These eight calculators answer two different questions about your body. The first
is about energy — how many calories you burn, and how many to eat to lose,
keep, or gain weight. The second is about body composition — what your weight
is actually made of, and where it sits. You do not need all eight. This page
explains how they fit together so you can pick the one that matches your question,
then hand the math to the tool.
Every number here is an estimate or a screening number, not a diagnosis or
medical advice. The formulas come from published research and public-health
guidelines, and each tool shows the exact formula it used. Your inputs stay in
your browser — nothing is sent to a server, and there is no signup.
Start with energy: BMR, then TDEE
Two calorie numbers get mixed up constantly, so it is worth separating them.
BMR (basal metabolic rate) is the energy your body burns at complete rest —
just keeping your heart beating, your lungs moving, and your cells running. It is
the floor. The BMR calculator estimates it with the
Mifflin-St Jeor equation (Mifflin & St Jeor, 1990), the formula most modern
sources treat as the best general-purpose predictor. If you know your body-fat
percentage, the tool can switch to the Katch-McArdle formula, which works from
your lean mass instead.
TDEE (total daily energy expenditure) is BMR plus everything else you do —
walking, working, training, fidgeting. It is the bigger, more useful number,
because it is roughly the calories you’d eat to maintain your current weight.
The TDEE calculator takes your BMR and multiplies
it by an activity factor (a common scale running from about 1.2 for a desk job
to 1.9 for twice-a-day training, descended from FAO/WHO activity-level bands). It
then shows a ladder of cut and bulk targets around your maintenance number.
So the order is simple: BMR is the resting baseline; TDEE adds your activity on
top. If you only want the resting number, use the BMR tool. If you want the
number to actually plan meals around, use TDEE. Most people want TDEE.
A caution that applies to both: these are population averages. Real
metabolisms vary, and the only exact way to measure energy burn is lab
calorimetry. Treat the result as a well-grounded starting point, not a verdict.
Eating to change weight: the deficit, and what to do with it
Once you know your TDEE, changing weight is about the gap between what you burn and
what you eat.
The calorie deficit calculator turns a target loss
rate into a daily calorie goal. It uses the long-standing “3500 calories ≈ one
pound” rule (Wishnofsky, 1958): a 500-calorie daily deficit lines up with roughly
a pound a week. Be honest with yourself about this rule, though — it is a starting
estimate, not a promise. Research by Hall and colleagues showed the linear rule
overstates long-term loss, because your metabolism adapts downward as you lose
weight, so the gap between the predicted and the actual loss widens over months.
Loss slows; the simple math does not. Use the deficit as a place to begin, then
adjust to what the scale actually does over a few weeks.
Two guardrails the tool builds in, worth understanding:
- A calorie floor. The tool will not push your target below about 1500
calories a day for men or 1200 for women — common floors cited by Mayo Clinic
and dietitian guidance to avoid undereating. If your deficit would dip under the
floor, the tool clamps it and tells you.
- A capped aggressive cut. The fastest preset is a 750-calorie deficit, not the
1000-calorie “extreme” some sites offer. Very large deficits are widely
discouraged because they are hard to sustain and can cost you muscle.
Once you have a calorie target, the macro calculator
splits it into grams of protein, carbohydrate, and fat. It defaults to a
protein-anchor method — set protein from your body weight, set a fat floor, and
let carbs fill the rest — using the constants 4 calories per gram of protein, 4 per
gram of carb, and 9 per gram of fat. You can also pick a preset ratio (balanced,
low-carb, high-protein, or keto). Macros are heuristics for planning, not a
prescription; individual response varies, and anyone with a kidney condition,
diabetes, an eating-disorder history, or a pregnancy should talk to a registered
dietitian before anchoring protein high.
The energy sequence, in one line: BMR → TDEE → deficit (if losing) → macros.
Each tool hands its output to the next.
Measuring the body: BMI vs. body fat vs. waist-to-height
The body-composition ring answers a different question: not “how much do I eat,”
but “what does this weight actually mean?” Three screens look at it three ways, and
the most useful thing to know is what each one misses.
BMI is weight divided by height squared. The
BMI calculator sorts adults into the standard
World Health Organization bands — underweight under 18.5, normal 18.5 to under
25, overweight 25 to under 30, then obesity classes I, II, and III at 30, 35, and
40 (the U.S. CDC uses the same numbers). BMI is fast, free, and needs only two
measurements, which is why it is everywhere. But it has a real blind spot: it
cannot tell muscle from fat. A lean, muscular person can land in “overweight” by
the number alone, and an older person who has lost muscle can look “normal” while
carrying excess fat. BMI is a screening number, not a diagnosis — a place to
start a conversation, not the end of one. That is exactly why this hub does not
treat BMI as the headline; it is a feeder into the better screens below.
Body fat percentage measures the thing BMI only guesses at — how much of you is
fat. The body fat calculator uses the US Navy
circumference method (Hodgdon & Beckett, 1984): height, neck, and waist for men,
plus hip for women, run through a log-based formula. No scale or caliper needed. It
sorts the result into descriptive bands (essential, athletes, fitness, average,
obese) from the American Council on Exercise. The honest catch: a tape-measure
estimate carries roughly ±3 to 4 percentage points of error and is not a
substitute for a DEXA scan or hydrostatic weighing. It is a reasonable home screen,
not a lab result, and it drifts at the very lean and very heavy extremes.
Waist-to-height ratio (WHtR) may be the most useful of the three and the least
known. The waist-to-height calculator divides your
waist by your height — both in the same units — and checks it against the
Ashwell / NICE bands (Ashwell & Gibson, BMJ Open 2016; NICE guideline NG246):
under 0.4 is below the healthy range, 0.4 to under 0.5 is healthy, 0.5 to under 0.6
is increased central-fat risk, and 0.6 and up is substantially increased. The
plain-English version is the well-known rule of thumb: keep your waist under half
your height. WHtR’s edge over BMI is that it looks at where fat sits — fat
around the middle is the kind most linked to cardiometabolic risk — and the same
thresholds apply to adult men and women without separate cutoffs.
How to choose between them:
| You want to know… | Use | What it can’t do |
|---|
| A quick weight-for-height flag | BMI | Tell muscle from fat |
| How much of you is fat | Body fat % | Beat a DEXA scan (±3–4%) |
| Whether belly fat is a concern | Waist-to-height | Capture overall fatness |
Read together, they tell a fuller story than any one alone. If BMI flags you as
“overweight” but your waist-to-height is in the healthy range and your body-fat
estimate is low, the muscle-vs-fat blind spot is probably why. None of these is a
diagnosis — they are screening numbers to bring to a clinician, not replace one.
Training: heart-rate zones
The last tool is about effort, not size. The
heart-rate zone calculator turns your age into
five training zones, from easy recovery up to maximum effort. It defaults to the
Tanaka formula (Tanaka, Monahan & Seals, 2001: max heart rate ≈ 208 − 0.7 ×
age), a meta-analysis-backed estimate that is more accurate than the old “220 minus
your age” rule — though the classic rule is offered as a toggle, because people
look for it. If you add your resting heart rate, the tool upgrades to the
Karvonen heart-rate-reserve method (Karvonen et al., 1957), which personalizes
the zones to your fitness.
The same caution as before, only sharper: any age formula can be off by 10 to 20
beats per minute for a given person. And if you take beta-blockers, have a heart
condition, are pregnant, or are starting a new program, talk to a clinician before
training to a heart-rate target — for some people the number simply does not apply.
A common path looks like this:
- Get your baseline. Run the TDEE calculator
to find your maintenance calories (it computes BMR along the way). If you only
care about the resting number, the BMR calculator
is the tighter page.
- Pick a direction. Losing weight? The
calorie deficit calculator sets a daily target
and a realistic rate. Maintaining or gaining? Use your TDEE as-is.
- Split the calories. Feed that target into the
macro calculator for protein, carb, and fat
grams.
- Check the body, not just the scale. Use
body fat % and
waist-to-height alongside
BMI so a single number does not mislead you.
- Train by effort. Use the
heart-rate zone calculator to keep easy days
easy and hard days hard.
You will not use every tool, and you should not try to. Pick the question you
actually have, use that tool, and come back when the next question shows up.
One more note for a specific moment: if you are pregnant or trying to conceive,
none of these fitness numbers are designed for you — BMI and heart-rate formulas in
particular are not valid in pregnancy. The right starting point is our
cycle, ovulation & pregnancy tools, and a
clinician.
These tools give estimates and screening numbers — they are not medical advice
or a diagnosis. Formulas come from published research and public-health
guidelines (cited below) and may not fit athletes, older adults, pregnant people,
or anyone with a medical condition. For decisions about your health, talk to a
qualified clinician.