Health calculators that tell you what the number can't say
Twelve calculators for body metrics, nutrition, pregnancy dates and training zones. Every one shows the formula it used and the population it was built from, because an estimate you can't interrogate is worth very little.
A BMI figure, a calorie target or a due date is the output of a formula somebody published decades ago on a specific group of people. That does not make it useless. It does mean the honest version of the answer includes what the formula assumes — and most sites quietly leave that part out.
So each tool here gives you the number, the equation behind it, and a plain sentence about where it stops being reliable. No account, no upload, and nothing you enter leaves your device.
No tool matches that.
Body Metrics Which one do I need? →
- BMI Calculator
Calculate your Body Mass Index from height and weight, with the standard category ranges explained.
- BMR Calculator
Estimate your Basal Metabolic Rate — the calories your body burns at complete rest.
- Daily Calorie Calculator
Find your daily calorie needs for maintaining, losing, or gaining weight based on activity level.
- Body Fat Calculator
Estimate body fat percentage from tape-measure inputs using the U.S. Navy method.
- Ideal Weight Calculator
See an estimated healthy weight range from height using several published formulas side by side.
Nutrition Which one do I need? →
- Macro Calculator
Split your daily calorie target into protein, carb, and fat grams for any diet style.
- Water Intake Calculator
Get a daily hydration target from your weight, activity level, and climate.
- Protein Intake Calculator
Find a daily protein target in grams based on your weight, goal, and activity level.
Pregnancy
- Pregnancy Due Date Calculator
Estimate your due date and current week of pregnancy from your last period or conception date.
- Ovulation Calculator
Estimate your fertile window and likely ovulation day from your last period and average cycle length.
Fitness
- Heart Rate Zone Calculator
Find your training heart rate zones from age and resting heart rate using the Karvonen method.
- Calories Burned Calculator
Estimate calories burned during an activity from its MET value, your weight, and duration.
From the blog
Where to start
For body composition, BMI is the fastest screening figure and the crudest: it knows your height and weight and nothing else, so a muscular build reads as overweight. Body fat and ideal weight add more inputs and more nuance; use them together rather than picking one.
For eating, the chain runs BMR, then daily calories, then macros. BMR is what you burn at rest, daily calories applies your activity level, and the macro calculator splits that total into protein, carbohydrate and fat. Protein and water intake answer the two questions people ask most often on their own.
For training, heart rate zones turn a target effort into actual beats per minute, and calories burned estimates the cost of a session. Both are population averages, so treat a change over weeks as more meaningful than any single reading.
What these tools are not
None of this is medical advice, and none of it is a diagnosis. These are the same standard estimates a clinician would use as a starting point, calculated quickly and explained honestly — a way to arrive at a conversation better informed, not a replacement for having one.
If a result surprises you, or you are managing a condition, pregnant, or changing a medication, the number here is a prompt to ask a professional rather than an answer to act on alone.
Common questions
Is a BMI calculator actually useful?
As a population screening tool, yes; as a verdict on one person, no. BMI only knows height and weight, so it cannot distinguish muscle from fat and misreads very muscular or very tall people. It is a starting point that tells you whether a closer look is worth it.
How accurate are the calorie and macro numbers?
They come from the Mifflin-St Jeor equation and standard activity multipliers, which are well validated across populations but can be off by a few hundred calories for an individual. Use the figure as a starting point, then adjust based on what actually happens over two or three weeks.
Is my health data sent anywhere?
No. Every calculation runs in your browser. Your weight, height, cycle dates and training numbers stay on your device, are never transmitted, and are never stored by us.
How exact is the pregnancy due date?
It uses Naegele's rule, the same 280-day calculation a clinic starts from. Fewer than one in twenty births happen on the estimated date, so read it as the middle of a likely window rather than an appointment.
Can I use these instead of seeing a doctor?
No. They are estimates from published formulas, useful for orientation and for asking better questions. Anything that concerns you, and anything involving pregnancy, medication or a diagnosed condition, belongs with a professional who can see the whole picture.