The height between counter and wall units sounds like one number. It is not, because the wall units are not all at the same height and the hob area usually gets treated differently.
A backsplash is rarely one continuous rectangle. It is a series of sections at different heights, interrupted by windows, appliances and gaps between wall cabinets.
Break the run into sections and measure each one's width and height separately. Typical sections are: under the wall cabinets, behind the hob where there may be no cabinet above, beside or under a window, and any open stretch of wall where the splashback runs to a different height.
The standard height between a countertop and the underside of wall cabinets is commonly around 18 inches, but measure it rather than assume, because installers set wall units to suit the user and the room.
Behind a hob there is often no wall cabinet, because an extractor hood sits there instead. That section runs from the counter up to the underside of the hood, which is considerably higher than 18 inches.
Measure that height specifically. It is frequently the tallest section of backsplash in the kitchen and it is the most visible, because it sits directly in the eyeline.
Hood clearances above a hob are specified by the appliance manufacturer and vary by hob type, so that height is fixed by the specification rather than by preference. Take the measurement after the hood is positioned, or from the intended position on the plan.
Some backsplashes stop at the wall cabinets. Some run to the ceiling on an open wall. Some finish at a horizontal line partway up. The decision changes every height measurement, so make it first.
Measure around a window rather than through it. The sections beside a window and the section beneath the sill are separate pieces with their own dimensions, and the sill itself may need an edge treatment.
Electrical outlets and switches sit within the backsplash area and are cut around rather than deducted. Note their positions from a fixed end so the tile layout can be planned to avoid a cut landing awkwardly across an outlet.
Where the backsplash ends at an open edge, rather than running into a wall or a cabinet, that edge needs finishing with trim or a bullnose tile. Count those edges and measure their length, because it is a separate order item.
Total the area of all sections. Backsplashes are small areas, so the numbers are modest, and this is one of the few tiling jobs where a single box or two of tile covers the whole thing.
Waste allowance runs higher than the area suggests, though. A backsplash is almost entirely edges and cuts: around outlets, under cabinets, beside windows, at the ends. Small format and mosaic tile in particular runs 12 to 17 percent waste, and a backsplash is exactly where those formats get used.
Buy a full extra box rather than calculating tightly. On an area this small the cost of one more box is trivial, and tile is batch-fired so a later top-up may not match. Tile measurement in general is covered here.
Backsplash is the last thing to go on, and it should be the last thing measured.
The counter is the bottom line and the wall cabinets are the top, so both need to be installed and final before the heights are real. Measuring from a plan gives you an estimate good enough for budgeting, and it will be wrong by whatever the cabinets moved during fitting.
Also check the wall is flat once the units are in. A backsplash sits between two fixed horizontal lines, and any bow in the wall shows as an uneven grout line at the top or bottom. The full kitchen method is here, and the countertop below it here.
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