The drawer box, not the counter height
Ask what limits the knee opening under a counter and most people say the counter height. It is nearly always the apron. A worktop at thirty six inches with an inch and a half top has thirty four and a half inches of air under it, and then a four and a half inch drawer box takes it to thirty. The counter has not moved. The opening has lost an eighth of itself to a drawer that holds tea towels.
That is worth knowing because the two are not equally expensive to change. Lowering a counter means a new worktop, new cabinet heights, a plumbing move and a surface that everyone standing at it now finds low. Taking a drawer out and putting a shallow rail or a bracket under the top instead is a cabinet job. The calculator reports both so you can see what each is worth before deciding which one to spend on.
Depth is measured to the obstruction, not to the wall
The clear depth under a counter is rarely the worktop depth. There is a backsplash, or a trap, or a rail across the back of the cabinet, or the supply pipes, and the knees stop against whichever of those comes forward furthest. Measure to that, not to the wall, and enter it. Under a sink this is usually the trap, and it is the reason a sink that looks like it has plenty of room underneath has almost none.
The toe recess buys some of it back, but only at the bottom. A three inch recess gives three more inches of depth for the feet, below the recess height, and nothing at all for the knees. Both numbers are reported because they answer different questions: the knee depth decides whether somebody can get close enough to work, and the toe depth decides whether the footplates go anywhere.
Reach falls off as a circle
This is the part that surprises people. If somebody can reach twenty four inches across a flat counter, they cannot reach twenty four inches to a shelf a foot up. The reach is a radius, so what is left at height h is the square root of the reach squared minus h squared, and the loss accelerates. The first foot of height costs a few inches of depth. The second foot costs many.
| Height above the worktop | Depth left, on a 24 in reach | Depth left, on a 30 in reach |
|---|---|---|
| At the worktop | 24" | 30" |
| 6" | 23-1/4" | 29-3/8" |
| 12" | 20-13/16" | 27-1/2" |
| 18" | 15-7/8" | 24" |
| 24" | nothing | 18" |
Those two columns are worked from the circle, not from any published figure, and they are there to show the shape of the fall-off rather than to be used as anybody standard. Put your own measurement in and the calculator does the same arithmetic on it. The practical reading is that a wall cabinet whose first shelf is a foot and a half above the counter is a different proposition from one at six inches, and moving the shelf down is usually cheaper than moving the cabinet forward.
What holds the top up over the gap
Take a base cabinet out and the worktop over that gap is unsupported. That is a real structural question with a real failure mode, and the obvious fix — a leg at each end of the opening — puts the legs exactly where the knees were going. The usual answers are brackets fixed back to the wall, a steel plate under the top, or a cantilever off the cabinets on each side, and which one applies depends on the material and the thickness.
The allowable span field on this page is empty because that figure belongs to the fabricator. Stone, solid surface, laminate on a board substrate and solid timber all behave differently, thickness changes the answer again, and the substrate and the way the ends are held change it a third time. Ask them for the number for the exact top you are buying. A worktop that fails over an opening fails onto whoever is sitting in it.
Reaching is not using, and this page has no opinion on either
Every figure above compares your dimensions to your own measurements. None of it is a requirement, none of it is a compliance check, and nothing here says a counter is accessible or usable. Reaching a tap with a fingertip is not turning it. Reaching an empty shelf is not lifting a full pan off it. Strength, grip, balance, what the day is like and what the chair does are all part of the answer and none of them are dimensions.
So the reach field ships empty and stays that way until you measure it, and what a particular person needs is a conversation with them and with their occupational therapist. If a code figure applies to this building at all — and in a private house it often does not — it comes from your building department or an accessibility specialist, not from here. Where the building is rented or open to the public, the legal question is separate from the building department one and belongs to a lawyer.
Questions people ask
What is the cheapest way to get more knee height under an existing counter?
Take out whatever is hanging below the worktop across the opening. On most kitchen runs that is a drawer box, and it is commonly four to five inches of pure loss. Replacing it with a shallow rail, or supporting the top on brackets fixed back to the wall instead, gives all of it back without touching the counter height, the plumbing or the rest of the run. The calculator shows the gain on your own numbers so you can compare it against the cost of lowering the counter.
Where exactly do I measure the reach from?
From the front edge of the worktop, sitting at the counter exactly the way it will be used, pushed in as far as it will go. That matters because getting the knees under the counter moves the whole body forward, and a reach measured while parked a foot back is not the reach anyone will have in use. Take it more than once and on more than one day. It is not a fixed number and it is not the same for two people.
Why does a shelf get harder to reach faster than it gets higher?
Because reach is a radius rather than a straight line. What is left at a given height is the square root of the reach squared minus the height squared, so the first six inches of height cost very little depth and the last six cost most of what is left. It is the reason a wall cabinet shelf raised a foot to clear something on the counter can go from usable to unreachable without moving back at all, and the reason lowering the shelf is often better value than moving the cabinet.
Can I just remove a base cabinet and leave the worktop spanning the gap?
Only if the fabricator says the material and thickness will span it, and the support does not go where the knees do. That is why the span field on this page belongs to them: stone, solid surface, laminate on board and timber all answer differently, and so does the same material at a different thickness. The usual supports are brackets back to the wall, a steel plate under the top or a cantilever off the cabinets either side, and all three keep the opening clear in a way that a leg at each end does not.
Does this tell me whether the counter is accessible?
No, and it is not able to. It compares dimensions you measured against dimensions you measured, which is a useful thing but not a verdict. Whether a layout works for a person is a judgement for that person and their occupational therapist, and it involves strength, grip, balance and equipment rather than only geometry. Whether a code figure applies to the building at all depends on what kind of building it is, and that question goes to your building department or an accessibility specialist.