Metric vs. US Units: How Statura Handles the Conversion

Most of the world measures body weight in kilograms and height in centimetres. The United States uses pounds, feet and inches. Health calculators have to serve both, and how they handle it determines whether the results agree.

Almost every published clinical formula is specified in metric, because that is the convention of scientific literature. So the honest approach is to convert your input to metric, calculate, and convert the result back for display — which is what every calculator on this site does.

The alternative, maintaining separate formulas per unit system, is where discrepancies between calculators usually originate.

These are exact or standard values, and we use the full precision rather than the rounded versions commonly seen.

BMI is the clearest illustration of how unit handling works. In metric it is weight in kilograms divided by height in metres squared. In US customary it is 703 × weight in pounds divided by height in inches squared.

That 703 is not arbitrary and it is not a different formula. It is the conversion factor folded into the arithmetic: converting pounds to kilograms and inches to metres produces a multiplier of approximately 703.07.

So both versions are the same formula. A correct conversion gives the same BMI either way — and if two calculators disagree on your BMI in different units, one of them has made an error.

The most common source of disagreement between calculators is not the formula but when they round.

Rounding intermediate values compounds error. If 5 ft 9 in is converted to 175 cm rather than 175.26, and the weight is rounded too, the resulting BMI can differ in the first decimal place — enough to move someone across a category boundary that is itself arbitrary.

We round only at final display. Every intermediate value keeps full floating-point precision, and the conversion factors above are used in full rather than truncated to two decimals.

Feet-and-inches height is the fiddliest input to handle, because it is two numbers representing one quantity, and people enter it inconsistently.

The calculators take feet and inches separately and combine them: total inches = (feet × 12) + inches, then × 2.54 for centimetres. Entering 5 ft 13 in rather than 6 ft 1 in still produces the right answer, because the arithmetic does not care.

One thing to watch is decimal feet. 5.5 feet is 5 ft 6 in, not 5 ft 5 in — a mistake that costs an inch. Entering the two fields separately avoids the ambiguity entirely, which is why we ask for them that way.

Several calculators display results in both systems regardless of which you selected. This is deliberate rather than clutter.

Some values are more intuitive in one system than the other. Water intake is easier to picture in litres for some people and in cups for others, so the Water Intake Calculator shows litres, fluid ounces and cups together.

It also helps when comparing against sources that use the other system. Published research is nearly always metric, while much popular guidance is imperial — showing both means you can check a result against either without doing arithmetic.

Your unit choice is remembered across calculators for the session, so switching from BMI to Body Fat to Calories does not mean reselecting it three times.

It is stored locally in your browser and never sent anywhere — the same approach as everything else on the site.

Switching units mid-calculation converts what you have already entered rather than clearing the form, so you can check a result both ways without re-typing.

A misplaced conversion in most contexts is an inconvenience. In health calculations it can move someone across a category boundary and change what they conclude about themselves.

Consider a BMI of 24.8 versus 25.1. Those figures are physiologically indistinguishable, and one sits in "normal" while the other reads "overweight". A conversion error of one inch in height is enough to produce that swing — which is a good argument both for careful arithmetic and for not treating the category boundaries as meaningful to a decimal place.

The same applies to drug dosing, which is where these conversions originated. A number of well-documented clinical incidents have traced back to weights recorded in pounds and treated as kilograms — an error factor of 2.2 in a dose.

None of which is a reason for anxiety about your BMI. It is a reason to read a result as a position within a range rather than as a verdict delivered to one decimal place.

Two further wrinkles are worth mentioning for completeness. The UK commonly uses stones and pounds for body weight — 1 stone = 14 pounds — which is a third convention layered on the imperial system. Our US customary mode uses pounds only, so UK readers should multiply stones by 14 and add the remainder.

And the pound itself has a precise definition: since 1959 the international avoirdupois pound has been exactly 0.45359237 kg by international agreement. The commonly used 2.2 conversion is a rounding that introduces about 0.2% error — small, but enough to matter at higher body weights.

These are the kinds of details that separate calculators which agree with each other from those that quietly do not.