Batts are manufactured to fit between studs and joists at standard spacings, so the useful measurement is the cavity, not the wall. Get the spacing wrong and every piece needs cutting.
Framing is built to standard centres, commonly 16 inches or 24 inches on centre, and batt insulation is manufactured in widths to friction-fit between them.
Measure centre to centre across several studs rather than a single gap, then divide. A single gap can land on an irregular bay, particularly near a corner, a window or where two framed sections meet, and building a whole order on that one reading means every batt is the wrong width.
Buying batts that match your actual spacing means they friction-fit and stay put. Buying the wrong width means cutting every single piece lengthwise, which is slow, wasteful and produces gaps that undermine the point of insulating at all.
Insulation works by trapping still air, and a gap lets air move around the barrier. A poorly fitted batt with gaps at the edges performs considerably worse than its rating suggests, which is why the spacing measurement matters more than it first appears.
Measure the depth of the framing cavity, from the face of the sheathing to the front edge of the stud.
That depth caps how much insulation can fit without compressing it, and compressed insulation loses performance because you have squeezed the air out of it. Batts are sold in thicknesses matched to common framing depths for exactly this reason.
The R-value you can achieve is therefore constrained by the framing you have, unless you add insulation outside the frame. That is a specification question rather than a measuring one, but the cavity depth is what starts the conversation.
For batts, the most reliable count is the number of cavities and their height.
Count the bays along each wall and measure the height of the cavity, floor to ceiling for a wall or the joist length for a floor or ceiling. Batts come in fixed lengths, so a cavity height of 8 feet against a batt length of 93 inches tells you whether each bay takes one piece or one plus an offcut.
Square footage still matters for ordering, since insulation is sold by coverage area per bag or roll, but the cavity count is what tells you whether the pieces fit cleanly.
For loose fill or blown insulation the logic reverses: area and target depth are what matter, and the framing spacing is irrelevant because the material fills whatever shape it lands in.
Attic floors are usually the highest-return area to insulate and are measured as floor area times the joist spacing count.
Watch for the parts that are not simply floor: the hatch, any boarded storage area, and the edges where the roof slope meets the joists and there is no room for full depth. Those edges are also where insulation most often gets pushed into the eaves and blocks ventilation, which is a fitting issue rather than a measuring one but worth noting while you are up there.
For a sloped ceiling, the area is larger than its plan area exactly as a roof is, so apply the pitch multiplier. The multiplier table is here.
Insulation is sold by the square footage each pack covers, so divide your total area by the pack coverage and round up.
Allow extra where there are lots of interruptions: windows, doors, services running through cavities, or irregular framing. Each one produces a cut piece and an offcut that may not fit the next bay.
Compression during transport means bags expand when opened, so a delivery looks smaller than the coverage claims. That is normal. Drywall goes on over it and is measured off the same walls, and the framing measurements are covered here.
For the wider picture on choosing a laser measure, including what the specifications actually mean, start on our front page.