The second trip to the hardware store is almost always caused by a bad measurement. This guide covers exactly how I measure rooms for paint, tile, and flooring — and the small differences between them that catch people out.
What you need
A 25-foot tape measure, a pencil, and a scrap of paper. That’s it. A laser measure is faster for bigger rooms but a tape measure is perfectly accurate if you use it right. I keep a short stub of pencil in my tape measure holster for exactly this purpose.
How to measure a rectangular room
Measure the length and width at floor level, along the baseboard. Not waist height, not diagonally across the middle of the room — along the wall, as close to the floor as you can get the end of the tape. Floors are occasionally not square, so take the measurement at the widest point of each dimension.
Write both numbers down. Multiply them: that’s your floor area in square feet. A 12-foot by 14-foot bedroom is 168 square feet of floor.
Alcoves and bay windows: measure them separately and add them in. A dining area that pokes into a bay window gets its own length-times-width measurement, then you add it to the main room total. For paint, if the alcove has its own walls, add those wall areas too. If you’re tiling the whole floor including the bay, add the bay floor area. Don’t try to cram an irregular room into one measurement — it never works cleanly.
How to measure an L-shaped room
Split it into two rectangles. There are usually two obvious ways to cut an L-shape; pick whichever gives you the cleanest rectangles with the fewest awkward leftover bits. Measure each rectangle separately, then add the areas together.
A common mistake: people try to measure the outer dimensions of the L and then subtract the missing corner. That math works, but it’s easier to get wrong. I always go with adding rectangles rather than subtracting a corner — one less thing to mess up under a store’s fluorescent lights when I’m writing down the numbers.
Paint vs tile vs flooring — the differences that matter
This is where things get specific, because these three materials cover different surfaces.
For paint, you measure the walls, not the floor. Measure the perimeter of the room (all four wall lengths added together), multiply by the ceiling height, and that gives you total wall area. Then subtract for doors (roughly 20 sq ft each) and windows (roughly 15 sq ft each for a standard window). I usually subtract half the door and window area for the first coat and all of it for the second, because paint still covers those surfaces to some degree in the first pass. The paint calculator handles all of this automatically.
For tile, you measure the floor (or the specific wall you’re tiling). Floor area only — ceiling height is irrelevant. The tile calculator then adds a waste factor on top, because cuts at the edges and occasional breakage mean you need more tiles than the clean area calculation suggests.
For flooring (hardwood, laminate, vinyl plank), you measure the floor area just like tile. But flooring usually ships in fixed-width planks and you often can’t use cut pieces from one row to start the next, depending on the pattern. The flooring calculator accounts for this — the waste factor for flooring tends to run higher than tile (10-15% vs 7-10%) because of the directional nature of plank installation.
For any area-based calculation, check the concrete calculator page for how I handle slab area — concrete is measured the same way (length x width) but you also need depth, which adds a third dimension.
Common mistakes
Measuring at waist height. Walls aren’t always perfectly plumb, and furniture or trim can push your tape off the actual floor dimension. Always measure at the floor.
Forgetting which surface you’re covering. For paint: walls. For tile and flooring: floor. Sounds obvious, but I’ve seen people bring wall measurements to a flooring store and wonder why their estimate is 40% too high.
Not accounting for closets. If you’re painting the walls of a room that has a closet, the closet walls need paint too. Measure the closet separately and add it. If you’re tiling a kitchen floor and the pantry opens off it, the pantry floor is separate area. Don’t assume your main-room measurement catches everything.
Rounding down. Always round your measurements up to the next foot, not down. The small area lost by rounding down adds up to a tile or a section of planks that you’ll be short on. Better to have a little extra than one more checkout trip.
FAQ
How do I measure an irregular room that isn’t rectangular or L-shaped?
Break it into as many rectangles as you need. A room with a diagonal cut corner, for example, becomes a rectangle plus a right triangle; the triangle area is half of base times height. For anything genuinely odd-shaped (a round turret room, a room with angled ceilings), I’d measure the bounding rectangle and then subtract the obviously uncovered sections. It takes a few minutes but it beats guessing.
Should I measure in feet or inches?
Feet with decimal fractions, or feet and inches — but be consistent. If one wall is 12 feet 6 inches, write it as 12.5 feet (or 12’6″), not just 12. Half a foot on a 14-foot wall is 7 square feet of floor area, which is meaningful for tile and flooring counts. I measure in feet and tenths because it’s easier to multiply than feet-and-inches.
What if my walls aren’t straight?
Older houses especially can have walls that bow in or out by half an inch or more. Take your measurement at the longest point of each wall — the widest point of a bowing wall, the longest stretch of an uneven one. This gives you the worst-case coverage requirement, which is what you want when buying materials. You’d rather have a little extra tile than fall one tile short on the last row.
Do I need to measure every room myself, or can I use the floor plan?
Floor plans are a good starting point, but I wouldn’t trust them alone. Builder drawings often show nominal dimensions (the intended size) rather than the as-built size, and they never account for wall thickness changes or bumped-out spots. Take your own measurements and use the floor plan only as a sanity check that your numbers roughly match.
Written and maintained by Sam Walsh. Last updated: July 2026.