counting app covers what to look for.

The adjusting part depends on data, and data depends on showing up every day. Treating your daily log like a habit tracking app checkbox is what makes the two-to-three-week review possible. And if you're ever eating to build instead, macro calculator for muscle gain covers the surplus side.

Frequently asked questions

How big should my calorie deficit be for fat loss?

A moderate deficit of about 10 to 25 percent below your maintenance calories works for most healthy adults, which is often 300 to 500 calories a day. Smaller deficits are slower but easier to sustain and gentler on training and muscle. Larger ones are harder to stick with. Pick a size you can keep up for several weeks, then adjust from your weight trend.

Are online calorie deficit calculators accurate?

They're a reasonable starting point, not a precise answer. The Mifflin-St Jeor resting-burn formula most calculators use lands within about 10 percent of measured values for most adults. The activity multiplier adds more uncertainty, since the categories are vague. Use the output for two to three weeks, then let your actual weight trend tell you whether to adjust.

Why am I not losing weight in a calorie deficit?

Usually the deficit is smaller than the calculator suggested. Common causes are an activity level set too high, eating back exercise calories your watch reports, or untracked bites, drinks and cooking oil. Water retention can also hide fat loss for a week or two. Review your weekly average over three weeks before trimming 100 to 200 calories.

Does AllStrong calculate my calorie target?

Yes. AllStrong estimates your TDEE from your stats and activity level and turns it into calorie and macro targets, with a progress bar that fills as you log meals. Both the TDEE estimate and the targets are on the free plan. Typed and spoken meal logging are free as well, while barcode scanning and photo logging are Premium features.