Every number here is an estimate
All five calculators do the same kind of thing: they put your height, weight, age, sex or a date into a published formula and return an estimate. A formula does not know your muscle mass, your frame, your thyroid function or that you changed jobs last month and now sit down all day. So the useful reading is the trend, not the single figure. Weigh yourself on the same scale at the same time of day and watch what happens over a few weeks.
The numbers that settle things come from somewhere else. Body composition comes from a scan or a clinic measurement, metabolic questions come from blood work, and gestational age comes from an ultrasound. Nothing on this site is a diagnosis or a substitute for a clinician or a dietitian. If you are pregnant, breastfeeding, managing a condition, taking medication, or have a history of disordered eating, take these figures to a professional before acting on them.
Which formula each tool uses, and why two sites disagree
When another calculator gives you a different answer, it is usually not an arithmetic error. It is a different formula or a different parameter. Here is what sits behind each page:
- BMI: weight in kilograms divided by height in metres squared. The formula is the same everywhere; what varies between sites is the cut-offs used to label the result. The BMI calculator also prints the healthy weight range for your height.
- BMR: the Mifflin-St Jeor equation, published in 1990 and the one usually recommended for adults over the older Harris-Benedict equation from 1919. Sites still using Harris-Benedict tend to return a higher figure for the same person. See the BMR calculator.
- TDEE: BMR multiplied by an activity factor, from 1.2 for sedentary up to 1.9 for extra active. This is where sites disagree most, because you pick the band yourself and the wording of the bands differs. Moving one band can change the answer by several hundred calories. See the TDEE calculator.
- Body fat: the Deurenberg formula, which derives a percentage from BMI, age and sex. It never measures fat, so it tends to overestimate people with a lot of muscle and it will disagree with a tape-based formula such as the US Navy method. See the body fat calculator.
- Due date: Naegele's rule, the first day of your last period plus 280 days. If you know the conception or IVF date, conception plus 266 days is the closer estimate. See the due date calculator.
BMI cut-offs are not the same everywhere
The English BMI page uses the World Health Organization adult bands: healthy from 18.5 up to 25, overweight from 25 up to 30, and obesity at 30 and above, split into classes at 30, 35 and 40. Those are the bands most international sources quote, and they are screening bands for populations rather than a verdict on one person.
They are not universal. A WHO expert consultation noted that many Asian populations show raised metabolic risk at a lower BMI, and several health services have since adopted lower action points for some groups. Taiwan's health authority, whose table the Chinese version of this site uses, treats 24 as the start of overweight and 27 as the start of obesity, and reads 22 to 27 as the healthy range for people over 65. So the same BMI of 24.5 lands in different categories on the two language versions of this site. Neither is wrong; they are answering to different national guidance. Follow whichever your own health service uses.
BMI also says nothing about where weight sits. A waist measurement adds the part it misses, and a common rule of thumb is to keep your waist under half your height.
The order to work in: BMR, then TDEE, then a goal
If you are planning intake, the sequence matters. Calculate BMR first to get what you burn at rest, then TDEE to add your activity on top, and only then set a target relative to TDEE. The TDEE page prints the two common targets for you, roughly 500 calories below for fat loss and 300 above for building muscle, so there is no second calculation to do. After two weeks, weigh yourself and adjust from the result. That correction is worth more than agonising over which activity band to pick, because real expenditure varies between individuals by more than the bands do. Eating below your BMR for a sustained period is generally not advised without professional supervision.
The due date calculator sits outside that chain. It has nothing to do with calories, and its output is an estimate too. Naegele's rule assumes a regular 28 day cycle, so it drifts for people whose cycles are longer or irregular. A first trimester ultrasound dates a pregnancy more reliably than any arithmetic, and if your clinician revises your due date after a scan, that date is the one that counts and the one every later appointment is measured against.
Common questions
Why does another site put my BMI in a different category?
The formula is identical everywhere, so the number should match. The labels do not, because cut-offs are set by national guidance. The English pages here use the WHO adult bands of 18.5, 25 and 30. Some health services use lower thresholds for certain populations, and the Chinese version of this site uses the Taiwanese table of 18.5, 24 and 27. BMI calculator
Should I calculate BMR or TDEE first?
BMR first, since TDEE is BMR multiplied by an activity factor. BMR is what you would burn lying still all day, and TDEE adds walking, work and training, so daily intake targets are set against TDEE rather than BMR. The TDEE page shows the BMR it calculated, so you can start there if you prefer. TDEE calculator
Why is my TDEE higher on another calculator?
Usually one of two reasons. The other site may use the older Harris-Benedict equation, which tends to return a higher BMR, or it may describe the activity bands differently so you select a higher multiplier. The bands run from 1.2 to 1.9, and one step is worth several hundred calories. BMR calculator
My gym scan gives a different body fat percentage. Which one is right?
Trust the measurement over the formula. The Deurenberg formula used here infers a percentage from BMI, age and sex and never measures fat directly, so it overestimates muscular people in particular. A DXA scan, calipers or a clinic bioimpedance device measure something real. Use the formula for tracking a trend, not for a precise figure. Body fat calculator
Can I just eat below my BMR to lose weight faster?
That is not the usual advice. Common guidance starts at 300 to 500 calories below TDEE, with enough protein and some resistance training. A larger deficit, or any plan while you are managing a health condition or taking medication, is a conversation to have with a doctor or dietitian rather than a decision to base on an online estimate.
My clinic gave me a different due date. Which should I follow?
The clinic date. This calculator applies Naegele's rule to the date you enter, while a clinician dates the pregnancy from an early ultrasound measurement, which is more reliable. Standard practice is to let the scan override the period based date when the two differ by more than a set margin, and to keep that date for the rest of the pregnancy. Due date calculator
Is any of this medical advice?
No. These are general purpose estimates from published formulas, meant for self tracking and for giving you something concrete to discuss with a professional. They are not a diagnosis and should not be used to change medication, treatment or a prescribed diet.
Is anything I enter stored or sent anywhere?
No. All five calculators run in your browser. There is no account, nothing is uploaded, and nothing is kept once you close the page.