9 Free Calculators
Know your numbers. BMI, ideal weight, body fat, ovulation, calories, and pregnancy milestones. 9 free health calculators — instant results, no sign-up.
Every tool on this page is a population-level estimate applied to an individual. That distinction matters more than any of the numbers. A formula built from the average of thousands of people describes a typical body, and yours may not be typical — which is not a flaw in the calculator, it is a limit on what arithmetic can know about a person.
Used well, these tools are useful for one specific job: giving you a starting point and a way to track direction over time. A BMI of 27 does not diagnose anything, but watching your own numbers move across months tells you whether what you're doing is working.
| Tool | What it estimates | Where it misleads |
|---|---|---|
| BMI | Weight relative to height | Cannot distinguish muscle from fat. Muscular athletes routinely score "overweight." |
| Calorie needs | Daily energy expenditure | Activity multipliers are self-reported and usually overestimated by 20–30%. |
| Ideal weight | A reference range for your height | Formulas from the 1970s that ignore frame size and body composition entirely. |
| Body fat % | Composition from tape measurements | ±3–4% error versus a DEXA scan; measurement technique dominates the result. |
| Ovulation window | Likely fertile days | Assumes a regular cycle. Irregular cycles make the prediction close to meaningless. |
| Due date | 40 weeks from last period | Only about 4% of babies arrive on the estimated date. |
Two practical adjustments make these estimates substantially better:
For calorie targets specifically, treat the calculator's output as a hypothesis. Hold it for two to three weeks, track your actual weight trend, and adjust from what really happened rather than from what the formula predicted. The formula gives you a starting point; your own data gives you the answer.
These tools are for general reference and are not a substitute for medical advice, diagnosis, or treatment. Talk to a qualified healthcare provider before making decisions about medication, pregnancy, significant weight change, or supplementation — especially if you have an existing condition.
Is BMI actually useful, or should I ignore it?
It is useful as a rough population screening tool and close to useless as an individual diagnosis. BMI does not measure body fat, distinguish muscle from fat, or account for where fat sits on your body — and abdominal fat carries more metabolic risk than the same weight elsewhere. Waist circumference and waist-to-height ratio track health risk better for most people.
Why does my calorie estimate differ across websites?
Different calculators use different equations. Mifflin-St Jeor is the current standard and is generally the most accurate for the general population; Harris-Benedict is older and tends to run high; Katch-McArdle uses lean body mass and is better if you know your body fat percentage. A 200–300 calorie spread between sites is normal and expected.
How accurate is a tape-measure body fat estimate?
Expect roughly ±3–4 percentage points against a DEXA scan, and the error is fairly consistent for a given person. That makes it poor for knowing your true number but genuinely good for tracking change, provided you measure the same spots the same way each time.
Do you store my health information?
No. All of these calculators run entirely in your browser. Nothing you enter — weight, measurements, cycle dates, pregnancy information — is sent to a server, stored, or shared with anyone. Closing the tab discards it.
How accurate are due date calculators?
The standard method (Naegele's rule) adds 280 days to the first day of your last menstrual period and assumes a 28-day cycle with ovulation on day 14. Only about 4% of births occur on the estimated date, and roughly 80% happen within two weeks either side. An early ultrasound dates a pregnancy considerably more accurately.
Should I eat back the calories I burn exercising?
Usually not all of them. Both fitness trackers and exercise calorie estimates tend to overstate burn, sometimes by 20% or more, and the calorie calculator's activity multiplier may already account for your training. Eating back roughly half of estimated exercise calories is a common, more conservative approach.
Browse Other Categories