You want to know how many calories you need in a day: to keep your weight, or to lose or gain slowly, without crash dieting. You do not need a special diet to find that out. You need two numbers, your basal metabolic rate (BMR) and your total daily energy expenditure (TDEE), and an honest choice about how active you are. This guide walks through both with worked examples, then covers the pace of change, why there is a floor you should not go under, and the mistakes that most often throw the estimate off.
Step 1: Your BMR, the calories you burn doing nothing
BMR is the energy your body uses at complete rest, to keep your heart beating, your lungs breathing and your organs working. The most widely used equation is Mifflin-St Jeor, published in 1990 from measurements on 498 healthy people. In its simplified form: 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years), then + 5 for men or − 161 for women.
A worked example (illustrative): a 45-year-old man, 175 cm, 95 kg.
- 10 × 95 = 950
- 6.25 × 175 = 1,093.75
- 5 × 45 = 225
- BMR = 950 + 1,093.75 − 225 + 5 = 1,823.75, about 1,824 kcal a day.
That number is an estimate for a typical person of that sex, age and size. Your own body can be above or below it. Treat it as a starting point you will check against real life, not as a measurement.
Step 2: From BMR to TDEE, picking your activity level honestly
TDEE is the calories you burn in a whole day, including movement. The usual way to estimate it is BMR multiplied by an activity factor. The five common ones are 1.2 for a sedentary day, 1.375 for light exercise 1 to 3 days a week, 1.55 for moderate exercise 3 to 5 days, 1.725 for hard exercise 6 to 7 days or a physical job, and 1.9 for very hard training or a very physical job. These are widely used approximations, not exact values for any one person.
The choice matters more than most people expect. Take the same man with a BMR of 1,824:
- Sedentary (1.2): 1,824 × 1.2 = about 2,189 kcal.
- Light (1.375): about 2,508 kcal.
- Moderate (1.55): about 2,827 kcal.
Picking moderate when the honest answer is sedentary overstates his needs by about 638 kcal a day, which is more than the whole cut most people try to make. A good test: count the days you really do exercise that raises your breathing, for a continuous stretch of time, in a normal week, not the best week of the year. A desk job with a walk twice a week is sedentary or light, not moderate. If you are between two levels, take the lower one.
As a sense check, the NHS gives typical daily intakes of about 2,500 kcal for men and 2,000 for women, and says needs vary with age, weight, height and how much exercise you do. If your result is wildly far from those, recheck your inputs before you trust it.
Step 3: Choose a pace you can live with
If you eat about what you burn, your weight tends to stay put. Eating less than you burn is what leads to weight loss, as the NHS puts it, because your body draws on stored fat. The question is how much less.
The NHS advice is to aim to lose 0.5 to 1 kg (1 to 2 lb) a week, steadily, rather than suddenly. A daily shortfall of 500 kcal is 500 × 7 = 3,500 kcal a week, which is a gentle pace at the slow end of that range. For the man above at the sedentary level, that means about 2,189 − 500 = 1,689 kcal a day.
Think in months, too. The NHLBI suggests an initial goal of losing 5% to 10% of your starting weight over about six months. For 95 kg that is roughly 4.8 to 9.5 kg. Reaching that and keeping it matters more than reaching a perfect number quickly.
To gain slowly, add a small amount to maintenance. Our calculator adds 250 kcal a day, which is our own cautious choice rather than a figure from a medical source. If you are underweight or want to build muscle, talk to a doctor or dietitian rather than relying on a formula.
Why there is a floor you should not go under
A bigger cut looks faster on paper, and that is the trap. The NHS says very low calorie diets, under 800 or 1,200 kcal, can be supported by a dietitian but are not suitable or safe for everyone, and are usually recommended only when weight is affecting other health conditions and you need to lose weight quickly. So we use 1,200 kcal a day as a hard floor: our calculator will never suggest less, because anything below that belongs with a doctor or dietitian.
Here is how the floor works in practice (illustrative). A 40-year-old woman, 155 cm, 55 kg, sedentary: BMR = 550 + 968.75 − 200 − 161 = 1,157.75. TDEE = 1,157.75 × 1.2 = 1,389. A 500 kcal cut would give 889 kcal, far below the floor, so the suggestion is held at 1,200, a cut of about 189 kcal. That is slower, and that is fine. A slower pace is safer, and a small deficit sustained for months beats a harsh one you abandon in two weeks.
A second case: a 60-year-old woman, 150 cm, 50 kg, sedentary. BMR = 500 + 937.5 − 300 − 161 = 976.5, TDEE = 976.5 × 1.2 = about 1,172 kcal. That is already below 1,200, so no reduction is suggested at all. If she wants to lose weight, the right move is a conversation with her doctor or a dietitian, not a smaller number.
Step 4: Check against your own body and adjust
The formula gives you a first guess. Then you test it:
- Eat at your target for three to four weeks, and weigh yourself at the same time of day, once or twice a week.
- Look at the trend, not single days. Water and food weight make daily readings jump around.
- If the trend is not moving at all, our suggestion is to adjust by about 100 to 200 kcal and give it another few weeks. Do not jump to a big change.
- Never adjust below the floor.
Movement helps too. The WHO recommends adults do at least 150 minutes of moderate-intensity physical activity a week. It supports health regardless of the number on the scale, and it is part of why choosing a higher activity level honestly, once you have actually earned it, is fine.
When the formula is not enough
The equation uses only four things: sex, age, height and weight. It was built from measurements on healthy people, so it knows nothing about your muscle mass, an illness, a condition that changes how you use energy, or medicines that affect appetite or weight. If any of those apply to you, the number is even less certain than usual, and the sensible move is to plan with your doctor or a dietitian and use the calculator only as a rough reference. The same is true if your result and your real-life weight trend disagree for several weeks in a row: trust the trend.
Common mistakes
- Choosing too high an activity level. The biggest error, as the example above shows.
- Treating the result as exact. It is an estimate with an error of its own.
- Cutting more because you are impatient. More deficit is not more safety.
- Using the numbers for the wrong person. The calculator is for adults aged 18 to 80, not for children, pregnancy or breastfeeding.
- Ignoring medical conditions. If you have diabetes, an eating disorder or any condition that affects your diet, plan with your doctor first.
This article is general information and a rough estimate, not medical advice or a diet plan.
The next step
Enter your own sex, age, height, weight and honest activity level in the calorie calculator and compare the result with the NHS typical figures. Choose the maintain option first and eat around that number for two weeks to see what your weight does. If you are not sure where your weight sits to begin with, what your BMI result means and where it falls short is the place to start. If you run a clinic and want patients to search by speciality and location and book without a phone call, Softwiro's MyClinic does that; to discuss it, contact us.
Sources
- A new predictive equation for resting energy expenditure in healthy individuals — Mifflin et al., Am J Clin Nutr 1990. The BMR equation (498 healthy subjects; the simplified sex-specific form).
- Understanding calories — NHS. Typical daily intakes (about 2,500 kcal men, 2,000 kcal women), that needs vary, and that eating less than you use leads to weight loss.
- Obesity: treatment — NHS. A steady loss of 0.5 to 1 kg a week, and that diets under 800 or 1,200 kcal are not suitable or safe for everyone.
- Calories and weight loss — NHLBI (NIH). An initial goal of 5% to 10% of your weight over about six months.
- Physical activity — WHO. At least 150 minutes of moderate-intensity activity a week for adults.
The activity factors (1.2 to 1.9) are in wide use and are attributed to McArdle's Exercise Physiology; we could not confirm them from a primary source, so they are approximate.