Every other space in a kitchen is a box you can measure at its edges. Under the sink, the obstruction is in the middle. A waste pipe comes down where you would most like to put something, the trap swings out to one side, the shutoff valves stand off the back wall, and a disposal unit takes a corner of the ceiling. Measure the cabinet and you get a number that describes almost none of the space you can use.
So this page is a measuring procedure and nothing else. It names no product and gives no product dimensions, because this site lists no under-sink model. Nothing here was tested by hand, and no price, rating or stock figure appears.
Measure the clear box, not the cabinet
The only measurement worth writing down is the largest rectangular space that is actually empty from the floor of the cabinet up. Not the cabinet. Not the opening. The empty part.
In most sink cabinets that is not one space, it is two: a strip on one side of the waste pipe and a wider one on the other. Measure them separately and write down two sets of three numbers. A single width-by-depth-by-height for the whole cabinet describes a space that is not there, and it is the number that makes people buy a tray which cannot go in.
Take each area the same way. Width between whatever bounds it on the left and right, which may be a cabinet wall on one side and a pipe on the other. Depth from the back of the door opening to whatever stops you at the back, which is often a valve rather than the wall. Height from the cabinet floor up to the lowest thing above that area, which is usually the basin, sometimes the trap, and under a disposal is the disposal.
Make a list of what is in there before you measure anything
Walk the cabinet with a torch first and write down what you find, because each one takes space in a different direction:
- The waste pipe and trap. The trap is the widest part and it does not run straight down; it turns. Measure to the outside of its widest point, not to the pipe above it.
- Shutoff valves. They stand out from the back wall, so they set your usable depth on the side they are on, not the wall behind them.
- A disposal unit. It hangs from the basin and eats the height on one side entirely. Anything you put beneath it is limited by the bottom of the unit.
- Hinges and the door. A hinge mounted on the cabinet wall takes space when the door closes, and a door that does not fold flat narrows the opening you have to pass things through.
- The cabinet floor itself. Sink cabinet bases are not always flat or level, and some have a leak pan or a raised lip at the front. A tray sits on whatever is actually there.
No measurement on that list has a typical value worth printing. Trap positions, valve heights and disposal sizes vary by cabinet and by the plumber who last worked on it, and a figure invented here would be worse than no figure. Measure yours.
The opening is a second measurement, not the same one
A thing that would sit happily in the space still has to get into the space. The door opening on a face-frame cabinet is narrower than the cabinet, sometimes by a lot, and a rigid tray has to pass through it at some angle that clears the frame, the hinges and the pipe on the way in.
Measure the opening on its own: width between the frame edges, height from the floor of the opening to its top edge. Then ask whether the thing you are considering can be tilted or turned to get through. If it only fits going in flat, and the trap is in the way of flat, the answer is no even though all three numbers matched.
What the size check will and will not say about this space
The size check on this site compares rectangles. For an under-sink space it will do exactly that and no more, and it says so rather than implying a clean answer. Two confirmations are yours to make, and until you tick them the result lists both under its verdict:
- Confirm hinges, pipes, runners and other obstructions are clear.
- Confirm the product can enter the opening in the stated orientation.
It treats anything you type in as standing on the floor of the space, so mounting, support and moving clearances are left to you. And where a measurement lands exactly on a dimension with nothing to spare, it refuses to call that a pass and asks for a manual check instead.
This site does not build a three-dimensional map of your plumbing, does not suggest moving a pipe, and does not suggest drilling the cabinet. What it can tell you is whether a rectangle you measured is big enough for a rectangle someone published. In this cabinet that is a smaller claim than usual, and it is the honest limit of the tool.
Where a maker’s page runs out
It is fair to ask why this site lists no under-sink product when the category is full of them. Here is a worked example of the problem. For the opposite case, where a page does state the figure and states it twice in two units that disagree, see the stated minimum drawer height.
Rev-A-Shelf sells a well-known under-sink pull-out, the 4SBSU series. Its series page says the range is "available in three sizes for sink base 30", 36" & 42"." That is the entire dimensional content: three cabinet sizes the series is meant for. On the day it was checked, the page’s Installation Instructions, Specs and Physical Specifications sections were unpopulated and the product was marked Coming Soon. No minimum opening, no depth, no height, no pipe clearance.
"Sink base 30 inches" is a cabinet size, not a measurement of your cabinet, and certainly not a measurement of the space left after your plumbing. A record here needs published figures for the product and, for a mounted unit, the exact model’s installation requirements. A series page that gives a cabinet size and nothing else cannot support a fit statement, so no record was made. When a maker publishes the model-level instructions, that changes.
Where the plan fails, in order
The cabinet is measured instead of the clear space. The most common one, and the most expensive, because everything that follows is built on a number that describes a space containing a pipe.
The space is treated as one rectangle. Two usable areas either side of a trap are not one area of their combined width, and nothing on the market spans a trap unless it was built to.
The opening is forgotten until the box arrives. Fits the space, will not go through the door: the one failure that survives a perfect set of measurements.
A cabinet size is read as a product size. "For sink base 30 inches" tells you which cabinet a maker had in mind. It does not tell you the product’s own dimensions, and it says nothing at all about your plumbing.
What to write on the sticky note
For each clear area separately: width, depth, height, and which obstruction bounds it on each side. Then the opening: width and height. Then one sentence about how a tray would have to be angled to get in.
That is more writing than a drawer takes, and it is the reason this space is worth measuring properly once. The measuring guide covers the tape technique, and the size check will compare each area with a product you found, while telling you plainly which parts of the question it cannot answer.
Sources
- Rev-A-Shelf, 4SBSU series page: "The 4SBSU Series is made of our classic Maple hardwood with a UV cured clear finish to ensure an acceptable match to any cabinet. This handy stow-away is available in three sizes for sink base 30", 36" & 42"." On the date checked, the Installation Instructions, Specs and Physical Specifications sections were unpopulated and the product was marked Coming Soon; no minimum opening, depth, height or pipe-clearance figure is published on that page. rev-a-shelf.com (checked 2026-09-07; evidence B-EV-03). A research example, not a recommendation: this product is not a listed record here.
- Inch & Drawer, How to Measure a Drawer for an Organizer, the Drawer Organizer Size Check and how figures get onto this site (own site). The two confirmations and the mounted-product wording quoted above are the size check’s own result text.