The elbow sets it, not the person
Desks are sold to people by their height, and the fit is decided by their elbow. Those are not the same measurement and they do not track each other reliably. Sitting height — how much of a person is above the seat — varies between people of identical stature by several inches, and the elbow rides on top of that. Add a chair that can be anywhere within six inches of travel and the standing-height number has stopped being informative altogether.
This is why two people who are both five foot ten can measure a seated elbow height of 26 and 29 inches, and why a desk that is obviously right for one of them is obviously wrong for the other. Neither person is unusual. The stature figure just never contained the information.
Travel span is not the failure mode
Adjustable desks are advertised on their range, and the range is rarely what goes wrong. Take the defaults here: a seated elbow at 27.5 inches and a standing elbow at 43, a mat adding three quarters of an inch under the standing position, and a keyboard sitting 0.9 inches proud of the surface. That gives a seated surface of 26.6 inches and a standing surface of 42.85 — 16.25 inches of travel needed. A desk quoting 25.5 to 50.5 has 25 inches of travel, far more than enough, and it covers both ends with 1.1 inches to spare at the bottom and 7.65 at the top.
Now imagine the same desk with a thicker top fitted, so the bottom of its range is 28 instead of 25.5. The travel span is unchanged. The desk now misses the seated position by 1.4 inches and no setting recovers it, because the problem was never the span. The bottom of the range is the number that fails, and it is the number least often quoted prominently.
The keyboard is part of the desk
The surface height and the keying height are two different figures, and the difference between them can be more than an inch either way. A keyboard resting on the desk lifts the keys by its own thickness. A tray slung underneath drops them, sometimes by two inches or more, which is why a tray occasionally rescues a desk whose bottom setting is too high. Enter the tray as a negative number and the seated figure moves up by the same amount, which is the desk height that now produces the same keying height.
Fixed desks and the floor
On a fixed desk the chair is the only adjustment left, and raising it has a consequence that has nothing to do with the desk. At the defaults — a 29 inch desk, a keyboard 0.9 inches proud, an elbow 10 inches above the seat — the seat has to sit at 19.9 inches for the elbow to meet the keying surface. If the back of your knee is 17.5 inches off the floor, that leaves 2.4 inches of air under your feet.
The desk did not move, so the chair did, and the floor stayed where it was. A platform that thick puts a surface back under your feet at the height they used to be. Whether that arrangement suits you is not a question arithmetic answers; the arithmetic only tells you the number.
What this page will not do
It will not tell you a correct desk height. There is no such figure that applies to a person, and the published ones disagree with each other in any case. Everything here starts from four measurements you take yourself and is arithmetic from there. If the reason you are reading this is that something hurts, that is a medical question and belongs with a clinician, not with a calculator.
Questions people ask
Why does the page not work the desk height out from my height?
Because the relationship is too loose to be useful. The measurement that decides the fit is elbow height, and elbow height depends on how much of your stature is torso and where the chair is set, neither of which is contained in a single standing-height number. Two people who measure the same standing can want desk surfaces three inches apart, and the calculator would be manufacturing precision it does not have. Four measurements take about two minutes and give an answer that is actually about you.
How do I measure seated elbow height on my own?
It is genuinely awkward alone, because the measurement changes if you lean to read the tape. The practical way is to sit as you normally do with the forearm level, rest a straight edge on top of the forearm against a wall or a door frame, mark where it touches, then stand up and measure the floor to the mark. Take the chair seat height at the same sitting — the two numbers are only consistent if they were taken together, and the calculator uses the difference between them as a body dimension.
The desk has plenty of travel but the result says it does not fit. Is that a bug?
No, and it is the most common real outcome. Travel span and range position are independent. A desk with 25 inches of travel that starts at 28 inches will never reach a 26.6 inch seated position, no matter how much range sits above it. The calculator reports the shortfall separately at each end for exactly this reason. If the span itself is short, that is a different failure and the page says so in different words.
Does the anti-fatigue mat really matter?
It matters by its own thickness and by nothing else, but that is not nothing. Standing on three quarters of an inch of foam raises your elbow by three quarters of an inch, so the standing surface height goes up by the same amount. Enter the compressed thickness rather than the figure on the label, because the label describes the mat with nobody on it, and a soft mat can lose half its stated thickness under a person.
Is a standing desk better for me than a seated one?
That is a health question and this page does not answer health questions. There is published research and it is argued about; nothing here endorses a conclusion, states a benefit, or claims that any arrangement prevents or relieves anything. What the calculator does is arithmetic: given the elbow heights you measured, here are the desk heights that match them and here is whether a particular desk reaches both. If you want to know what a change would do for you specifically, ask a doctor or a physiotherapist.