Health Calculators

What these numbers can and cannot tell you

Population formulas, individual bodies

BMI, ideal body weight and calorie formulas were derived from population averages. They describe groups well and individuals only roughly. A muscular person can be classed overweight by BMI while being metabolically healthy, and the formula is not wrong so much as being asked a question it was never built to answer.

Estimated needs are a starting point, not a prescription

Basal metabolic rate equations carry a margin of error of roughly ten percent in either direction even when applied correctly. Use the number as an opening hypothesis, then adjust against what actually happens over several weeks.

Activity multipliers are the largest source of error

Most people overestimate their activity level, and the multiplier is applied to the whole calculation. Choosing "very active" when "moderately active" is honest can inflate a daily target by several hundred calories, which is enough to reverse the intended direction entirely.

Rate of change matters more than the target

The same goal weight reached over six months and over six weeks are different interventions with different outcomes. Most of the tools here let you set a rate, and a slower rate is nearly always the one that holds.

None of this is diagnosis

These calculators do not know your medical history, medications, or the conditions that change how any of these formulas apply. They are for orientation before a conversation with a clinician, not a substitute for one.

Which one do you need?

You want a healthy weight range

Ideal body weight, which leads with the BMI range rather than a single number.

You are planning calorie intake

TDEE for maintenance, then the calorie calculator for a deficit or surplus.

You are training for a race

Running pace, which uses Riegel rather than multiplying your 5K pace by the distance.

You are pregnant or planning

Pregnancy due date — and enter your real cycle length, which moves the date day for day.

You are tracking fitness intensity

Heart rate zones, based on your measured resting rate where you have one.

Where these go wrong

• Overstating activity level

  The activity multiplier is applied to the entire calculation, so an optimistic setting can inflate a daily target by several hundred calories. Most people are one level lower than they choose.

• Reading BMI as a body composition measure

  BMI uses only height and weight. It cannot distinguish muscle from fat, which is why it misclassifies athletes and can miss risk in people at a normal weight.

• Recalculating too often

  Daily weight swings are mostly water, food volume and timing. Judging a change against a weekly average is the only way to see the trend rather than the noise.

• Treating the output as a medical opinion

  These formulas do not know your medication, history or conditions. They are a starting point for a conversation with a clinician, not a replacement for one.

Common questions