Calorie Deficit Calculator

What a calorie deficit is

A calorie deficit means you burn more energy than you eat. Your body needs a certain number of calories every day just to keep your heart beating, your brain running and your temperature steady. That baseline is your basal metabolic rate, or BMR. Multiply it by how active you are and you get your maintenance calories, the amount that keeps your weight exactly where it is.

Eat less than maintenance and you lose weight. Eat more and you gain. There is no trick beyond that, which is why every diet that works, keto, intermittent fasting, low-fat, Mediterranean, works for the same underlying reason: it puts you in a deficit you can stick to.

The calculator uses the Mifflin-St Jeor equation, which is the formula dietitians and the NIH Body Weight Planner are built on. It needs four things: your sex, because men and women have different lean mass at the same height; your age, because metabolism slows gradually; your height and weight, which set your body's size; and your activity level, which is the biggest swing factor of all. Two people with identical bodies can differ by 1,000 calories a day in what they burn purely because of how much they move.

The output is a daily target, not a promise. Your real burn varies day to day with sleep, stress, hormones and how much you fidget. Treat the number as the middle of a range, not a rule.

Worked example: a 40-year-old woman at 180 lb

Take a woman who is 40 years old, 5'6" tall and 180 lb, doing light exercise one to three days a week. Here is the whole calculation:

• Weight in kilograms: 180 lb x 0.4536 = 81.6 kg
• Height in centimeters: 66 in x 2.54 = 167.6 cm
• BMR = (10 x 81.6) + (6.25 x 167.6) - (5 x 40) - 161 = 1,503 calories
• Light-activity multiplier: 1.375
• Maintenance: 1,503 x 1.375 = 2,067 calories a day
• Deficit for 1 lb a week: 500 calories
• Daily target: 1,567 calories

That target is well above the 1,200 calorie floor, so it is a reasonable place to start. Push the goal to 2 lb a week and the target drops to 1,067 calories, under the floor, and the calculator flags it. Drop her activity to sedentary and maintenance falls to 1,804, so the same 1 lb a week target becomes 1,304 calories.

Notice how much the activity setting moves things. The difference between sedentary and light activity is 263 calories a day, or about 2.3 lb a month. Choosing that dropdown honestly matters more than any other single input.

Worked example: a 35-year-old man at 220 lb

Now a man who is 35, 6'0" and 220 lb, training three to five days a week.

• Weight in kilograms: 220 x 0.4536 = 99.8 kg
• Height in centimeters: 72 in x 2.54 = 182.9 cm
• BMR = (10 x 99.8) + (6.25 x 182.9) - (5 x 35) + 5 = 1,971 calories
• Moderate-activity multiplier: 1.55
• Maintenance: 1,971 x 1.55 = 3,055 calories a day
• Target at 1 lb a week: 2,555 calories

He can lose 2 lb a week at 2,055 calories and still be comfortably above the 1,500 floor, so he has room to move faster than the woman in the first example. That asymmetry is real: bigger bodies burn more, so the same deficit bites less. It is also why two people on the same diet plan get different results.

The trap for him is the opposite one. A 3,055 maintenance number feels generous, so it is easy to eat back the deficit with a single takeout meal and a couple of beers, 1,200 calories without trying. The deficit only exists if the total stays under the line.

How fast is safe to lose weight

Guidelines from the CDC and most medical bodies put sustainable loss at 1 to 2 lb a week, or roughly 0.5 to 1% of body weight per week. Faster than that, and a growing share of what you lose is muscle and water rather than fat.

The CDC's guidance on losing weight is deliberately unglamorous: aim for slow and steady, and protect muscle by staying active. Two pounds a week is the ceiling most sources name for adults, and only for people with a lot of weight to lose.

One more thing to expect: the deficit you need shrinks as you lose weight. A 180 lb woman at 2,067 maintenance who drops to 160 lb now maintains on about 1,933. The same 1,567 calorie target that produced 1 lb a week now produces about 0.75 lb. Recalculate every 10 lb or so, or the loss quietly stalls.

Calories by activity level: a quick reference

The activity multiplier is the input people get wrong most often. Here is the same 180 lb, 5'6", 40-year-old woman at every level, with a 500 calorie deficit applied.

Read the spread: 1,304 to 2,356 calories a day for the same person, just from activity. Pick the level that matches a normal week, not your best week. If you train hard three days and sit the rest, that is light-to-moderate, not active.

And be honest about the top of the scale. "Extra active" is a roofer, a mover or a competitive athlete, not someone who lifts for an hour and then sits at a desk. Choosing it when it does not apply inflates your maintenance number and hides your real deficit.

Why the scale stalls even when you are in a deficit

Fat loss is not the same as weight loss. Body weight moves up and down by 2 to 5 lb within a single day because of water, food still in your gut, sodium, glycogen and hormones. In a 500 calorie deficit you are losing about 0.15 lb of fat a day, which is far smaller than normal daily water swings. That is why the scale can show nothing for two weeks and then drop 2 lb in three days.

Three things cause most stalls. First, under-counting: cooking oil, butter, sauces, drinks and "just a bite" are the usual culprits, and studies of self-reported intake find people underestimate by 20 to 30%. Second, unplanned activity dropping: when you eat less, you unconsciously move less, which can wipe out 100 to 200 calories of the deficit. Third, water retention from a high-salt meal, a hard workout or the week before a period.

The fix is not to cut further, it is to measure more carefully for two weeks and compare weekly averages rather than daily numbers. If your four-week average has not moved at all, drop your target by 100 to 150 calories or add 2,000 steps a day. Small adjustments beat drastic ones, because drastic ones get abandoned.

Protect your muscle while you cut

When you eat less, your body can burn fat or muscle, and which one it chooses depends mostly on two things: how much protein you eat and whether you lift weights. Lose weight without either and a meaningful share of the loss is lean tissue, which lowers your maintenance calories and makes the next pound harder.

A practical target is roughly 0.7 to 1 gram of protein per pound of body weight while dieting, spread across the day. For a 180 lb woman that is 125 to 180 grams, which is a lot, it means a protein source at every meal and a plan, not luck. Strength training two or three times a week, even 20 minutes, tells your body to keep the muscle it has. Walking is excellent for adding to your deficit without wrecking recovery.

The other lever is sleep. Short sleep raises hunger hormones, lowers the hormone that signals fullness, and reliably pushes people toward more calories the next day. The Dietary Guidelines for Americans cover the nutrition side in detail, and the WHO fact sheet on obesity and overweight sets out the public-health picture. Neither will tell you what to eat for dinner, that part is yours.

When a deficit is the wrong tool

A calorie deficit is the right answer for most people who want to lose fat. It is the wrong answer for some. If you are pregnant or breastfeeding, the goal is usually to maintain or even increase intake, not cut it. If you are under 18, you are still growing and should not be dieting without a pediatrician's guidance. If you have type 1 diabetes, kidney disease, thyroid disease or a history of an eating disorder, the plan needs to come from a clinician, not a calculator.

There is also a point where a deficit stops helping. Extended severe restriction can slow your metabolism, disrupt sleep and mood, reduce training performance and lead to binge-restrict cycles that undo months of work. If you have been dieting hard for months and feel exhausted, cold, and obsessed with food, that is a signal to take a maintenance break at your new maintenance calories for a few weeks, not to cut deeper.

Finally, a reminder about what this page is. Every number above is a reference estimate, not medical advice. The formula is standard and the arithmetic is exact, but it describes an average body, and you are not one. Use it to get a sensible starting target, then adjust based on what the scale and the mirror do over four weeks. If anything about your health changes while dieting, stop and talk to a professional.