ProjectCalc.

Home › Interiors & flooring › Carpet and rug calculator

Carpet and rug calculator

Carpet off a 4 m or 5 m roll from your room size, with the seams, underlay, gripper rod and door bars that go with it — and the largest standard rug size that fits the same room.

Your measurements

Waste and tolerance preset

These buttons change the waste and tolerance defaults further down, and nothing else. Every figure stays visible in its box and you can type over any of them. They are rules of thumb for a typical job — assumptions, not measurements of yours.

  • DIY figures assume 150 mm trimming and seam allowance: less confident trimming along walls that are not square.
  • Trade figures assume 100 mm allowances: the stock figure for a fitter working to the skirting.

The boxes still hold the site defaults. Pick a preset, or type your own figures.

Room size
Metres. This is the direction the carpet runs, so measure the longest wall to the longest wall. An L-shaped room is two rectangles — run each one through and add the lengths together.
Metres. Measure the width in two places and take the larger figure — this is the measurement that decides which roll width you need and whether you get a seam.
Roll and fitting
Carpet is woven in a fixed width and sold by the length off it, so the roll width decides both the seams and what you pay. Leave this on the recommendation unless your supplier only stocks one width.
mm of extra length per piece for trimming at each end to the skirting. 100 mm is plenty; go to 150 mm on a very out-of-square room.
mm of extra width at each seam, for edge trimming and matching the pile. Only used when the room is wider than the roll and needs two or more pieces.
Number of openings in the room. Each one takes a 900 mm gap out of the gripper line and wants a door bar across the threshold.
Underlay
m² per roll. Sold in 10 m², 15 m² and 20 m² rolls; a 15 m² roll does a small bedroom with the offcut to spare.
Roll-fit plan with the strips across the room A room in plan with the carpet laid as strips across the room width, off a roll of fixed width: the seams between the strips are marked, the part of the roll you pay for but do not lay is the offcut, and the rug that fits is drawn inside the room. Fill the form in and the plan redraws with your own roll width, strip count and rug size. Roll-fit plan — strips and seamsrug that fitsoffcutRoom widthRoom lengthRoll widthStrips = room width ÷ roll width, raised until the seams fitYou pay for the roll width, not the room width — hence the offcut
Roll-fit plan: strips across the room with their seams, the offcut you still pay for, and the rug that fits inside. It redraws to your own room as you fill the form in.

Results

Fill in every measurement above and your results will appear here.

Save, share or print

How the carpet calculator works

Carpet is not priced by the floor area. Broadloom carpet is woven in a fixed width — 4 m and 5 m are what UK merchants stock — and you buy a length off the roll, so you pay for the full roll width along the full length whether the room fills it or not. A 3.6 m wide bedroom off a 4 m roll leaves a 400 mm strip running the whole length, and that strip is in the price. That single fact is what makes the roll choice the most important decision on the order.

So the calculator costs both standard widths and recommends the one that needs less carpet. Where both fit in one piece the 4 m roll wins, because the 5 m roll only adds carpet you cannot lay. Where the room is wider than 4 m the 5 m roll usually wins, because it covers the width in one piece and avoids a seam — and a seam is not just a line in the floor: two pieces have to be trimmed straight, matched for pile direction and joined, and a badly matched seam shows under a low light for the life of the carpet.

Each piece is measured as the room length plus a trimming allowance of around 100 mm, because carpet is cut in at the skirting on site. Where a seam is unavoidable, each one also takes a seam allowance of about 100 mm of width, for trimming the edges and matching the pile. The result is quoted as one continuous length off the recommended roll.

Underlay, gripper and the threshold

Underlay covers about the same area as the carpet and is normally sold in 10, 15 and 20 m² rolls; it is the cheapest part of the job and the part that decides how the carpet feels underfoot and how long it lasts, so it is not the place to save money. Underlay is not glued to the floor — it is laid over the gripper and held with tape or staples at the seams.

Gripper rod runs around the perimeter, right up to the skirting but stopping short at every doorway, which is why the calculator takes one 900 mm gap off the perimeter for each opening. Gripper is sold in 1.2 m lengths. At each doorway a door bar (a threshold strip) finishes the edge and holds the carpet down where it meets the next floor. Multiply the perimeter by nothing and the doorways by one — that is the whole calculation.

The rug rule of thumb

A rug is not a small carpet: it is a piece of furniture. The usual rule is that a rug should be at least two-thirds the width of the room, so that the front feet of the sofa and chairs sit on it and the back feet stay off — that is what makes a seating area look like one group rather than a rug floating in the middle of the floor. The standard UK sizes are 120 × 170, 160 × 230, 200 × 290 and 240 × 340 cm, and the calculator picks the largest one that fits your room with about 150 mm of floor left showing all round. In a bedroom, the same rule means the rug should run at least a third of the way down each side of the bed.

Carpet and rug FAQs

Which carpet roll width should I buy, 4 m or 5 m?

Buy the narrowest roll that still spans the room width, because you pay for the whole roll width whether you lay it or not. A 5 m × 4 m room comes off a 4 m roll as one piece 5.1 m long and 20.4 m² of carpet. The same room off a 5 m roll is the same 5.1 m long but 25.5 m², so 5.1 m² of carpet is bought and never laid.

What if the room is wider than 4 m?

Then the 5 m roll usually wins, because it avoids a seam. A 4 m × 4.5 m room off a 5 m roll is a single piece 4.1 m long, 20.5 m² of carpet. Off a 4 m roll the same room needs two pieces laid side by side: 8.2 m of carpet, 32.8 m² paid for, and a seam down the middle of the floor.

How much waste does the roll width create?

The waste is the strip of roll width the room does not fill, running the whole length. A 4 m × 3.6 m bedroom off a 4 m roll is 4.1 m of carpet and 16.4 m² bought for a 14.4 m² floor — about 2 m² spare, most of it a 400 mm strip off one side. That is normal, and the offcut does a wardrobe floor or a landing.

How much underlay and gripper do I need?

Underlay covers about the same area as the carpet: a 5 m × 4 m room takes roughly 20 m², so two 15 m² rolls. Gripper rod runs around the perimeter less the doorways — 18 m of perimeter for that room, 17.1 m once a 900 mm doorway is deducted, which is 15 lengths of 1.2 m. Add one door bar per doorway to finish the threshold.

What size rug do I need for a living room?

A rug wants to be at least two-thirds the width of the room so the front feet of the sofa and chairs sit on it rather than beside it. In a 5 m × 4 m living room that means the 240 × 340 cm size, which is about 41% of the floor with a 150 mm gap to the skirting. The standard UK sizes are 120 × 170, 160 × 230, 200 × 290 and 240 × 340 cm, and the calculator picks the largest one that fits your room.

Does carpet come in a standard width?

Yes. Broadloom carpet is woven in a fixed width and you buy a length off it, and 4 m and 5 m are the widths UK merchants stock, with some ranges at 3.66 m (12 ft). The roll width decides the pieces and the seams, so it matters more than the price per square metre: cheap carpet in the wrong width can cost more than a better carpet that fits in one piece.

Regs & planning for this calculator

Carpet and rugs sit inside the rules on impact sound and on what a lining may be.

Other calculators