A calorie calculator answers a narrower question than a diet plan: about how much energy this adult body uses in a day, and what a smaller number would look like. The resting piece is BMR, basal metabolic rate. This page uses the Mifflin–St Jeor equation, which clinics and most TDEE calculators still use as a starting estimate. For men it is 10 times weight in kilograms, plus 6.25 times height in centimetres, minus 5 times age in years, plus 5. For women the same sum uses minus 161 instead of plus 5. Type 80 kg, 180 cm, and 30 years as a man and the page does 800 + 1,125 − 150 + 5, which is 1,780 kcal. The same body as a woman is 1,614 kcal. Those two constants are why the form asks for sex. They are not a judgment about anything else.
BMR is not the number you eat to stay the same weight. A day includes walking, work, and whatever training you actually do. TDEE, total daily energy expenditure, is BMR times an activity factor. The factors on this calorie calculator are the usual set: 1.2 for a mostly seated day, 1.375 for light effort on one to three days a week, 1.55 for moderate effort on three to five days, 1.725 for hard effort most days, and 1.9 for a physical job or two sessions in a day. The worked example at moderate effort is 1,780 × 1.55 = 2,759 kcal. That is the maintenance estimate, the TDEE. It is not a laboratory measurement. Two people with the same form answers can burn different amounts because one fidgets, one stands, and one has a different amount of muscle. If you already track weight for a fortnight at a steady intake, that scale is better evidence than the factor.
A calorie deficit calculator is the maintenance figure minus the gap you choose. The form starts at 500 kcal. On the example, 2,759 − 500 is 2,259. The result also prints a 250 kcal line and a 500 kcal line so you can see both without retyping. A common classroom rule treats about 7,700 kcal as a kilogram of body weight. A 500 kcal daily gap is then about 0.45 kg a week. The page shows that product and calls it rough, because water, glycogen, and a week of uneven meals move the scale by more than the arithmetic. A deficit of 0 is maintenance. The field stops at 1,500 kcal so a typo cannot silently print a target of zero. If the target falls under about 1,500 kcal for a man or 1,200 for a woman, the page still shows the number and adds a warning. That warning is not permission and not a ban. It is the usual floor people quote before they talk to a clinician.
Use the form from the top. Leave kilograms and centimetres, or switch to pounds, feet, and inches. Height in the metric fields is centimetres: 180, not 1.80. Inches are the leftover after feet, so 5 feet 11 inches is feet 5 and inches 11. The conversions are the exact ones, 1 pound = 0.45359237 kg and 1 inch = 2.54 cm, the same factors as the BMI calculator and the unit converter. Age runs from 15 to 80. Under 18 the result is labelled as an adult formula used on a teenager, which is the wrong tool. Children and adolescents need a chart that knows age and sex in a different way. Pregnancy is a different budget again. The activity menu is a description of the week, not a medal. If you are unsure, sedentary and light are the less flattering guesses, and a flattering factor is how these calculators overshoot.
What the TDEE calculator does not know is the rest of a person. It does not know body fat, thyroid disease, medication, or a job that is heavy on some days and idle on others. It does not adjust for the way appetite changes when the gap is large. Athletes who carry a lot of muscle often need more than the equation, and the BMI page has the same blind spot: a high BMI can be muscle, and a "normal" BMI can hide a low intake. Use one number as a start, eat in a way you can repeat, and let two or three weeks of mornings on the same scale tell you whether the deficit is real. This site will not turn the figure into a menu, a supplement, or a programme.
Nothing you type is uploaded. There is no account and no saved history. Close the tab and the figure is gone. That is deliberate. A calorie target is not something this site should keep. The page is also not medical advice and not a promise that 2,259 kcal will change your weight by Friday. If a doctor has already given you a target, use that target. The loan calculator is unrelated arithmetic if you arrived from the directory. Each tool does one job. If you only wanted the unit change, the kg to lbs page uses the same pound definition without pretending to know your expenditure.