You can measure a room for carpet yourself, and for a plain carpet in a square bedroom you’ll probably get close. But the number that decides what you actually pay isn’t the floor area — it’s how your room sits against a 4m or 5m roll. That’s the bit almost every guide leaves out, and it’s why two rooms of identical size can come back with very different quotes.

Here’s how to measure properly, in metric, the way it’s actually done in the UK — and where DIY measurements tend to come unstuck.

What you need

A decent tape measure, 5m or longer, is the only essential. A pencil and a scrap of paper to sketch the room on. That’s genuinely it. A laser measure is quicker across a big room and no more accurate than a tape in a normal bedroom, so don’t buy one for a single job. Graph paper helps if the room has bays or alcoves, because sketching it is easier than holding four numbers in your head. Measure in metres and centimetres. UK carpet is sold by the square metre and the rolls come in metric widths, so working in feet just means converting everything back later.

Measuring a straightforward room

Measure the length and the width at the widest points, wall to wall, and include any doorway recesses the carpet will run into. Multiply the two for the floor area. A room 4.2m long and 3.6m wide is 15.1m². Simple enough. Two things worth doing while you’re down there. Measure both ends of the room rather than assuming it’s square — in a Victorian terrace it very often isn’t, and a 40mm difference corner to corner matters when the carpet’s cut. And measure into the doorway, because carpet normally runs to the centre of the door so it’s not visible from the other room.

The bit nobody tells you: roll widths

This is where DIY calculations go wrong, and it has nothing to do with your arithmetic. Carpet comes off a roll, and in the UK that roll is almost always 4m or 5m wide. You buy a length off that width. You don’t buy “15.1 square metres” — you buy, say, 4.3m off a 4m roll. So the width of your room decides which roll it comes off, and that decides the waste:

Room	Floor area	What you’d buy	Waste
4.2m × 3.8m	16.0m²	4.3m off a 4m roll	Very little
4.2m × 4.3m	18.1m²	4.3m off a 5m roll	0.7m across the full length

The second room is only 2m² bigger, but it needs a wider roll and you’re paying for most of that extra width whether you use it or not. A room that’s 4.1m wide is one of the more frustrating sizes in this trade for exactly that reason. None of this means you’ve done anything wrong. It just means floor area on its own doesn’t tell you the cost, which is why an online calculator and a fitter’s quote can look so different.

Awkward rooms

Most Bristol houses have at least one room that isn’t a rectangle. Break it into rectangles, measure each one and add them up. Bay windows. Measure the bay as its own section — depth from the main wall, and width at its widest point. Don’t try to measure the angles; a fitter templates those on the day. Chimney breasts and alcoves. Measure the full room as if the chimney breast weren’t there, then measure the breast itself and subtract it. That’s usually simpler than measuring two alcoves separately, and it’s safer, because carpet often needs to run behind where a hearth was. Doorways. Add the depth of each doorway recess. On a room with four doors off it, that can be half a square metre you’d otherwise be short. Rooms out of square. Take the longest length and the widest width. Carpet is cut to the biggest dimension and trimmed in.

Stairs

Stairs are measured differently and they’re the part DIY calculations get most wrong. The rough method: measure the depth of one tread plus the height of one riser, add about 50mm, and multiply by the number of steps. A typical step comes out around 450mm, so thirteen steps is roughly 5.9m of length. Width is usually a straight run off the roll, plus whatever the landing needs. Winders — the wedge-shaped steps on a turn — don’t follow that formula at all. Each one is cut individually and needs more carpet than a straight tread. If your staircase turns, treat any figure you calculate as a rough minimum. More on specifying stair carpet properly in our guide to the best carpet for stairs.

Waste, and why it isn’t optional

Add 10% to whatever you’ve calculated for a plain carpet. For a patterned carpet, add 15–20%, because the pattern has to line up across every join and that means cutting into the repeat. The reason this matters more than people expect: if you run short and have to order more, the new piece comes from a different dye lot. Carpet is made in batches and the colour drifts between them. You’ll see that join in daylight for as long as you own the floor, and there’s no fixing it afterwards. Order it all at once, from one batch, and keep the offcuts.

Why a fitter’s measurement differs from yours

When we measure, we’re not just working out the area. We’re working out how to cut the room out of a roll with the fewest joins, with the pile running the same way throughout, and with any join falling somewhere you won’t look at it. Pile direction matters more than people realise. Carpet has a nap, and two pieces laid in opposite directions will look like two different shades under the same light even though they came off the same roll. We’re also looking at the floor. Around one in six old floors we lift has damp or rot underneath somewhere — usually at a back door threshold or under an old washing machine — and that changes the job before any carpet arrives. More on that in what we find under Bristol floors.

When it’s worth just getting it measured

Measure it yourself if you want a rough budget before you start looking, or you’re carpeting one plain bedroom. Get it measured properly if the room has a bay, the house is Victorian and nothing is square, you’re doing stairs, you’ve picked a patterned carpet, or the carpet runs through more than one room. Our measure is free and there’s no obligation. We bring samples so you can see them in your own light, and you get a fixed price before anything is ordered — see what’s included in our carpet fitting service, or our guide to what carpet fitting costs in Bristol.

Frequently asked questions

How do I measure a room for carpet in metres?

Measure the length and width at their widest points, wall to wall, in metres, and multiply them for the floor area. Include any doorway recesses the carpet will run into, and measure both ends of the room rather than assuming it’s square.

What width does carpet come in?

In the UK, almost always 4m or 5m. You buy a length off that width, which is why the width of your room matters as much as its area — a room 4.1m wide needs a 5m roll and most of the extra 0.9m is waste.

How much extra carpet should I allow?

Around 10% for a plain carpet, and 15–20% for a pattern, because the pattern has to match across every join. Order it all in one go: a reorder comes from a different dye lot and the colour won’t match.

Do I subtract the chimney breast?

Measure the room as though it weren’t there, then measure the chimney breast and subtract it. It’s simpler than measuring each alcove, and carpet often needs to run back into where a hearth used to be.

How do I measure stairs for carpet?

Tread depth plus riser height plus about 50mm, multiplied by the number of steps — roughly 450mm per step. Winders on a turn are cut individually and need more, so treat any figure you calculate as a minimum if your staircase turns.

Why is a fitter’s quote different from an online calculator?

Calculators work out floor area. A fitter works out how your room cuts out of a 4m or 5m roll, with the pile running one way and joins placed where you won’t see them. Those are different sums, and the second one is what you actually pay for.

Can I just measure it myself and order online?

You can, and for one plain bedroom it often works out fine. The things that catch people out are pattern match, rooms that aren’t square, bays and stairs. If any of those apply, have it measured — a wrong order can’t usually be returned once it’s cut.

Free carpet measure in Bristol and Bath

Seamless Floors supplies and fits carpet across Bristol, Bath and the surrounding areas. The measure is free, the quote is fixed, and we’ll tell you honestly if a room is one of those awkward widths before you commit to anything. Call 07901751755 or use the contact form to book a free measure.