Why two calculators give different answers
Different conventions, different standards, rounding, and genuine error — how to work out which result to trust when two tools disagree.
Disagreement usually is not error
Put the same numbers into two online calculators and you will fairly often get two different answers. The instinct is that one of them is broken. Most of the time neither is — they have answered slightly different questions, and neither said which.
That is worth knowing because it changes what you do next. If the cause is a convention, you need to find out which one applies to you. If it is genuine error, you need a different tool. Telling them apart is a skill worth having, and it comes down to five causes.
1. Different conventions for the same word
The commonest cause, and the least visible. A word that seems unambiguous turns out to have two accepted meanings.
A year. Some tools use 365 days, some 365.25 (the Julian year, averaging in leap years), a few 365.2422 (the tropical year). Over one year the difference is hours; over a century it is weeks. Thetime converter uses 365.25 and says so.
A megabyte. 1,000,000 bytes or 1,048,576, depending on whether the tool follows the SI definition or the older binary habit. That is a 7% gap at the megabyte and about 10% by the terabyte — which is exactly why a 1 TB drive reports as 931 GB.
A horsepower. Mechanical horsepower is 745.7 W; metric horsepower — PS, CV, pk — is 735.5 W. The same engine gets two different numbers depending on the market it is sold in.
A month. 30 days, 30.4375 days, or an actual calendar month of 28 to 31. There is no right answer, which is why thetime converter declines to offer months at all rather than pick one silently.
2. Different published standards
Where several authorities have addressed the same problem, tools implement whichever their author knew about.
Ideal body weight is the clearest example: the Devine, Robinson, Miller and Hamwi formulas all exist, all are published, and all give different answers for the same person — by several kilograms. None is wrong. A calculator naming only one of them looks authoritative and is simply making a choice on your behalf.
The same happens with PCB trace current (IPC-2221 versus the newer IPC-2152, which accounts for more of the board and generally permits narrower traces), with wind chill, and with metabolic rate equations. When two tools disagree substantially and both look careful, look for the name of the method first.
3. Rounding, and where it was applied
Two calculators using identical formulas can differ in the last digits because of when they rounded.
Rounding an intermediate result and then continuing propagates the error through everything after it. Carrying full precision and rounding once at the end does not. The difference is invisible in one step and compounds through a chain — which is why a multi-stage calculation done by hand from displayed values often disagrees with the same calculation done internally.
A related trap: displayed precision is not accuracy. A tool showing eight decimal places on an input you measured with a tape measure is telling you nothing beyond the second. The digits are real arithmetic on an imaginary level of certainty.
4. Unstated assumptions
Many calculators quietly fix a variable you did not know existed.
A gravity calculation might use 9.8, 9.81 or 9.80665 m/s². An air-density figure might be sea level at 15 °C or something else. A pressure conversion might be gauge or absolute — differing by a whole atmosphere, so a tyre at 32 psig is 46.7 psia and a tool that does not say which will be out by 14.7.
Standard resistor values are another: a tool generatingE-series values from the formula produces 2.61 and 8.25, while the published IEC 60063 lists say 2.7 and 8.2 because the lower series were fixed by convention before the mathematics was tidied. Both are defensible; only the published list is purchasable.
5. Genuine error
And sometimes a calculator is simply wrong. A transcribed constant, an inverted formula, a unit conversion applied in the wrong direction. These are real and more common than you would hope, particularly on sites where the tools were generated quickly and never checked.
The tell is usually magnitude. A convention difference moves an answer by a few percent; a broken calculator is typically out by a factor of ten, a thousand, or a sign. If two results differ by 3%, look for a convention. If they differ by 100×, look for a bug.
Working out which to trust
A practical order of checks, cheapest first:
- Check the units on every input. Degrees against radians, gauge against absolute, mass against force. This is the single most productive check and it costs seconds.
- Find a known case. Every field has anchor values — water boils at 100 °C, one cubic yard is 27 cubic feet, 0 dBm is 1 mW, a 12/12 roof pitch is exactly √2. Feed in a case you already know the answer to and see which tool reproduces it.
- Sanity-check the magnitude. Does the answer have roughly the size you expected? Most real errors announce themselves here.
- Look for the named method. A tool that names its standard, its constants and its assumptions has given you something to check. One that produces a bare number has not.
- Do it by hand once. Slow, and conclusive.
What this site does about it
The reason every calculator here names its formula, its constants and the standard it follows is precisely this problem. A result you cannot check is a result you have to take on faith, and there is no good reason to ask that of a reader.
Behind the pages, the formulas are checked when the site is built: each module carries assertions that must pass before anything is published, so a mistyped constant fails the build rather than reaching you. That process has caught real errors — a surface-gravity table that disagreed with published values, a unit switch that silently changed an answer by a factor of 4,187.How we verify describes the method, including what it cannot prove.
It cannot prove a formula is the right one for your situation. That judgement stays with you — which is why the assumptions are on the page rather than buried in the code.