The slider that moves the seat, not the chair
Most chair controls raise, lower or tilt the whole seat. Seat depth is the exception: it carries the seat pan itself out and in, which changes how far the cushion reaches along your thighs, so one chair can fit a long leg or a short one. The control is called the seat slider, and where a chair has one it is usually a lever beneath the front of the seat; some chairs use a turn-knob instead, and many leave the adjustment out altogether, so the pan is a fixed length you cannot change. Whether your chair adjusts at all is the first thing to settle before you try to set it.
Set it by the gap behind your knees
Set the height first, because the seat has to be at its working height before the depth reads true, and our guide to office chair height follows the HSE's order for that. Then sit as far back as the chair allows, so you are against the backrest rather than perched forward, and judge the fit from there. The task-chair maker Merryfair gives the target plainly on its own adjustment guide: "check the gap between the front edge of the seat and the back of your knees. You should fit two to three fingers in that space", and "if your chair has a seat depth slider, pull it forward until that two-to-three-finger gap appears". Because you are matching the seat to your own leg, the makers give it as a hand-width rather than a figure in centimetres, since the length that fits one person is wrong for the next.
Too deep, too shallow
Which way to nudge the slider depends on how that gap looks. If the seat runs too far forward, its front lip reaches the backs of your knees before you are fully back in the chair, and you end up sitting forward off the backrest to stay clear of it. If it runs too short, your thighs overhang the front with nothing beneath them, so your weight rests on less of the cushion than the seat was built to carry. Sliding the pan back closes an over-long seat and drawing it forward fills in a short one, and you stop when the backs of your knees clear the edge with the cushion still under most of your thigh.
Why a slider is worth looking for when you buy
A seat pan with no slider is a single depth, picked for an average leg, and it suits the people who sit close to that average. A slider widens the span one chair can cover, so it earns its place if your legs are longer or shorter than the norm, or if more than one person shares the seat. It is also the adjustment a budget chair is likeliest to drop, ranking below height, arms and lumbar support, so on a cheaper model it is often what you trade away; our office chairs under 300 guide weighs chairs with that order in mind. If you are choosing between two chairs and your legs sit outside the average, the one with a working slider is the safer fit.
FAQ
Q: Which lever on the chair is the seat depth one? A: It is usually a second lever under the seat, set towards the front or the front-left, while the lever under the right-hand side is almost always the height. Lift or squeeze it to release the pan, shift your weight to slide the seat, then let go to lock it. If the only control you can find is the height lever, or the chair takes a knob rather than a paddle, check the maker's diagram, because some chairs put the depth release in an odd spot and some do not fit one at all.
Q: Is seat depth the same as seat height? A: No, they are separate controls that are easy to mix up. Height raises and lowers the whole seat to set your arms and feet against the desk; depth leaves the height alone and moves the pan forward or back to suit the length of your thigh. Set the height first and the depth after, because changing the height later shifts where your legs meet the cushion and undoes the depth you had.
Q: What if my office chair has no seat depth adjustment? A: Then the pan is a set length and you fit yourself to it. Sit right back and see where the gap behind your knees falls: if it is already a couple of fingers wide, the fixed depth happens to suit you. If the seat sits too deep, the workaround Merryfair points to is a firm cushion against the backrest, which pushes you forward so the edge clears your knees. If it is too shallow there is little to add, which is the honest reason to look for a slider on the next chair.