The short answer
Set the standing desk so the keyboard is about level with your elbows while your shoulders stay relaxed, elbows remain near your sides, and wrists stay straight. The right desktop height varies with the person, keyboard and footwear, so use your typing posture as the check rather than a single height chart.
How high should a standing desk be?
There is no one desk height that fits everyone. Begin with the keyboard around elbow height. Let your upper arms hang comfortably by your sides, relax your shoulders, and keep your wrists in line with your forearms as you type. OSHA describes keyboard placement near elbow height and calls for relaxed shoulders and straight wrists.
How do you set the desk height?
Stand in the shoes you normally wear at your desk. Move the keyboard and mouse close enough that you do not reach. Raise or lower the surface until your elbows are near the keyboard height without lifting your shoulders or bending your wrists. Type for a few minutes, then make small adjustments; the keyboard position matters more than the desktop number on the controller.
Where should the monitor go?
Adjust the monitor after the keyboard feels comfortable. Place it in front of you so you can see the screen without tilting your head sharply up or down. If bringing the keyboard to a comfortable height makes the monitor too low, use a stable monitor riser or adjustable arm rather than raising the keyboard again.
Should you stand all day at a standing desk?
A sit-stand desk is most useful when it makes changing position practical. Alternate between sitting, standing and other tasks as comfortable; standing continuously is not the goal. OSHA recommends opportunities to alternate postures and vary work activities.
What if the desk cannot reach a comfortable height?
Check whether the desk range works for your standing and seated positions with your actual keyboard, monitor and shoes. A separate keyboard tray, monitor support or footrest may help the setup fit. Compare stability, lifting capacity, leg clearance and warranty before replacing the desk.
Sources
OSHA: computer workstation guidancePractical guidance for fitting a chair, desk, monitor and keyboard to the person using them. It is not a product endorsement.