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About this tool
Seat height, seat depth, matching desk height and footrest size from your lower-leg measurement.
Seat height is set by one measurement — popliteal height, the distance from the floor to the crease behind your knee while you sit with your feet flat — plus the heel thickness of the shoes you wear. This calculator turns that into a seat height, a usable seat depth, the desk height that matches it, and the footrest you need when the desk is fixed and the chair has to come up to meet it. It also checks whether your thighs clear the underside of the desk, and states the knee space OSHA's computer-workstation guidance asks for: at least 51 cm wide and 38 cm deep.
Open Chair Height Calculator on AltFTool — it loads instantly in your browser.
Enter the values you already know.
Fine-tune the options to match your scenario.
Read the result and use it in your planning or reporting.
Popliteal height is the dimension chair standards use, and it is easy to measure at home with a tape.
When the desk cannot move, the tool raises the seat to the desk and sizes the footrest that has to follow.
Flags when the thighs will not clear the desk apron, which is what makes tall people sit sideways.
Set the seat to your popliteal height plus your shoe heel — for a 43 cm popliteal height and 2 cm heels, that is 45 cm. Sitting at that height puts your feet flat on the floor with your thighs roughly level and no pressure from the seat edge behind the knee.
Add your seated elbow height to the seat height. For a 172 cm adult, seated elbow height is about 23 cm above the seat, so a 45 cm seat pairs with a desk around 68 cm — noticeably lower than the 73 to 75 cm most fixed desks are built at.
You need one whenever the desk is fixed above the height your legs want, because the chair has to come up to reach the desk and your feet leave the floor. With a 74 cm desk and a 45 cm ideal seat height, the seat rises to about 51 cm and the footrest needs to be roughly 6 cm tall.
OSHA's computer-workstation guidance asks for knee space at least 20 inches (51 cm) wide and 15 inches (38 cm) deep at knee level, plus enough height that your thighs clear the underside with a couple of centimetres to spare. Drawers, cross-braces and thick aprons are the usual culprits when that fails.
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