You cannot fix an out-of-square box at the hinges. The doors inherit the shape of the box forever.

Measure both diagonals on every box, before final tightening

The method is exactly as simple as it sounds and suppliers spell it out: as you assemble each box, use a tape to measure from one corner to the opposite corner, then measure the other diagonal, and if the two are not identical, gently adjust the box until they are. A rectangle with equal diagonals is square, and there is no other quick way to know. The part that matters is when: do it before you tighten everything down, and if you are using glue, do it in the minutes before the glue grabs. Every door, drawer and shelf in that cabinet is going to reference the shape you lock in, so this is a gate to pass through rather than a check to perform at the end.

Hinge adjustment cannot rescue a box that is not square

Worth understanding because it explains why people burn whole afternoons on the wrong problem. Modern cabinet hinges adjust in several directions and they are genuinely capable, which creates the impression that door alignment is always solvable at the hinge. It is not, because the hinge can move the door and cannot change the shape of the opening the door has to fit. A supplier describes the outcome plainly: six hours of hinge fiddling and still an eighth of an inch of gap. If a door gap tapers from top to bottom on a new cabinet, that is the box telling you it is a parallelogram, and the honest options are to take it apart or to live with it.

The back panel is what holds the box square

This is the part almost everybody underestimates. The thin back panel is not a dust cover, and a supplier states its function directly: the back panel gives the cabinet structure and helps it stay square. It drops into quarter inch grooves cut into the sides and bottom, and once fully seated it acts as a diaphragm that stops the box racking. So three things matter. It has to go in the right way around, with the unfinished face outward. It has to seat fully in every groove rather than being forced or left proud in one corner. And it has to be fitted to a box that is already square, because a back panel installed into a racked box locks the error in permanently rather than pulling anything back into shape.

Dry fit the first of each type, and not out of caution

The reason to dry fit is practical rather than nervous. A supplier explains that it lets you verify the part count and confirm the cam locks or screws align, and that it gives you a rehearsal which reduces the panic that sets in once the glue starts to tack up. That last part is the real value: assembly with glue is a timed exercise and the first attempt is where you are slowest. The specific error it catches is worth quoting, since the same source says it happens more often than people expect: getting halfway through a glue-up and realizing you are holding the left-side panel for a different cabinet size. Dry fit one of each type, then batch the rest.

Stop tightening a quarter turn past resistance

Overtightening is a separate failure with its own consequences, and guidance names them: stripped threads, cracked panels and weakened joints. The instruction is refreshingly concrete. Tighten until you feel resistance, then give it a final quarter turn and stop. On a power drill, use a low torque setting or the clutch rather than trusting your wrist, because cam locks and screws in engineered panels strip much faster than the same fastener in solid lumber and the damage is not reversible. A stripped cam lock in a side panel is a cabinet with one fewer working joint, and it will show up later as a box that will not hold square under load.

Clamps are not optional, they are the second person

Assembly guidance is consistent that clamps hold everything square while you glue or screw, which is precisely the job a second pair of hands does. Two clamps are enough for a small project and four make a real difference, with bar clamps and corner clamps both worth having for larger boxes. Corner clamps in particular do the specific thing this page is about, which is holding ninety degrees while a joint is fastened, so the box arrives square rather than being adjusted into square. This is also the cheapest tooling decision in the project and the one most likely to be skipped by somebody assembling a kitchen for the first time.

Gang a wall of bases together on their backs

The best technique on this page comes from Tom Silva at This Old House, and it connects the assembly stage to the installation stage in a way nothing else does. Lay the base cabinets on their backs, clamp a whole wall’s worth together into effectively one long box, join them with inch and a quarter bandhead screws, then tip the entire assembly up and move it into place. The reasoning is that it produces a consistent, straight installation and reduces the number of adjustments needed once the cabinets are in position. It is much easier to get eight cabinet faces into one plane on a floor, with clamps and gravity helping, than to align them one at a time against a wall.

A sixteenth of an inch is the tolerance, because it multiplies

The number suppliers work to is a sixteenth of an inch, and the reason is accumulation rather than perfectionism: because cabinets have to align across the entire kitchen wall, any small gap will multiply. Eight boxes each a sixteenth out is half an inch by the end of the run, and that is the error the countertop fabricator eventually measures and the eye reads along the door line. It also means the assembly surface matters. A box built on an uneven garage floor starts out twisted no matter how carefully the joints are fastened, so clearing a genuinely flat area is part of the setup rather than housekeeping.