About the Carpet Calculator
Carpet is the one floor covering you cannot buy by simply measuring the floor. Tiles, laminate and wood come in small pieces that fill any shape; carpet comes off a big roll of a fixed width, and the shop cuts you a length. Whatever the room's own width, you pay for the full width of the roll times the length cut, and the fitter trims away what does not fit.
That makes the choice of roll width and direction important. A room that fits within the roll's width can be covered in one piece with no seams. A room slightly wider than the roll needs a second strip — and suddenly almost twice as much carpet.
This carpet calculator lays the roll both ways across the room, adds a trimming allowance to every cut, picks the way that uses less carpet, and tells you how much to buy, how many seams there will be, how much is trimmed off, and roughly what it will cost.
How to Use the Carpet Calculator
Choose metres or feet.
Enter the room's length and width, measured at the widest points and into any doorways where the carpet will finish.
Enter the roll width. It defaults to 4 m in metric and 12 ft in feet; 5 m and 15 ft rolls are also common.
Adjust the trimming allowance if needed; it defaults to 10 cm (4 in) per cut.
Optionally, enter the price per square metre or square yard to estimate the cost.
How the Carpet Is Worked Out
strips = room width ÷ roll width, rounded up
cut = room length + trimming allowance
carpet = strips × cut × roll width
seams = strips − 1
(tried both ways round; the smaller wins)Step-by-Step Example
A 5.5 × 3.6 m room, 4 m roll, 10 cm trim.
Along the length: 1 strip × (5.5 + 0.1) × 4 = 22.40 m²
Along the width: 2 strips × (3.6 + 0.1) × 4 = 29.60 m²
Choose: 22.40 m², one piece, no seams
Floor area: 5.5 × 3.6 = 19.80 m²
Trimmed off: 2.60 m², about 13%A 5 × 6 m room is different: it is wider than a 4 m roll either way, so it needs two strips of 5.1 m — 40.8 m² of carpet for a 30 m² floor, with one seam. A 5 m roll would cover it in one 6.1 m piece: 30.5 m², no seams.
Roll Widths Change Everything
The table shows how much carpet a 5 m long room needs on a 4 m roll as its width grows:
| Room width | Strips | Carpet to buy | Floor area |
|---|---|---|---|
| 3.0 m | 1 | 20.4 m² | 15.0 m² |
| 4.0 m | 1 | 20.4 m² | 20.0 m² |
| 4.2 m | 2 | 34.4 m² | 21.0 m² |
A room just 20 cm wider than the roll needs a second strip either way — even laid across the room, it jumps from 20.4 to 34.4 m². That is why it pays to check whether a 5 m (or 15 ft) roll is available in the carpet you like, or whether running the carpet the other way round fits the roll better. The calculator does the second check automatically.
Seams and Pile Direction
If the room does need more than one strip, a seam joins them. Good seams are nearly invisible, but they are a weak point in heavy traffic, so fitters keep them out of doorways and walkways where possible and run them in the direction of the main light rather than across it.
Every strip must have its pile running the same way, or the two sides of the seam will look like different shades. For the same reason, offcuts are not normally turned round to fill gaps; the calculator takes each strip at full length rather than assuming leftovers can be pieced in.
Underlay, Gripper and Doorways
Underlay and gripper are measured from the floor, not the carpet. Underlay is laid to the floor area; gripper — the spiked wooden strip that holds the carpet at the walls — runs round the perimeter, less the doorways. Each doorway needs a threshold strip where the carpet meets another floor. The calculator gives the floor area and perimeter for these.
Measuring for Carpet
Measure each wall rather than assuming the room is square, and use the longest length and widest width, because the carpet has to reach the furthest corner. Measure into the doorway to the middle of the door when it is closed, which is where the threshold strip will sit.
Include alcoves, bay windows and cupboards you want carpeted. A bay adds to the length of the cut even though it adds only a little floor, so it often shows up as extra waste. Stairs are measured separately: each tread plus riser, plus the nosing, is a strip of its own, usually cut from the same roll.
Understanding Your Result
Carpet to buy is the roll width times the total length cut.
Cut tells you how many pieces, how long, and which way they run.
Seams gives the number of joins.
Waste shows how much of the carpet is trimmed off.
Cost estimates the price if you entered one.
Worth knowing gives the underlay area and gripper length.
When Should You Use This Calculator?
Before visiting a carpet shop, to know roughly what to expect.
When choosing between roll widths, to see which covers the room in one piece.
For an awkward room just wider than a standard roll.
Checking a quote, which should list the size of the cut.
Common Mistakes
Buying by floor area. You pay for the whole roll width.
Forgetting the trimming allowance. Walls are rarely straight.
Seams in doorways. Put them where they will not be walked on.
Mixing pile directions. Both sides of a seam must match.
Ignoring the other direction. Turning the carpet can save a strip.