The room is almost never the problem. The doorway is, or the stair turn, or the corridor outside the lift. Those are the measurements people skip, and they are the ones that strand a sofa on a landing.
Furniture that fits a room perfectly is useless if it cannot reach the room. Work the delivery path backwards from where the item will stand to the front door, and measure every constriction on the way.
The diagonal readings here are exactly what a laser measure makes easy and a tape makes awkward, because you can shoot corner to corner without needing a second person to hold anything. Measuring alone is covered here.
For any large item, the constraint is its smallest cross section against the tightest point on the route. A sofa can usually be tipped onto its end or its back, so the relevant figure is often its height and depth rather than its length. Ask the retailer for the depth and height, not just the length.
Once the route is clear, the room measurements are the easy part, but there are four that people forget.
Wall length between obstructions, not wall to wall. The usable run for a sofa is from the edge of the chimney breast to the edge of the door architrave, and that is always shorter than the wall.
Radiator and skirting projection. A radiator sticks out three to four inches, and furniture pushed against the wall will sit that far forward. On a tight fit this is the difference between working and not.
Window sill height and depth, if anything is going under a window. A sill lower than the back of a sofa changes what fits.
Door swing arcs. Mark on your sketch how far each door opens into the room. A wardrobe that fits the wall perfectly and blocks the door is a common and expensive discovery.
Fitting furniture to the exact inch produces rooms that technically work and are unpleasant to live in.
Useful working figures: allow around 30 to 36 inches for a main walking route through a room, and about 18 inches between a sofa and a coffee table, which is close enough to reach and far enough to get past. Dining chairs need roughly 36 inches behind them to pull out and sit down, and closer to 44 inches if people have to walk behind a seated diner.
For a bed, allow at least 24 inches on any side you need to walk down, and remember that a made bed with a duvet is wider than the mattress dimension you were quoted.
The cheapest step in this entire process, and the one that catches what measurements miss.
Mark the footprint on the floor with painter's tape before ordering. Take the width and depth from the retailer's specification, tape a rectangle onto the floor, and live with it for a day. Walk the route past it, open the doors, sit down where the chair would be.
This exposes the things a number cannot: that the walking line is now awkward, that the door catches, that the room feels crowded even though everything fits. For a large piece it is worth an hour, and it is far cheaper than a return.
Half of this problem is on the retailer's side and the listed figures are not always what you expect.
Check whether the depth quoted includes cushions, which plump beyond the frame. Check whether the height is to the back or to the top of a headrest. Check whether the width includes arms that flare. And ask whether the legs unscrew, because on a sofa that is frequently the two inches that gets it through a door.
When the fit is genuinely tight, the number that matters is the diagonal of the item's cross section rather than either dimension on its own, because that is what has to clear the narrowest point when the item is turned. The general room method is here, and turning your measurements into a plan makes furniture layout much easier to reason about.
If you are still deciding which laser measure to buy at all, our front page is the shorter version of everything on this site.