Slabs, footings, pads and foundations are all the same calculation: area times depth. What varies is the depth, and the depth is set by what the concrete has to carry.
Concrete fills whatever you build for it, so the dimensions that matter are the inside faces of the formwork rather than the area you marked out.
Measure the form length and width at both ends. Formwork is timber and it is rarely perfectly straight over a long run, and it bows outward under the pressure of wet concrete, so the finished slab is usually fractionally larger than the form measured empty.
Check the diagonals. A slab that is out of square by two inches across a garage floor is a problem you want to find before the pour, not after, and the diagonals are the only reading that tells you.
Four inches is the long-standing residential standard for patios, walkways and driveways taking cars and light SUVs, laid over a properly compacted granular base.
Five to six inches is usual for garage floors and heavier duty driveways, and five inches is a common recommendation on driveways in colder climates for extra margin.
Treat those as typical figures rather than a specification for your project. Subgrade, climate, expected loads and local code all bear on it, and this is a question for a contractor or your building department rather than a generic article.
Most slabs are not a uniform slab. There is usually more concrete somewhere.
A thickened perimeter edge is common on slabs that carry a wall or take edge loading. It runs around the whole boundary and it is easy to leave out entirely. Measure its width and depth and treat it as a long thin box around the perimeter.
Footings are trenches: length times width times depth. Measure each run separately and add. A footing 40 feet long, 18 inches wide and 24 inches deep is 40 times 1.5 times 2, which is 120 cubic feet, or 4.4 cubic yards.
Pads and piers are individual boxes or cylinders. Count them, measure one, multiply.
Calculate each element at its own depth and add the volumes. Averaging a slab with a thickened edge into one depth produces a number that is wrong in a direction you will discover on pour day.
It is tempting to deduct for a column base or a drain that sits within the slab area. Unless it is genuinely large, leave it in. The subgrade will absorb more than you expect and the deduction is usually smaller than the hollows it is competing with.
Cubic yards equals length in feet times width in feet times depth in inches, divided by 324. For a standard 4 inch slab there is a shortcut: divide square footage by 81.
Then add around 10 percent, which for concrete is closer to mandatory than advisory. The ordering side, including truck sizes and short load charges, is covered here.
Where a slab is part of a framed structure, a few measurements are worth taking at the same time because you are already there with the tool.
Stud and joist spacing is standardised, commonly 16 inches on centre with 24 also used, and knowing which you have determines sheet layout for anything that fixes to it. Measure centre to centre across several studs rather than one gap, because a single measurement can land on an irregular bay.
Wall plate lengths, opening positions and header spans all come off the same sketch. Taking them in one pass is considerably faster than returning for each trade.
For anything structural, span tables and code determine the sizes rather than your measurements. What you are producing is the dimensions those tables get applied to. Insulation is measured off the same cavity spacing, and drywall off the same wall dimensions.
Re-measure the forms once they are built and braced, not just when they were set out. Forms move during construction, and the number you order should come from the thing the concrete is actually going into.
It is a five minute check and it is the last opportunity to catch an error while it is still cheap. Driveways are covered here, and patios here.
If you have not settled on a unit yet, the laser measures we recommend are gathered on our front page, eight of them, each with the reason it is there.