Home Measuring
How to Measure a Room So the Numbers Hold Up
A step-by-step method for measuring a room correctly, including alcoves, bay windows, and how to catch out-of-square walls before they cost you money.
Decoreto · 21 August 2026 · 8 min read · every photograph here was rendered for this article
Which tool should you actually use?
A laser distance measure is worth the $20–$40. Point it at the far wall, press once, done — accurate to about ±2mm (1/16 in) over 10 metres (33 ft), and you get a reading with one hand while the other holds your notebook. A fabric or steel tape works too, but only for spans under about 3 m (10 ft); beyond that it sags, twists, or you read it at an angle and add a centimetre without noticing.
Don't trust a phone AR measuring app for anything you're about to pay for. They're fine for "will the sofa roughly fit" but they drift by 3–5 cm (1–2 in) on real walls, especially near corners and skirting boards — exactly where furniture and built-ins need to be exact.
If you only own a tape, measure everything twice from opposite ends of the wall and use the smaller number for anything that has to clear an opening, the larger number for anything that has to fill a gap.
What order should you measure the room in?
Do it in this sequence, every time: perimeter wall lengths first, ceiling height second, then every door and window opening (width, height, and sill height off the floor), then diagonals last as a check. Measuring in this order means if a number looks off later, you know roughly when it happened.
Measure wall lengths about 90 cm (35 in) up from the floor, not at skirting-board height. Skirting boards bulge out 1–2 cm (about half an inch) from the wall face, and a laser or tape resting on top of one gives you a wall that's shorter than it really is at head height — which is where most furniture actually sits.
Measure ceiling height in at least two corners, not just the middle. Old floors and ceilings are rarely parallel; a 1–2 cm difference across a room is normal in anything pre-1980, and it matters if you're ordering tall wardrobes or a built-in bookcase that has to reach the ceiling.
How do you measure an alcove or a chimney breast correctly?
Measure the width at the back of the alcove and again at the front opening — plaster is rarely flat, so these two numbers often differ by 5–12 mm (about 1/4 to 1/2 in). If you're ordering something that has to slide in (a wardrobe, a fitted cupboard), use the smaller of the two widths. If you need clearance for something that doesn't need to touch both sides, use the larger.
Measure the depth on both the left and right side walls of the alcove, not just once in the middle. A 3 mm difference over 60 cm doesn't matter for a bookshelf; it matters a lot for a fitted wardrobe door that needs to close flush.
Check the floor too — measure height from floor to ceiling at both front corners of the alcove. Floors settle unevenly near chimney breasts more often than people expect, and a 1 cm slope across a 60 cm alcove will show up as a visible gap under a tall unit.
How do you measure a bay window without getting the wrong shape?
Treat a square bay as three separate wall segments, not one curved measurement. Measure each pane's wall length individually, then measure the projection — the distance from the main wall line to the front face of the bay. Finally measure the angle between the side panes and the main wall if you can (a cheap angle finder is under $10); most bays sit at 30°, 45°, or 90°, but builders don't always follow the standard.
For a curved bay window, measure the chord — the straight-line width across the opening — and the projection depth at the centre point. Don't try to force this into a rectangle on your plan; a curved bay drawn as a straight wall will throw off both the floor area and any curtain or blind order that needs the actual curve length.
If you're briefing a builder on bay-window furniture (a window seat, a fitted bench), give them the projection depth and the angle, not just the width — the angle changes how much depth you actually get to work with at the ends versus the centre.
How do you check if your walls are actually square?
Measure both diagonals of the room, corner to corner. In a true rectangle, the two diagonals are equal. If they differ by more than about 1 cm (3/8 in) over a room 4 m (13 ft) or larger, at least one wall is out of square — common in older houses, extensions, and anything built before spirit levels were standard.
For a single corner, use the 3-4-5 rule: measure 3 units along one wall from the corner, 4 units along the other wall, then check the distance between those two points. If it's exactly 5 units, the corner is a true 90°. This takes two minutes and catches problems that ruin fitted kitchens and flooring layouts if you find them after the units are cut.
This matters more than people think when ordering fitted furniture. A wardrobe built for a 90° corner will leave a wedge-shaped gap against a wall that's actually 87°, and that gap is very visible at eye level even if it's only 1–2 cm at the far end.
How do you turn raw measurements into a usable plan?
Sketch the room freehand first, corner by corner, and write every number directly on the sketch as you take it — wall lengths, opening widths, alcove depths, diagonal check. Don't wait until you're back at a desk to remember which number went with which wall; you won't.
Once you have the numbers on paper, transfer them into a tool that calculates area for you rather than doing it by hand. Decoreto lets you draw the walls point by point using your own measurements — when the outline closes, it works out the room name and floor area automatically, which is a fast way to catch a transcription error before you order flooring by the square metre.
Compare the tool's calculated area against your own rough estimate (length × width, ignoring alcoves and bays). If they're off by more than about 5%, you've mistyped a number or misread a diagonal — go back to the room and re-check before you commit to an order.
What mistakes send people back to re-measure everything?
Skirting boards are the most common one. A standard skirting is 10–19 mm (about 3/8 to 3/4 in) deep, and measuring on top of it instead of at wall height overstates every room by a small but real amount — enough to make a wardrobe that looked fine on paper stick out past a door frame.
Measuring around furniture instead of clear wall is the second. A sofa pushed against a wall reads as "wall length minus sofa," and it's easy to forget to add the sofa's footprint back in. Always measure the wall itself, not the visible gap.
Forgetting protrusions is the third: radiators, skirting-board thermostats, and light switches at floor level all eat into usable width, often by 5–8 cm (2–3 in), and they don't show up on a tape measurement taken at 90 cm up. Walk the room at skirting height too and note anything sticking out.
Measuring a wall once, at one height, is the fourth. Older walls bow by up to a centimetre over a 3 m span. Take one reading at floor level and one at 1.5 m up on any wall over 3 m long, and use the smaller number if you're fitting something rigid against it.
How do you double-check your numbers before you trust them?
Add up the four wall lengths and compare that total to the room's rough perimeter based on length × 2 + width × 2 — they should match closely once you account for any bays or alcoves. If they don't, you've either mismeasured a wall or missed a jog in the layout.
Re-measure the two most critical dimensions — the ones a piece of furniture actually depends on — with a second tool, even if it's just running a tape over a wall you already lasered. Two independent readings that agree are far more trustworthy than one careful reading you can't verify.
Take a photo of the tape or laser reading in place for anything you're about to order, especially alcove widths and bay angles. It costs nothing and it's the fastest way to settle a dispute with yourself three weeks later when the wardrobe arrives and you're second-guessing the number you wrote down.
If the job involves a fitted kitchen, built-in wardrobes, or structural work, get a professional to re-measure before ordering. Your numbers are good enough to brief a builder or shortlist furniture; they are not a substitute for a site survey once real money is committed.
Common questions
Should I measure to the wall or to the skirting board?
Measure to the wall face, taken about 90 cm (35 in) up, above the skirting board. Skirting adds 10–19 mm of extra depth at floor level that will make the room look bigger than it actually is at furniture height.
How accurate do my measurements need to be for buying furniture?
Within 1 cm (about 3/8 in) for anything that has to fit into a fixed space like an alcove or between two walls. For general room layout and rugs, within 2–3 cm is fine.
Can I just use my phone to measure a room?
For a rough sense of scale, yes. For anything you're about to order or give to a builder, no — AR measuring apps typically drift 3–5 cm on real walls, especially near corners.
What's the minimum kit I actually need?
A laser distance measure (or a tape measure over 5 m/16 ft), a notebook, and an angle finder if the room has a bay window or an out-of-square corner. That covers every case in this guide.
How do I measure ceiling height accurately?
Measure at two or more corners, not just the centre of the room. Older ceilings and floors are rarely parallel, and a 1–2 cm difference is common — it matters for anything tall that needs to reach the ceiling.
What if my walls clearly aren't square?
Measure both room diagonals corner to corner. If they differ by more than about 1 cm over a room 4 m or larger, at least one wall is out of square — note the actual angle with an angle finder before ordering fitted furniture for that corner.