Medical Disclaimer

This is health education, not medical advice

HealthTools publishes general health information and calculation tools. Everything here is educational. It is not a substitute for professional medical advice, diagnosis or treatment from a doctor, registered dietitian, physiotherapist, midwife, pharmacist or other qualified clinician who knows your history.

Using this site does not create a clinician-patient relationship between you and HealthTools or anyone who writes for it. We do not know you, we cannot examine you, and we cannot take responsibility for decisions about your care. If you contact us we will answer questions about our sources and methods, but we will not interpret your results or advise on your health, as explained on our contact page.

What a calculator result actually is

Every calculator here applies a published equation that was derived from studying groups of people. That is worth understanding properly, because it explains what the number can and cannot tell you.

• These are population averages. An equation fitted to a study sample describes the typical relationship across that group. It does not describe you individually, and for any given person the real value can sit well away from the estimate.
• They carry known error, and it is not small. The margins are set out calculator by calculator in the next section. None of them is tight enough to justify treating a result as a measurement of you.
• They cannot see your circumstances. No calculator here knows your medical history, your medications, your diagnoses, your body composition, your pregnancy status, your lab results or your family history. All of those can change what a healthy number looks like for you.
• They were validated on particular groups. Where an equation was derived from a narrow population, it may perform poorly outside it. Each calculator page states the limitations of the method it uses.

Treat any result as a rough starting point for a conversation with a clinician. It is never a finding, a diagnosis or an instruction. Our editorial policy describes how we choose and check the formulas behind these numbers.

The specific limits of each calculator family

Different tools fail in different ways, and the honest way to present them is one family at a time. Each calculator page carries this detail alongside its result; it is collected here so you can see the whole picture before you rely on any of it.

Body composition

BMI is a ratio of weight to height squared. It cannot see body composition at all: it does not know whether a kilogram is muscle, fat, bone or fluid, and it does not know where fat is distributed. A muscular athlete and a sedentary person of the same height and weight get an identical figure and a very different health picture. BMI was built to describe populations, and it does that job well; applied to one person it is a screening prompt, not a finding. Circumference-based body-fat methods add the information BMI lacks, but they typically differ from a DEXA scan by roughly three to four percentage points, and they are sensitive to tape placement and tension. The same person measured twice can shift by more than a point. Ideal-weight equations were written for drug dosing rather than health targets, which is why several remain in use and why they can disagree by six kilograms for one person.

Nutrition and energy

Predictive energy equations estimate what a metabolic cart would measure, and they carry an error of roughly ten per cent for an individual even when applied correctly. On a 2,500-calorie requirement that is a band of about 250 calories a day in either direction. Activity multipliers are coarser still. They compress the whole range of daily movement into a handful of categories. Treat a calculated calorie target as a starting hypothesis to be corrected by two or three weeks of real weight data, never as a prescription. Hydration targets are general-population guidance and are affected by climate, activity, medication and medical conditions.

Fitness and training

Age-predicted maximum heart rate is the weakest number on this site, and it is worth knowing why. Every age-based equation describes an average, and individual variation around it is on the order of ten to twelve beats per minute. Two people of the same age can have genuinely different maximums, and the training zones built on that estimate shift with them. Anyone taking a beta blocker or another medication that blunts heart rate should disregard predicted zones entirely. One-rep-max estimates are most accurate in the three to ten repetition range and drift as repetitions rise; they are a programming convenience, not a tested maximum. Energy cost of activity is derived from published MET values averaged across people, and two people doing the same session can differ substantially.

Pregnancy dating

A due date is a window, not a date. Standard obstetric dating adds 280 days to the first day of the last menstrual period and assumes a regular 28-day cycle with ovulation on day 14, which describes a minority of cycles. Only a small fraction of births occur on the estimated date, and the clinically useful spread is several weeks wide. A first-trimester ultrasound is more accurate than any calculation, and a date from your midwife or obstetrician takes precedence over anything produced here.

Do not use these tools to make treatment decisions

Please do not use anything on this site to diagnose or rule out a condition, to start, stop or change a medication or its dose, to decide against seeing a clinician, to set an extreme calorie or weight target, or to delay investigating a symptom that concerns you. If a calculator result and your doctor's advice disagree, follow your doctor.

Situations that need professional guidance

General-population formulas are least reliable in exactly the circumstances where the stakes are highest. Please speak to a healthcare professional before acting on any result from this site if any of the following applies to you.

• Pregnancy, breastfeeding and the postpartum period. Energy, hydration and weight guidance in pregnancy and lactation differs substantially from general adult guidance, and none of the equations here are validated for it. Our due date calculator gives an estimate based on standard obstetric dating rules. An ultrasound-based date from your midwife or obstetrician takes precedence over it.
• Children and adolescents. Adult formulas do not apply to growing bodies. BMI in under-18s is assessed against age-and-sex growth percentiles, not adult categories, and these tools should not be used to set targets for a child.
• Eating disorders, or a history of disordered eating. Weight, body-fat and calorie figures can be genuinely harmful in this context. If numbers of this kind are difficult for you, please step away from these tools and speak to a clinician or an eating disorder support service.
• Cardiac conditions. Anyone with heart disease, arrhythmia, high blood pressure or a cardiac device should have training intensity set by a clinician. Predicted heart-rate zones are not safe to use as a target in these circumstances.
• Kidney or liver disease. Fluid and protein requirements are often medically restricted in these conditions, and general hydration or macronutrient targets may be actively unsafe.
• Medication affecting heart rate or fluid balance. Beta blockers and some other cardiac drugs blunt heart rate, which invalidates predicted training zones. Diuretics, lithium and several other medications change fluid needs. Your prescriber should set these targets.
• Diabetes, thyroid disorders and other metabolic conditions, where energy requirements and healthy ranges may differ from population norms.
• Athletes and people carrying a lot of muscle. BMI routinely places well-trained bodies in overweight or obese categories on weight alone, and circumference body-fat equations were not derived on this group either. Energy requirements for heavy training loads also sit well above what a general activity multiplier produces.
• Older adults, particularly with sarcopenia. Age-related loss of muscle mass changes the relationship between weight, body composition and health risk, so BMI and body-fat equations become less informative with age. Protein and energy requirements often rise rather than fall, which is the opposite of what a general-population equation suggests. This applies equally to anyone starting exercise after a long break, an injury or surgery.

In an emergency

No warranty of accuracy

We work hard to keep our formulas and content correct, and we verify each calculator against its source before publishing. Even so, we cannot guarantee that everything on this site is accurate, complete or current. Clinical guidance changes, published equations are revised, and mistakes happen. All content is provided as-is, without warranty of any kind, as set out in our terms and conditions. If you find an error, please report it to jonestitus194@gmail.com. We would rather know.

External links and advertising

We link to external sources, including research papers and clinical guidelines, so you can check our reasoning. We do not control those sites and are not responsible for their content. Advertising displayed on this site is served by third parties, and the appearance of an ad for a health product, service or programme is not a recommendation or endorsement by HealthTools. Our editorial policy explains how advertising is kept separate from what we publish.

Your use of this site

By using HealthTools you accept that any decision you make based on the information or calculations here is your own responsibility, and that the appropriate place to take a health question is a qualified professional who can assess you properly. If you do not accept that, please do not use these tools.

Questions about this disclaimer can be sent to jonestitus194@gmail.com.