Medical and Fitness Disclaimer
Every number this site produces is a mathematical output, not a clinical assessment. This page explains exactly what that means, which specific equations we use and where they are known to be wrong for certain people, and what to do instead.
Do not use any result on this site to diagnose a condition, start or stop medication, or change your diet or training program without consulting a qualified professional. If you have a health concern, speak with a physician, registered dietitian, or other licensed practitioner who can assess you in person.
What these calculators actually are
Each calculator on CalcToolsKit is an implementation of an equation that was published in a scientific paper, a clinical study, or an official guideline. They are deliberately simple: you enter a handful of numbers, and the page applies the formula exactly as its authors specified.
That simplicity is a feature, but it is also a limit. The equations were developed to classify populations — to tell whether a group of people, on average, falls into a category that carries elevated risk. They were not designed to assess one specific person on one specific day. When a study validates a method, it validates it against a population distribution, not against your individual physiology.
There is a second, subtler limitation. Almost every equation in this field was built from observational data: researchers measured large groups, found a statistical relationship, and expressed it as a formula. A statistical relationship is not the same thing as a mechanism, and a real individual's value can sit well away from the model's prediction without the model being "wrong" in the technical sense.
Known limits of the specific equations we use
We list these on each calculator page too, but they are worth collecting in one place.
| Calculator | Equation | Where it is weakest |
|---|---|---|
| BMI | Weight ÷ height² | Carries no information about body composition. A lean athlete and a sedentary person of the same weight and height get identical scores. Not recommended as a screening tool for Asian populations, who have higher cardiometabolic risk at lower BMI. |
| TDEE | Mifflin-St Jeor | An estimate of average daily energy expenditure, not a measurement of your specific burn. Activity multipliers are population averages; real-day variation is large. Results are least reliable for people with significantly different lean mass than average. |
| Body fat | US Navy circumference | Assumes a fixed relationship between your measured dimensions and your skeletal bone width. Assumes accurate neck and waist measurement. Tends to underestimate fat in very lean individuals and can be off by several percentage points depending on where you measure. |
| Ideal weight | Devine, Robinson, Miller, Hamwi | Derived from drug-dosing tables for a population of US soldiers, mostly young men. The coefficients were never designed as health or aesthetic targets and have not been revalidated as such. |
| Macro split | Several ratio conventions | No single ratio is established as optimal. Evidence favours protein ranges over specific percentages, and carb versus fat allocation depends entirely on goals and training state. |
| Target heart rate | Karvonen, percentage of max HR | Age-predicted maximum heart rate has a standard deviation of roughly 12 beats per minute, so any zone boundary is approximate. Zone boundaries shift with training, heat, hydration, and caffeine. |
| Running pace | Riegel, and equivalent models | Predicts average pace for a well-trained runner over a race distance. Large prediction error — on the order of minutes per mile — for the first weeks of a training plan or in poor conditions. |
| One-rep max | Epley, Brzycki, Lombardi | Valid only if derived from a genuine maximal set performed to failure with correct technique. A submaximal set produces an overestimate. Formula choice changes the result by several percent. |
Reference data: WHO body mass index thresholds; CDC child and adolescent BMI-for-age growth charts (2000 reference); ACSM guidance on target heart rate and resistance training estimation. Linked from each calculator page.
Who should not rely on these numbers
Be especially cautious with a calculator result if any of the following apply to you. In these situations the underlying population data probably does not describe you, and a number coming out of a formula can be actively misleading.
- Pregnancy. Standard body fat, BMI, and calorie formulas are all built on non-pregnant reference data.
- Children and adolescents under 18. This is why our BMI page includes a dedicated mode that uses CDC age- and sex-specific percentile charts instead of the adult cut-offs. Adult thresholds misclassify children frequently.
- Eating disorders or a history of disordered eating. Numerical output about weight, body fat, or calorie targets can be harmful. Please avoid weight-based calculators entirely and seek support from a qualified professional.
- Conditions affecting metabolism or body composition. Including thyroid disorders, diabetes, Cushing's syndrome, severe malnutrition, bariatric surgery, and conditions causing muscle wasting or fluid retention. Many formulas assume normal regulation.
- Advanced age. Equations derived from young and middle-aged adults extrapolate poorly to people over 75, where body composition shifts substantially.
- Very high or very low body fat percentage. Several methods, including the US Navy circumference method, lose accuracy at the extremes of the range.
- Undertraining or untrained lifting. Strength estimates assume a load you could genuinely lift to failure safely. For beginners, perceived exertion is a safer guide than any formula output.
What to do instead
Use these calculators for what they are good at: understanding a number, seeing how a change would move it, and having an informed conversation with a professional. Treat the output as a starting point, not a conclusion.
For anything that affects your health:
- Have a clinician measure what should be measured directly. Blood pressure, blood tests, and body composition imaging do not have the uncertainty of an estimate from a formula.
- Ask a registered dietitian to set calorie and macronutrient targets. They can adjust for your medical history, medications, and preferences in ways no online tool can.
- Ask a physical therapist or qualified coach about training loads and progression rather than computing a one-rep max from a single submaximal set.
- Discuss weight changes with your doctor rather than relying on a trend from a body-fat estimate.
If something on this site seems to conflict with what your clinician has told you, follow your clinician. They have access to your history and can run tests that we cannot.
If you think you have a medical emergency
Do not use an online calculator. Contact your local emergency number or go to the nearest emergency department immediately.
Corrections
We implement the published equations faithfully and test them against published worked examples, but we are a small independent project and mistakes happen. If you find a calculation error, a misquoted threshold, or an inaccurate limitation note, please tell us and we will fix it and note the correction.
Our Terms of Use also set out the licence governing use of these calculators and the limits of our liability.
Common questions about our results
Are your calculators accurate?
The equations are implemented exactly as published, and we test each one against worked examples from the original sources. The accuracy of the final number is a different question: it depends on how well the underlying population study describes the individual using it, which varies widely between individuals.
Why does my body-fat estimate differ from my doctor's?
Common reasons. Your clinician may have used a different equation, or a method that measures rather than estimates — DEXA, air displacement, or bioelectrical impedance. Circumference methods are sensitive to exactly where the tape sits and to posture. Hydrostatic weighing and DEXA are more accurate, so a small disagreement with a circumference estimate is normal and not a sign that either is broken.
Can I use BMI to decide whether I am healthy?
No. BMI is a population screening tool and a reasonable first pass, but it cannot distinguish muscle from fat and it does not describe cardiometabolic risk directly. Several national health bodies, including the WHO, now recommend that clinicians use additional measures — waist circumference, blood pressure, lipids, and glucose — before drawing conclusions about an individual's health.
Is the running pace prediction reliable for a beginner?
Not very. Riegel-type models predict what an already-trained runner will average over a sustained distance. In your first weeks of training, actual pace is usually slower than predicted and varies a great deal day to day. Use the prediction as a rough guide, not as a target to hit.
Should I trust my one-rep max for programming weights?
Only if it came from a genuine maximal set with correct technique, and even then build up conservatively. For most people, working a few percent below the estimated maximum is the safer approach, and the estimate itself has a wide margin of error because the formulas were fitted to average data rather than to you.
Why do you show limits for every calculator instead of just the answer?
Because a number without its error bars is misleading. Telling you that a body-fat estimate carries several percentage points of uncertainty is more useful than pretending the figure is exact. Each calculator page documents the specific failure modes of its equation so you know when to trust it and when to ask someone else.