Carpet Calculator
Carpet is not bought by the square metre. It comes off a roll of fixed width and is cut to length, so what you order is a length, and you pay for the full roll width whether the room uses it or not. This calculator works out the cut length, chooses the direction that wastes least, and tells you when the room is too wide for the roll and needs a join.
Your measurements
Check your order
Before ordering roll materials
- Compare usable roll width and length with the cutting plan, not just the total area.
- Check pattern, direction and matching requirements before combining offcuts.
- Confirm the backing, permitted adhesive or fixing method, and suitable cutting tools.
Tools for this job
Confirm the room layout and roll width before cutting. Match the blade and cutting method to the carpet backing; this is not a complete fitting kit.
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- Check room dimensions and cuts
Short tape measure
STANLEY 25-foot tape measure
For checking lengths, widths and short runs before ordering. This is a 25-foot tape; use matching units in the calculator and check the scale on the selected model.
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Utility knife
STANLEY Classic 99 retractable utility knives
For scoring or trimming sheet materials that allow knife cutting. Choose a blade suited to the material and support the work on a cutting surface.
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Already have suitable tools? Reuse them. These are optional product suggestions, not a complete equipment list or a hands-on comparison. Amazon may redirect Amazon.com links to local products or search results. Amazon.nl links search for alternatives, which may differ in brand, size and specification. Check suitability, seller, delivery and any import costs on your chosen store before ordering.
How much carpet do I need?
Carpet comes on a roll of fixed width, 4 m or 5 m in the UK and Europe and 12 ft or 15 ft in the US, and the shop cuts a length off it. The pile leans one way, so every piece must run in the same direction: the cut length is the room dimension along the roll, and the roll width has to cover the room dimension across it. If it does, you order one length. If the room is wider than the roll both ways, you need two strips seamed together, and the second strip costs the same full width as the first.
So the answer is a length, not an area: the room dimension plus a cutting allowance at each end, plus a little more if a doorway sits at the end of the cut. Multiply it by the roll width and you have the area you pay for; the difference from the room area is the offcut, normally 10 to 25% of the room. The calculator tries the cut both ways and picks whichever fits with the least waste.
The formula
Which roll width fits which room
A room just over 3.8 m wide is the classic problem: too wide for a 4 m roll once walls that are not straight are allowed for, so it needs a 5 m roll or a join. Not every range is made in 5 m, so check first.
| Roll width | Spans up to | Typical availability | Best for |
|---|---|---|---|
| 3.66 m (12 ft) | 3.46 m (11 ft 4 in) | US standard, some UK | Bedrooms, small living rooms |
| 4 m (13 ft 1 in) | 3.8 m (12 ft 6 in) | UK and EU standard | Most bedrooms and lounges |
| 4.57 m (15 ft) | 4.37 m (14 ft 4 in) | US wide | Larger US living rooms |
| 5 m (16 ft 5 in) | 4.8 m (15 ft 9 in) | UK and EU, many ranges | Rooms 3.8 to 4.8 m wide, avoiding a join |
Worked example
A living room 5 m long and 3.5 m wide, carpet on a 4 m roll, 10 cm allowance per edge, one doorway.
Try the cut along the length: the width to cover is 3.5 + 0.2 = 3.7 m, which fits inside 4 m, so one strip. Cut length = 5 + 0.2 = 5.2 m. Try it the other way: the width to cover would be 5.2 m, which does not fit, so that needs two strips of 3.7 m = 7.4 m. The first way wins.
Order 5.2 m off the 4 m roll, which is 20.8 m² to pay for. The room is 17.5 m², so the offcut is 3.3 m², a strip about 0.5 m wide down one side. Gripper rod is the perimeter of 17 m less the 0.9 m doorway = 16.1 m, and underlay is the room area plus 5%, about 18.4 m².
Make the room 4 m wide and neither direction fits a 4 m roll: the calculator switches to two strips of 4.2 m, 8.4 m in total and 33.6 m² to pay for, with a join. On a 5 m roll the same room is a single 5.2 m cut and 26 m².
When the room is wider than the roll
- The fill strip must run the same way as the main piece, so it comes from another full-width length. The calculator counts it as a whole strip; a fitter may cut the fill more economically from a shorter length.
- Put the seam where it is least visible: away from the window, so light does not rake along it, and under furniture if you can.
- A patterned carpet needs extra for the pattern match, usually one full repeat per strip. Add it to the doorway extension field.
What else the job needs
- Underlay: the room area plus a small allowance, rounded to whole rolls. The underlay calculator does the roll count.
- Gripper rod around the perimeter except across doorways, sold in 1.52 m lengths. Divide the gripper figure above by 1.52 and round up.
- Door bars: one per doorway, matched to the floor on the other side.
- Skirting is fitted before the carpet, not after: the skirting board calculator works from the same room size, and the same doorway deduction.
Measuring for carpet
- Measure the room at its longest and widest points, not along one wall. Alcoves and bays are measured to their deepest point, because the carpet is cut as one rectangle and trimmed.
- Measure into every doorway to the middle of the door, and add that to the length if the doorway is at the end of the cut.
- Note which walls have the window and the main door: that decides where a seam can go and which way the pile should run.
- Take the room dimensions to the shop, not just the cut length. A good retailer will check the plan and confirm which roll width their carpet comes in.
Sources and assumptions
The calculator compares rectangular strips cut from a fixed-width roll, allowing trimming at each end. Automatic orientation prioritises fewer joins and then a shorter combined cut length.
- Alternative Flooring: measuring and planning: The manufacturer explains why measuring the home, checking the subfloor, planning patterns and comparing roll widths matter before fitting.
Before using this estimate
- The plan does not include pattern repeats, a staircase, multiple connected rooms or reuse of every offcut.
- A geometric orientation may conflict with pile direction or the preferred seam position. Have the fitter confirm the layout and ordered cut lengths, especially for patterned carpet.
Source notes reviewed . See how we check calculations and handle corrections.
Planning your order
Check the inputs before buying: measure the surfaces you need, then apply an allowance and round to whole packs. The guides include worked examples you can follow alongside this calculator.
Published by OrderEnough. This is a planning estimate: check the product specification and the assumptions above before ordering. Last updated . Read how we source our figures on the methodology page.