Carpet Calculator

The Carpet Calculator helps you estimate the total amount of carpet you need for a room, including extra material for waste and pattern matching. Whether you are fitting a single bedroom or a whole house, this tool saves you from buying too little or too much carpet—both of which can be costly mistakes.

What the Carpet Calculator Does and When to Use It

The Carpet Calculator works out the floor area of a room or multiple rooms and then adds a waste allowance, typically between 5% and 15%. This waste factor accounts for the offcuts, joins, and pattern repeats that occur when fitting carpet. If your carpet has a large repeating pattern, you will need a higher waste percentage—sometimes up to 20%—to ensure the pattern lines up across the room.

Use this calculator whenever you are planning a carpet installation. It is ideal for standard rectangular rooms, L-shaped areas, and even rooms with alcoves or bay windows. You simply enter the room dimensions, choose your waste percentage, and the calculator returns the total square metres of carpet required, plus an estimate in square yards if you prefer those units.

Because carpet is sold in fixed widths (most commonly 3.66 metres in the UK, 4 metres in Europe, and 12 feet in the US), the calculator also accounts for the roll width. This prevents you from ordering a theoretical area that cannot be cut from a real roll.

The Formula Explained Variable by Variable

The core calculation is straightforward, but understanding each part helps you get accurate results. The basic formula for a rectangular room is:

Total carpet needed = (Room width × Room length) × (1 + Waste percentage)

Here is what each variable means:

• Room width: The shorter side of the room, measured in metres or feet. Always measure at the widest point.
• Room length: The longer side of the room. For irregular shapes, break the room into rectangles and measure each separately.
• Waste percentage: A decimal fraction added to cover offcuts. For a plain carpet in a simple rectangle, use 0.05 (5%). For patterned carpet in a room with many corners, use 0.10 to 0.15 (10%–15%).

For rooms that are not simple rectangles, the calculator sums the area of each rectangular section. For example, an L-shaped room is split into two rectangles. Their areas are calculated separately, then added together before the waste factor is applied.

Roll width adjustment: Carpet rolls come in standard widths. If your room width exceeds the roll width, you will need a seam. The calculator checks this. For instance, if the roll width is 3.66 metres and your room is 5 metres wide, you need two strips. The calculator then adjusts the total length accordingly.

Key measurement tips

• Always measure in the same unit throughout. If you start in metres, keep everything in metres.
• Measure twice. A 2-centimetre error across a 5-metre width becomes a 10-centimetre error by the time you multiply by length.
• For stair carpets, measure the tread (horizontal part) and riser (vertical part) separately. A typical stair has a tread of 25–30 cm and a riser of 15–20 cm.

Worked Examples with Concrete Numbers

Example 1: Simple rectangular living room

Problem: You have a living room that measures 4.2 metres wide by 5.8 metres long. The carpet you have chosen is plain and comes on a 4-metre-wide roll. You decide to add 8% waste for general offcuts. How much carpet do you need?

Step 1: Calculate the room area
Room area = 4.2 m × 5.8 m = 24.36 square metres.

Step 2: Apply the waste factor
Total before roll adjustment = 24.36 m² × (1 + 0.08) = 24.36 × 1.08 = 26.31 square metres.

Step 3: Check the roll width
The roll width is 4 metres. Your room width is 4.2 metres, which is wider than the roll. You will need two strips. Each strip is 4 metres wide, covering the room width of 4.2 metres with a small overlap. The room length is 5.8 metres, so each strip must be at least 5.8 metres long. Total length = 2 strips × 5.8 metres = 11.6 metres. Total area from the roll = 11.6 m × 4 m = 46.4 square metres.

Result: You need to order 46.4 square metres of carpet (or about 55.5 square yards). The waste is higher than 8% because of the roll width constraint, which is normal.

Example 2: L-shaped bedroom with patterned carpet

Problem: An L-shaped bedroom has a main section of 3.6 metres by 4.2 metres and an alcove of 2.1 metres by 1.8 metres. You want a patterned carpet with a large repeat, requiring 15% waste. The roll width is 3.66 metres. Find the total carpet needed.

Step 1: Calculate each section
Main section area = 3.6 m × 4.2 m = 15.12 m²
Alcove area = 2.1 m × 1.8 m = 3.78 m²
Total room area = 15.12 + 3.78 = 18.90 m²

Step 2: Apply the waste factor
Adjusted area = 18.90 m² × 1.15 = 21.74 m²

Step 3: Roll width adjustment
The main section width is 3.6 metres, which fits within the 3.66-metre roll. The alcove width is 2.1 metres, also fitting. However, the total length required is the room length (4.2 m) plus the alcove length (1.8 m) if they are aligned end-to-end. In this L-shape, you can cut the main section from a 4.2-metre length and the alcove from a 1.8-metre length. Total length = 4.2 m + 1.8 m = 6.0 metres. Total area from roll = 6.0 m × 3.66 m = 21.96 m².

Result: You should order 22.0 square metres of carpet (rounded up). In imperial units, that is roughly 26.3 square yards. The pattern repeat means you will have some leftover at the end—keep it for future repairs.

Common Mistakes to Avoid

Mistake 1: Forgetting to account for the roll width
Many people calculate only the floor area and order that exact amount. But if your room is wider than the carpet roll, you will need extra material. Always check the roll width before ordering. A 4-metre-wide room requires two strips from a 3.66-metre roll, doubling the length you need.

Mistake 2: Using the wrong waste percentage for patterned carpet
Patterned carpet requires extra material so the pattern lines up at the seams. For a large repeating pattern, 15% waste is a minimum. If you use only 5%, the pattern may not match, and you will end up with visible, ugly seams.

Mistake 3: Not measuring in the same unit
Mixing metres and centimetres, or feet and inches, leads to errors. For example, measuring a wall as 4.2 metres but then adding a centimetre measurement of 30 cm as 0.3 metres is fine, but if you treat 30 cm as 30 by mistake, your area becomes wildly incorrect. Convert everything to the same unit before multiplying.

Mistake 4: Ignoring doorways and obstacles
The calculator assumes a clean rectangular shape. Doorways, radiator pipes, and built-in cupboards reduce the actual carpet area. If you have a doorway 0.9 metres wide, you can subtract that area from the total, but do not subtract too much—you still need carpet to go slightly under the door for a clean finish.

Mistake 5: Ordering carpet exactly to the calculated area
Always round up to the nearest half metre or whole metre. Carpet fitters prefer a little extra to work with, and cutting a strip that is 0.05 metres too short means you have to buy another full roll. A 0.5-metre buffer is cheap insurance.

Frequently Asked Questions

How much waste should I add for a staircase?

For staircases, waste is typically higher because of the angled cuts and the need to match the pattern across steps. Add 10% to 15% for a straight staircase with plain carpet. For a curved or winding staircase, add 20% to 25%. Measure each step individually (tread plus riser) and multiply by the number of steps for the base area.

Do I need to account for underlay when calculating carpet?

No. The Carpet Calculator gives you the area of carpet only. Underlay is typically sold separately and is measured by the same room area, but you do not add waste for underlay because it is cut to fit without pattern matching. Order underlay for the exact room area, plus about 5% for fitting ease.

What if my room is not rectangular—for example, a circular bay window area?

For circular or curved areas, treat the section as a rectangle that fully encloses the curve. For example, a bay window that is a half-circle with a radius of 1.2 metres: the enclosing rectangle would be 2.4 metres wide by 1.2 metres deep. Calculate that rectangle area (2.4 × 1.2 = 2.88 m²) and then add 20% waste for the curved cuts. This method ensures you have enough carpet to work with, even if some is wasted on trimming the curve.