Measure the room completely before choosing a single cabinet. The services and the out-of-square walls decide what is possible, and both are easier to design around than to discover mid-installation.
This is the step that separates a kitchen that fits from one that needs scribing everywhere.
Measure each wall run at floor level, at counter height around 36 inches, and at wall cabinet height around 54 inches. In most houses these three numbers differ, sometimes by an inch or more across a run.
Base cabinets sit on the floor and are affected by the floor-level dimension. Wall cabinets hang higher and are affected by the wall at that height. A kitchen designed from one measurement will fight the room at the other two.
Also measure the diagonals of the room. Kitchens are frequently out of square, and knowing by how much tells you where the filler panels need to go.
Base cabinets are levelled on adjustable legs, so a sloping floor is manageable, but only if you know its extent before ordering. A drop of an inch across a run means the legs at one end are near the limit of their adjustment.
Shoot the ceiling from several points along the run and compare. Differences tell you the floor is not flat. Do the same horizontally to check whether walls lean, which affects how wall cabinets sit against them.
Assume all three are out until measured. Cabinet runs are built from rigid boxes of fixed dimensions, and every deviation in the room has to be absorbed somewhere. Knowing where it is lets you design the fillers in rather than improvising them.
These constrain the layout more than the wall lengths do, and they are expensive to move.
Measure the position of each from a fixed corner, along the wall and up from the floor. That gives you a point that can be transferred onto a plan.
Appliances have specified housing openings and they are not negotiable in the way a cabinet is.
Measure the intended opening width, height and depth, then compare against the appliance specification rather than its nominal size. A dishwasher described as 600mm needs slightly more than 600mm of opening to slide in and be serviceable.
Depth is the one people miss. An appliance needs clearance behind it for pipework, cabling and ventilation, and that clearance sits between the appliance and the wall. A unit that fits the opening dimensionally can still stand proud of the run because the services behind it will not compress.
For an integrated appliance, the door furniture adds to the front. For a freestanding one, check whether the door opening arc clears the cabinet beside it.
Measure the position of every opening along the wall from the corner, plus its width, and the height of any window sill above the floor.
Sill height determines whether a wall cabinet can sit beside a window or has to stop short, and whether the worktop passes under it cleanly. A sill lower than 36 inches means the counter runs into it.
Door swings matter too. A door opening into the kitchen sweeps an arc that no cabinet or appliance door can occupy.
Draw each wall as a rectangle and mark everything on it: openings, services, radiators, sill heights, the three width measurements.
A kitchen is designed from elevations rather than a plan, and a supplier or designer can work directly from good elevation sketches. Handing over a list of numbers means they have to ask you the questions the sketch would have answered.
Countertops are measured separately and later, corner cabinets are the hardest measurement in the room, and the backsplash comes after the units are in.
If you are still deciding which laser measure to buy at all, our front page is the shorter version of everything on this site.