Every fitness calculator does the same thing underneath. Somebody measured a group of people, fitted a curve to what they found, and now that curve is being applied to you. The number that comes out is where you would fall if you were average in all the ways the formula assumes — and nobody is.
That sounds like a reason to ignore them. It is not. It is a reason to use them for the question they are good at, which is not the one people ask.
The tool BMI Calculator Metric or imperial, with the healthy weight range for your height and Asian cut-offs.Three different ways of being wrong
The three most common calculators are wrong in three genuinely different ways, and knowing which is which tells you how much to discount.
BMI measures the wrong quantity. It is mass over height squared — it cannot see what the mass is made of, so muscle and fat count the same. The World Health Organization, whose thresholds everyone uses, calls it a “surrogate marker of fatness” and points at waist circumference for the rest. That is not a flaw in the arithmetic; it is the arithmetic doing exactly what it says. It was built to describe populations, and applied to one person it is a screening flag, not a finding.
A one-rep max estimate has no single answer. There are seven or eight published formulas, they were each fitted to different lifters doing different lifts, and they disagree with each other — mildly at three reps, substantially at twelve. Any calculator printing one confident number has picked a favourite and hidden six dissents. The honest version shows the spread, because the spread is the uncertainty.
A race prediction is right arithmetic on an unverifiable assumption. Riegel's formula is a clean curve and it assumes you are as trained for the far distance as for the near one. A fast 5K is real evidence about speed and no evidence at all about whether you have built the endurance a marathon asks for. The prediction is a ceiling, not a forecast.
The thing that makes them useful anyway
Here is the part that gets missed. If a formula is wrong about you, it is usually wrong in the same direction by the same amount every time you use it. A BMI that overstates you because you carry muscle overstates you by roughly the same margin next month. A 1RM formula that reads two kilos high reads two kilos high in March as well.
And a constant error subtracts itself out of a difference.
So the useful question is not "what is my number". It is "which way is my number going, and how fast". That one these tools answer well, and it is also the question that actually corresponds to what you are doing.
How to get a clean trend
The cancelling only works if the bias really is constant, which means keeping everything else the same:
- Same tool, same formula. Switching calculators mid-way replaces one bias with another, and the jump looks like progress or loss.
- Same conditions. Weigh at the same time of day. Test a lift with the same warm-up and the same rep range — an estimate from three reps and one from ten are not comparable even from the same formula.
- Long enough intervals. Week to week is mostly noise: water, sleep, what you ate. Month to month is signal.
- Write it down. Not because the numbers are precious but because memory quietly rounds toward whatever you expected.
When the absolute number does matter
Sometimes the level is the point, and then a formula is the wrong instrument.
A clinical threshold, a competition weight class, a qualifying standard for a race — those are decisions made on the value itself, and the right response is to be measured properly rather than calculated. A body composition scan says what BMI cannot. An actual single at the bar says what seven formulas are arguing about. A time trial says what Riegel is guessing.
The calculators are for the ninety-nine days between those, which is most of them.
Read the spread, not just the middle
One habit worth taking from all of this: when a tool shows you a range, the range is information rather than clutter. A one-rep max where the formulas agree within four per cent is telling you something quite different from one where they span twenty, and the second is not a worse tool — it is the same tool being honest about a set that was too long to estimate from.
Numbers that hide their own uncertainty are more comfortable and less useful. That is why the calculators here print theirs.
This is about how estimating formulas behave, not health advice. None of these numbers diagnoses anything, and decisions about weight, training load or an injury belong with somebody qualified who can see you.