How much carpet for a 4.2 m by 3.6 m room?

With 10 cm added to each measurement and carpet from a 4 m roll, one possible unjoined piece is 4 m wide by 4.3 m long: 4.3 linear metres from the roll. Check the pile or pattern direction first.

Can carpet be calculated from square metres alone?

No. The fixed roll width, room dimensions, pile direction and any joins decide the cut.

How much carpet do I need?

Carpet is normally bought from a roll of fixed width. The order is a cut length from that roll, not simply the floor area plus a waste percentage.

One rectangular room

Measure the longest length and widest width, including recesses the carpet will cover.
Add the fitting and trimming allowance you are using. Many UK measuring guides use 10 cm on each measurement; it remains an editable example, not a rule for every job.
Check which planned room dimension fits across the exact roll width offered for the carpet.
The other planned dimension is the cut length. Round it up only to the step in which that shop sells cut lengths.

For a 4.2 m × 3.6 m room, adding 10 cm gives 4.3 m × 3.7 m. The 3.7 m side fits across a 4 m roll, so the cut is 4.3 m. You buy a 4 m × 4.3 m piece: 17.2 m² for a floor measuring 15.12 m².

Direction matters

The pile, stripes or pattern normally run along the roll length. Turning the piece can change both the appearance and whether it fits. BuildBatch can show the smaller one-piece cut that fits, but that is not a recommendation about direction. Check the chosen carpet and fitting plan.

When this sum stops

A room wider than the available roll may need a join. Several rooms, L-shaped rooms, halls, landings, stairs and patterned joins need a cutting plan that keeps pile direction and pattern matching consistent. The first BuildBatch carpet calculation refuses those rather than guessing.

Underlay and fitting materials

Count underlay from the area printed on the chosen roll or pack, rounded up to whole packs. Gripper, adhesive, threshold bars and subfloor preparation depend on the carpet, underlay, floor and fitting method, so they remain checks until those choices are known.

Do it with BuildBatch

Open the carpet calculator, enter the rectangular room and replace the example roll figures with the carpet and shop you will use.