Drywall Calculator Metric: Sheets & Waste

A metric drywall calculator works out how many full sheets of plasterboard you need for a room or wall, based on the total surface area you enter, and it also estimates the waste you should add for cuts and mistakes. You input the area in square metres (or the wall length and height), and it returns the number of sheets in common metric sizes (e.g., 1200 × 2400 mm, 1200 × 2700 mm, 1250 × 2500 mm) plus the total area covered including waste. This page explains the simple maths behind the estimate so you can check the result yourself and order the right amount of board the first time.

Formula

The calculator uses a single formula:

Sheets needed = ⌈ (A × (1 + W/100)) / S ⌉

• A = total area to cover (square metres, m²)
• W = waste allowance (percent, %)
• S = area of one sheet (square metres, m²)
• ⌈ ⌉ = round up to the next whole number (you can’t buy a fraction of a sheet)

For a standard metric sheet of 1200 mm × 2400 mm, S = 1.2 m × 2.4 m = 2.88 m². For a 1250 mm × 2500 mm sheet, S = 1.25 m × 2.5 m = 3.125 m².

Worked Example

Using the default values from the calculator: area = 48 m², waste = 10%, sheet size = 1200 × 2400 mm (S = 2.88 m²).

1. Add waste: 48 × (1 + 10/100) = 48 × 1.10 = 52.8 m²
2. Divide by sheet area: 52.8 / 2.88 = 18.333…
3. Round up: ⌈18.333⌉ = 19 sheets

So the calculator returns 19 sheets and a total area of 52.8 m². If you bought exactly 18 sheets, you would cover only 51.84 m² – short by about 1 m².

Why Use a Metric Drywall Calculator?

Drywall (also called plasterboard, gypsum board, or wallboard) is sold in standard metric sheet sizes in most countries outside North America. The most common sizes are 1200 mm wide and 2400, 2700, or 3000 mm long. A 1200 × 2400 mm sheet covers 2.88 m², but because you have to cut around doors, windows, corners, and outlets, you will always need more than the exact area. The waste percentage (typically 5–15%) accounts for:

• Cuts that don’t perfectly fit (especially around corners and openings)
• Damaged sheets or mistakes during cutting
• Offcuts that are too small to use
• Slight variations in room dimensions (walls are rarely perfectly square)

Using a calculator like this prevents two common problems: ordering too few sheets (which delays the job) or ordering too many (which wastes money and leaves you storing leftover boards).

How to Choose the Right Sheet Size

The calculator lets you pick from four common metric sizes. Which one you choose depends on your ceiling height and the layout:

• 1200 × 2400 mm (2.88 m²) – the standard for walls up to 2.4 m high. Most rooms have ceilings at 2.4 m, so this size minimizes horizontal joints.
• 1200 × 2700 mm (3.24 m²) – for walls with a 2.7 m ceiling height. Using this avoids a horizontal joint at the top.
• 1250 × 2500 mm (3.125 m²) – common in some European countries; slightly wider and longer than the 1200×2400, covering more area per sheet.
• 1200 × 3000 mm (3.6 m²) – for very tall walls (3 m ceilings) or to reduce joints on long runs. Heavier and harder to handle alone.

If your ceiling height is between standard sizes, you can either use the next longer board and cut it down, or use a shorter board and add a small piece at the top (which is more work). The calculator’s waste percentage can be raised to 15% if you have many openings or a complex layout.

Common Mistakes and How to Avoid Them

The biggest mistake is forgetting to convert millimetres to metres when calculating sheet area. A 1200 × 2400 mm sheet is 1.2 m × 2.4 m, not 1200 × 2400 = 2,880,000 m². Always divide millimetres by 1000 to get metres.

Another mistake is using the floor area instead of the wall area. If you are drylining a room, you must measure the perimeter (sum of all wall lengths) and multiply by the ceiling height. For example, a 4 m × 5 m room with a 2.4 m ceiling has wall area = (4+5+4+5) × 2.4 = 18 × 2.4 = 43.2 m², not 20 m² (the floor area). The calculator expects the total surface area, so measure carefully.

A third issue is forgetting to subtract large openings like windows and doors. You can subtract them from the total area, but many professionals prefer to keep them in and let the waste percentage cover the extra. If you have a large garage door or picture window, subtract that area first, then add 10% waste.

Limits of the Formula

This calculator gives a good estimate, but it cannot account for every real-world factor. It assumes you are covering a flat, continuous surface. It does not know about:

• The orientation of the boards (vertical vs. horizontal) – which affects how many cuts you make
• The spacing of studs or joists – boards must be cut to fit the framing, which can create extra waste
• Curved walls, arches, or angled ceilings – these require many more cuts
• Fire-rated or soundproof board – those are the same size but often cost more, so you might order less waste

For a simple rectangular room with a flat ceiling, the estimate will be very close. For a complex shape, increase the waste to 15–20% or draw a cutting plan.

Cost and Ordering Considerations

While this calculator does not give prices (they vary by region and supplier), you can easily estimate cost once you know the sheet count. Multiply the number of sheets by the local price per sheet. Remember that you will also need joint compound, tape, screws, and corner beads – those are not included here.

When ordering, it is wise to buy all sheets at once from the same batch to ensure consistent colour and thickness. Also, check the manufacturer’s recommended storage: drywall must be kept flat and dry, so plan to store it indoors or under a tarp.

Alternative Methods

If you don’t have a calculator, you can do the math by hand using the same formula. Or you can use a simple rule of thumb: divide the total wall area by the sheet area (e.g., 2.88 m²) and add 10% – that’s the same thing this calculator does. Some contractors use a “sheets per square metre� factor: for 1200×2400 boards, that is 1/2.88 ≈ 0.347 sheets per m². Multiply your area by that and round up.

Another alternative is to use a cutting layout app that lets you draw the walls and see exactly where each board goes. That can reduce waste to 5% or less, but it takes more time to set up.

Frequently Asked Questions

How do I measure the area of a room for drywall?

Measure the length of each wall and add them together to get the perimeter. Multiply the perimeter by the ceiling height to get the wall area. For the ceiling, multiply length by width. Add wall and ceiling areas if you are doing both. Subtract large openings (doors, windows) if you want a more precise number, or leave them in and rely on the waste allowance.

What is a good waste percentage for drywall?

For a simple rectangular room with few openings, 5–10% is typical. For rooms with many corners, windows, or a sloped ceiling, use 10–15%. If you are a beginner, 15% is safer because you will likely make more cutting mistakes.

What is the actual size of a metric drywall sheet?

Common metric sheets are 1200 mm wide and 2400, 2700, or 3000 mm long. Some are 1250 mm wide. The thickness is usually 12.5 mm for walls and 9.5 mm for ceilings, but the area calculation does not depend on thickness.

Can I use this calculator for ceilings too?

Yes. The formula works for any flat surface. Just enter the ceiling area (length × width) and choose the appropriate sheet size. Ceilings often use 9.5 mm board, but the area calculation is the same.

How many sheets do I need for a 12 m² room?

If the room has 12 m² of wall area (not floor), with 10% waste and 1200×2400 sheets: 12 × 1.10 = 13.2 m²; 13.2 / 2.88 = 4.58, so you need 5 sheets. If you have a 12 m² floor area with 2.4 m ceilings, the wall area is about 43 m² (depending on layout), so you’d need about 17 sheets.

Should I round up or down?

Always round up to the next whole sheet. You cannot buy a partial sheet, and cutting a board to fit a small gap is usually easier than trying to join two small pieces. Rounding up also gives you a spare for repairs.

Related

• Board and Batten Calculator: Siding & Spacing
• Board Foot to m3 Calculator: Quick Conversion
• Board Foot to Piece Calculator: Lumber Math

in area | Wall/ceiling area | m² | 48 | 0
in waste | Waste allowance | % | 10 | 0
sel sheet | Sheet size | 1200x2400=2.88, 1200x2700=3.24, 1250x2500=3.125, 1200x3000=3.6
out sheets | Sheets needed | sheets | ceil((area * (1 + waste/100)) / sheet) | 0
out total_area | Total area incl. waste | m² | area * (1 + waste/100) | 1