How much carpet do I need?

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.

Worked example: a 5 m × 4 m room in 4 m-wide carpet

A 5 m × 4 m room in 4 m-wide broadloom, cut with a trimming allowance at each end.

How we got there

1. The 4 m roll spans the 4 m width in one piece — At 4 m wide the carpet covers the room with no seam — one piece, no joins.
2. Buy length: 5.1 m — The roll is cut long enough to trim at both ends, so the order is 5.1 m.
3. Area purchased: 20.4 m² — 5.1 m off a 4 m roll is 20.4 m² — about 0.4 m² more than the floor.
4. A 5 m roll would cost more — A 5 m roll would cover the same room for 5.1 m but 25.5 m² of carpet, wasting 5.5 m² — the narrower roll is the cheaper choice.
5. Underlay: 2 rolls; gripper: 15 lengths — A 15 m² underlay roll gives 2 rolls, and the 17.1 m gripper run is 15 × 1.2 m lengths, plus 1 door bar.

What you need to buy

The order for the worked example above, straight from this calculator's own shopping list.

• Carpet, 4 m wide broadloom — 5.1 m (1 piece at 5.10 m, no seam. Ordered as one length off the roll.)
• Carpet underlay (15 m² roll) — 2 rolls (≈ 20.4 m² — the floor plus the trimming and seam allowances.)
• Gripper rods (1.2 m) — 15 lengths (Covers 17.1 m of skirting line, excluding the doorways.)
• Door bars / threshold strips — 1 strip (One per doorway, cut to the opening.)
• Carpet tape, adhesive and gripper fixings — estimate only (Double-sided tape holds the underlay and the gripper is nailed or glued down; a fitter normally supplies these with the labour.)
• Rug, 240 × 340 cm (optional) — 1 (The largest standard UK size that fits 5.00 m × 4.00 m with a 150 mm gap to the skirting.)

Figures for the worked example only. Change the measurements above and the list recalculates.

Waste and over-ordering

Carpet is sold off the roll by length, so the waste is the trimming allowance at each end and any seam you are forced into. A room wider than the roll needs a join, which costs a little extra width. Order the carpet and the underlay together so the fitter can lay it in one visit.

Regs & planning for this calculator

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

• Approved Document E — impact sound A separating floor has a 62 dB L′_nT,w maximum (64 dB after a material change of use), and underlay is part of that build-up. Read the requirement.
• Approved Document B and BS EN 13501-1 — fire A floor covering is part of a lining, and linings carry a reaction-to-fire class that depends on the room. Read the requirement.
• Approved Document K — stairs Treads have to have slip-resistant surfaces, which is what a stair carpet or runner is doing. Read the requirement.