What is a calorie deficit?

A calorie deficit means eating fewer calories than your body burns each day. When you take in less energy than you use, your body draws on stored fat to cover the difference, and you lose weight. A deficit of about 500 calories per day produces roughly half a kilogram (one pound) of fat loss per week for most adults.

That single idea is the engine behind almost every diet ever invented — low-carb, keto, intermittent fasting, calorie counting, portion control. They are all just different ways of arriving at the same place: taking in less energy than you spend. The calculator above does the math for you, but understanding why it works is what keeps the weight off after it comes off.

How this calculator works

The tool runs a two-step calculation. First it estimates your Total Daily Energy Expenditure (TDEE) — the total number of calories you burn in a day, including sleeping, digesting, walking, and training. Then it subtracts your chosen deficit from that number to give your daily target.

To estimate the calories your body burns at rest, we use the Mifflin-St Jeor equation, which nutrition researchers consider the most accurate of the widely used formulas for healthy adults. It looks at your age, sex, height, and weight, then multiplies the result by an activity factor based on how often you train:

The most common mistake people make here is overestimating their activity. If you sit for most of the day but train hard three times a week, you are moderately active — not very active. When in doubt, pick the lower level. It is far easier to add calories back later than to wonder why the scale will not move.

How big should your calorie deficit be?

The right deficit is the largest one you can hold onto without feeling deprived, losing energy, or losing muscle. For most people that lands between 250 and 750 calories per day. Bigger is not better — an aggressive deficit strips away muscle alongside fat and rarely survives contact with real life.

Notice that even the “ambitious” setting tops out at 750 calories. There is a reason the calculator will not let you go higher, and it matters enough to have its own section.

Why we cap your target at 1,200 or 1,500 calories

If your chosen pace would push your daily target below 1,200 calories for women or 1,500 calories for men, the calculator automatically raises it back to that floor and shows you a warning. These are the minimums generally recommended by dietitians and public-health bodies for adults losing weight without medical supervision.

Eating below those levels day after day tends to backfire. It becomes very hard to get enough protein, vitamins, and minerals; hunger and fatigue climb; and your body defends itself by slowing your metabolism and shedding lean muscle — the exact tissue that keeps your metabolism high in the first place. The result is a plan that feels miserable and stops working within weeks.

If the calculator tells you that reaching your goal weight would require going below the floor, that is useful information, not a dead end. The sustainable path is to lower your deficit, extend your timeline, or — the option most people overlook — raise your TDEE by moving more, so you can eat more while still losing fat. Very aggressive weight loss belongs under the care of a doctor or registered dietitian, not a self-directed crash diet.

Why you might not be losing weight in a deficit

If the scale has stalled even though you believe you are eating in a deficit, one of a handful of things is almost always happening:

Your intake has crept up

Untracked bites, drinks, cooking oils, and weekend meals add up fast. Studies consistently show people underestimate their intake by 20–40%. A few days of honest tracking usually reveals the gap.

Your TDEE has dropped

As you lose weight, a lighter body burns fewer calories, so the deficit you started with shrinks over time. This is called metabolic adaptation, and it is completely normal. Recalculate your numbers every four to six weeks, or after every 5% of body weight lost, to stay on track.

You are holding water, not fat

New training, a salty meal, poor sleep, or (for women) the menstrual cycle can mask real fat loss behind day-to-day water shifts. Weigh yourself under the same conditions each morning and watch the weekly average, not the daily number.

You are gaining muscle while losing fat

If you are new to resistance training or returning after a break, you may be building muscle at the same time you lose fat. The scale barely moves while your body composition improves. This is a good outcome the bathroom scale simply cannot see — track waist measurements and how your clothes fit alongside your weight.

A note on the “3,500 calories = 1 pound” rule

You will see this rule everywhere, and the calculator uses a version of it (roughly 7,700 calories per kilogram of fat) to project your timeline. It is a reasonable planning estimate, but treat the projected goal date as a target, not a promise. Real weight loss is never a straight line — it comes in stops and starts, with plateaus and whooshes. As long as the weekly average trends down over a month, the plan is working.

Frequently asked questions

How many calories should I eat to lose weight?

Subtract 250 to 750 calories from your TDEE. For most adults a 500-calorie daily deficit is the sustainable sweet spot, producing about half a kilogram (one pound) of loss per week. Enter your details in the calculator above to get your exact number rather than a generic guess.

Is a 500 calorie deficit safe?

For most healthy adults, yes — a 500-calorie deficit is considered a moderate, sustainable pace. It only becomes a problem if it drops your total intake below about 1,200 calories a day for women or 1,500 for men, which is why the calculator caps your target at those minimums.

Should I eat back the calories I burn during exercise?

Usually no. The activity multiplier in the calculator already accounts for your regular training, so eating back workout calories on top of that often erases your deficit. If you do a genuinely unusual amount of extra activity, add back only a portion of it.

How long should I stay in a calorie deficit?

Stay in a deficit until you reach your goal weight, then transition gradually to maintenance calories. On very long journeys, many people benefit from short planned “diet breaks” at maintenance every couple of months to ease hunger and reset hormones before continuing.

Your next steps

Once you know your calorie target, two things determine whether you keep muscle and feel good while the fat comes off: protein and meal timing.

• Set your protein target with the Protein Intake Calculator — higher protein protects lean muscle in a deficit and keeps you fuller.
• Just want your maintenance number? Start with the TDEE Calculator.
• Prefer to control when you eat rather than count every calorie? The Intermittent Fasting Planner pairs naturally with a moderate deficit.

For the science behind keeping the weight off, read our guides on [LINK: metabolic adaptation article] and [LINK: how to break a weight-loss plateau article].

Reviewed by Dr. Julian Vance. Dr. Vance writes on metabolism, longevity, and evidence-based fat loss for RegenStep. Calculations on this page use the Mifflin-St Jeor equation and standard activity multipliers used in clinical nutrition practice.

This article and calculator are for general educational purposes and are not medical advice. Calorie needs vary between individuals, and estimates are approximations. If you are pregnant or breastfeeding, under 18, have a history of disordered eating, or manage a condition such as diabetes or thyroid disease, talk to a doctor or registered dietitian before starting any calorie-restricted plan.