A calculator cannot tell you that you measured the wrong thing. It will take a floor area you meant as a wall area and hand back a confident, precise, useless number.
Measuring decides whether your buy list is right, and it takes about ten minutes.
What to bring
A 25-foot tape measure, a phone or paper to write on, and a pencil. A laser measure is faster for large or empty rooms and worth borrowing for a whole house, but you don’t need one, and it is less reliable in bright rooms and around clutter.
Write everything down as you go. If you hold four dimensions in your head while climbing over furniture, a 12 becomes a 21.
Wall area, for paint and wallpaper
Measure the perimeter, then the height, then multiply.
- Measure the length of each wall along the floor, at the baseboard.
- Add them together. A 12 × 16 room gives 12 + 16 + 12 + 16 = 56 linear feet.
- Measure floor to ceiling in two or three places. Older houses are rarely level, and a half-inch of variation across a long wall matters more than it sounds.
- Multiply: 56 × 8 = 448 square feet.
Measure the walls, not the floor. The floor of that room is 192 square feet and the walls are 448. Using floor area is the most common estimating error, and it leaves you more than half short.
Round up, never down. If a wall measures 11 feet 8 inches, call it 12. You are buying materials in whole units anyway, and the arithmetic is a poor place to economise.
Floor area, for flooring, tile and mulch
For a rectangular room, length × width in feet. For anything else, break it apart.
L-shaped rooms split into two rectangles. Draw the shape, cut it into rectangles along the shortest line that works, measure each one, and add the areas. Don’t average anything. Measure both.
Alcoves, bay windows and closets are separate rectangles. Include them if you are flooring them.
For round or curved areas, such as a circular patio or a round bed, use radius × radius × 3.14. Measure from the centre to the edge in a couple of directions and use the larger.
Genuinely irregular shapes, like a curved garden bed or an odd-angled deck, can be approximated by covering them with rectangles and triangles on a sketch. A triangle is base × height ÷ 2. It doesn’t need to be exact, since a few percent over is what the waste allowance is for anyway.
Openings: when to subtract
For paint, leave one or two doors and windows in the total. They act as a buffer, and the paint you don’t use on the window goes into touch-ups.
Start subtracting when openings are a real fraction of the wall, roughly 10% or more. Standard sizes:
| Opening | Approximate area |
|---|---|
| Interior door | 21 sq ft (3 ft × 6 ft 8 in) |
| Exterior door | 24 sq ft |
| Standard window | 15 sq ft (3 ft × 5 ft) |
| Large picture window | 35-50 sq ft |
| Sliding patio door | 40 sq ft |
For wallpaper, always subtract openings. It is expensive, sold in whole rolls, and the pattern repeat already builds in generous waste.
For flooring, subtract only what won’t be floored: a hearth, a permanent island, a built-in. Don’t subtract a doorway, because you floor through it.
Ceiling height, and the things that change it
Measure floor to ceiling, not baseboard to crown moulding. If you have crown moulding and are painting up to it, measure to the bottom of it.
Vaulted or sloped ceilings need a different approach. Measure the height at the lowest point and at the highest, and use the average for the sloped wall. That is a genuine approximation, so add a little.
Stairwells are the hardest surface in any house to measure and the one most often underestimated. Measure the tallest wall at its full height, and expect the paint to go further than you think because you will be working from a ladder and wasting more.
Take a second pass at anything expensive
For paint at forty dollars a gallon, one careful pass is fine. For hardwood flooring at six dollars a square foot, measure the room twice, on different days if you can, and compare. A 4% error across 400 square feet of oak is a hundred dollars.
Double-check any dimension you were uncomfortable taking: the one where the tape sagged, or you were reading around a sofa, or you estimated the last foot. That one is probably wrong.
Write it down like this
For each room, record:
- Each wall length, individually, not just the total
- Ceiling height, and where you measured it
- Number and size of doors and windows
- Floor dimensions, broken into rectangles if the room is not one
- Anything unusual: a slope, an alcove, a chimney breast
Keep the individual wall lengths. If you later decide to paper one wall and paint the rest, you have the numbers. If you only kept the perimeter, you are measuring again.
The checklist
- Walls: perimeter × height. Floor area is not wall area.
- Measure ceiling height in several places. Rooms are not square.
- Break irregular rooms into rectangles and add them.
- Subtract openings for wallpaper always, and for paint only above about 10%.
- Round up to the next whole foot.
- Measure twice for anything expensive per square foot.
- Write down each wall separately, not just the total.
With those numbers in hand, every calculator on this site takes them directly: paint, flooring, wallpaper, decking, fencing, mulch, gravel and BTU.