How to use this calculator
- 1Measure the room length and width at the widest points, into door recesses and bay windows.
- 2Choose the roll width the carpet is sold in: usually 4 m or 5 m in the UK and Europe, 12 ft or 15 ft in the US.
- 3Keep the trim allowance of about 10 cm (4 in); add the pattern repeat if the carpet is patterned.
- 4Leave the direction on "whichever needs less carpet", or pick one if you want the pile to run a certain way.
- 5Add doorways to get the gripper length. For a cost, type the shop price per m², ft² or yd² under Prices at the end of the form.
The formula
- 01Pieces = (room side across the roll + trim) ÷ roll width, rounded up
- 02Piece length = room side along the roll + trim (+ pattern repeat if there is a seam)
- 03Carpet length to buy = pieces × piece length
- 04Area you pay for = carpet length × roll width
- 05Underlay = room area × (1 + extra %)
- 06Gripper = 2 × (length + width) − doorway widths
Carpet comes off a roll of fixed width, and you buy the full width for every metre or foot you cut. That is why a carpet order is not simply the floor area: a 3.6 m wide room covered from a 4 m roll still pays for 4 m of width. The calculator works out both directions — pieces running along the length or along the width — and picks the one that needs less carpet.
If the room (plus trim) is wider than the roll, you need two or more pieces joined with a seam. Every piece must have its pile running the same way, so a fitter cannot simply turn an offcut sideways to fill a gap. With patterned carpet each extra piece must be moved along to line up the pattern, which costs up to one pattern repeat per piece.
Underlay is laid butted, not overlapped, so it only needs the room area plus a little extra for trimming. Gripper rod goes round the edges of the room but not across doorways, where a door bar is fitted instead.
Worked example
A 5 m × 4.2 m room is 21 m². With 100 mm trim it is too wide for one piece of a 4 m roll either way.
Running the pieces along the width needs 2 pieces of 4.3 m: cut 8.6 m from the roll, which is 34.4 m² of carpet with 1 seam.
You also need 23.1 m² of underlay and 17.6 m of gripper rod.
Tips before you buy
- Check whether the carpet also comes in a wider roll. A 4.2 m wide room on a 5 m roll needs no seam and can cost less than two pieces from a 4 m roll.
- Measure every room separately, even if they share a carpet, and ask the shop whether offcuts from one room can cover a small space like a landing or cupboard.
- Plan seams so they run towards the main window and away from doorways; light across a seam makes it much more visible.
- Buy underlay and gripper at the same time, and a door bar for each doorway where the carpet meets another floor.
- Keep a large offcut after fitting — it is ideal for repairing a stain or burn later.
Frequently asked questions
How do I work out how much carpet I need?
Measure the room at its widest points and add about 10 cm (4 in) each way for trimming. If that width fits within the roll width, you need one piece as long as the room plus trim. If not, you need two or more pieces, and you pay for the full roll width of each one.
What widths does carpet come in?
In the UK and Europe most carpet is sold in 4 m and 5 m widths. In the US the standard is 12 ft, with some ranges in 13 ft 6 in or 15 ft. The widths available depend on the range, so check before you choose.
Why is the carpet I pay for bigger than my room?
Because carpet is cut from a roll of fixed width and needs extra for trimming at the walls. A 3.5 m wide room still uses the full 4 m of a 4 m roll, and the offcut strip is usually too narrow to use elsewhere.
How much extra carpet should I allow?
Carpet retailers ask for at least 10 cm (about 4 in) on each measurement for trimming and fitting. Patterned carpet needs more when there are seams — up to one pattern repeat for each extra piece so the pattern lines up.
How much underlay do I need?
The room area plus about 5–10 % for trimming, especially in rooms with bays or alcoves. Underlay pieces are butted together and taped, not overlapped, so you do not need as much extra as for the carpet itself.
How do I convert square feet of carpet to square yards?
Divide square feet by 9. Many US stores price carpet per square yard: choose "per yd²" next to the carpet price under Prices, and switch the unit next to the result to yd² to see the area that way.
Can I use this for sheet vinyl or lino?
Yes. Sheet vinyl and lino are also cut from a roll of fixed width — usually 2, 3 or 4 m in the UK and Europe — so choose the roll width and read the length to buy. Ignore the underlay and gripper, which are for carpet only.
Also searched as: how much carpet do I needcarpet estimatorcarpet square yards calculatorcarpet roll calculatorcarpet and underlay calculator
Sources
- Carpet roll widths (4 m and 5 m) and how roll width affects ordering — Flooring Hut
- US carpet widths (12 ft, 13.5 ft, 15 ft) — Flooring America
- Measure at the widest point and add at least 10 cm — Online Carpets
- Underlay: add 5–10 % — Online Carpets
- Sheet vinyl roll widths 2 m, 3 m and 4 m, cut to length — Online Carpets
Formulas last reviewed: September 30, 2026 ·How we check our formulas