How Many Tiles Do You Need? The Maths Behind Tile Wastage

You've measured your bathroom floor three times. You've done the length times width calculation. You've even added 10% for wastage because is what the internet told you to do. Then you get to the end of the job and realise you're six tiles short. Now you're driving to three different hardware stores hoping they still stock the same batch.

Use this method to calculate tile quantities properly and avoid running short.

Start with the basics: floor area

Measure the length and width of your space in metres. Multiply them together. That gives you square metres.

For a rectangular room, this is easy. For an L-shaped room, break it into two rectangles and add them together. For a room with alcoves or recesses, measure each section separately.

Example: Your bathroom is 2. 4 metres by 3 metres. That's 7. 2 square metres of floor.

Account for the stuff in the way

You don't need to tile under the vanity, the toilet, or the bathtub if they're already installed. But you should tile under them if this is a new build or complete renovation.

Here's the thing though: even if you're saving tiles by not tiling under fixtures, you should still buy enough to cover those areas. Future you will thank present you when a pipe leaks and you need to lift a tile, or when you want to change the vanity and find the old one was covering mismatched tiles.

Tile sizes and coverage

Tile coverage isn't about square metres. It's about how many physical tiles you need.

A 600mm by 600mm tile covers 0. 36 square metres. You need 2. 8 of them per square metre. A 300mm by 300mm tile covers 0. 09 square metres. You need 11. 1 of them per square metre.

But here's where it gets tricky. You don't buy 2. 8 tiles. You've to buy whole tiles. And is where wastage calculations come in.

Why 10% wastage isn't always enough

The standard advice is to add 10% for cuts and breakages. That works for simple layouts in small rooms. It doesn't work for:

• Diagonal or herringbone patterns: You waste more tiles on angled cuts
• Large format tiles: One broken tile is a bigger percentage of your total
• Rooms with lots of corners: More cuts mean more waste
• Feature strips or niches: These create extra cutting around edges

For diagonal layouts, add 15% instead of 10%. For herringbone or other complex patterns, consider 20%. For large format tiles in a small bathroom, add 15% because one broken tile is significant.

Pattern matters more than you think

A straight lay is the most efficient pattern. You cut tiles at the edges and is it. A brick pattern (also called running bond or subway pattern) is nearly as efficient. You still have straight cuts, offset by half a tile.

Diagonal patterns look great but they waste tiles. Every edge cut becomes a triangle instead of a rectangle. You throw away the other half of the tile.

Herringbone looks amazing but it's the most wasteful. You're cutting every tile at 45 degrees and the geometry means you don't always use the offcuts elsewhere.

Pro tip: If you're set on a complex pattern, buy one extra box of tiles and keep the receipt. If you don't open it, most stores will accept returns.

The batch number problem

Tiles are dyed in batches. Batch 47 might be slightly different to batch 48. Not obviously different, but put them side by side and you will notice.

This is why buying extra tiles later is risky. Even if the store still stocks the same tile, it might be from a different batch. The colour difference is often subtle or it's often glaring under your bathroom lighting.

Always check all your tiles have the same batch number before you leave the store. If you're buying a large quantity, open a few boxes and compare.

Worked example: a real bathroom

Let us run through a typical Australian bathroom renovation.

Floor: 2. 2m by 2. 8m = 6. 16 square metres
Walls: Two walls at 2. 2m by 2. 4m high, two walls at 2. 8m by 2. 4m high = 24 square metres
Minus window: 0. 8m by 1. 2m = 0. 96 square metres
Minus door: 0. 9m by 2. 1m = 1. 89 square metres

Total wall area to tile: 24 - 0. 96 - 1. 89 = 21. 15 square metres
Total area overall: 6. 16 + 21. 15 = 27. 31 square metres

Now add wastage. For a straight lay on floor and walls with standard 300mm by 600mm tiles, add 10%: 27. 31 times 1. 1 = 30. 04 square metres.

If your tiles come in boxes of 1. 44 square metres, you need 20. 8 boxes. Round up to 21 boxes.

But here's the thing: if those boxes are heavy and you're working alone, or if your tiles are coming from overseas with a 12 week lead time, you might want to add an extra box for insurance. That's a judgement call based on your tolerance for risk and your access to transport.

When to break the rules

Sometimes the standard calculations don't fit your situation.

Feature walls

If you're doing a feature wall with mosaic tiles or a different colour, calculate separately. Mosaics often come on sheets and the coverage is different. You might need more or less wastage depending on the sheet size.

Niches and recesses

A shower niche looks simple but it creates four extra edges need tile cuts. Each niche might add half a square metre to your wastage calculation even though the niche itself only covers a small area.

Stepped floors

If your bathroom has a hob or step down to a shower area, vertical face needs tiles too. Don't forget to measure the height of the step and add it to your calculations.

The leftovers question

You will have leftover tiles. Even with perfect calculations, you will have cuts don't be used elsewhere and possibly a few breakages. Here's what to do with them.

Keep at least two full tiles. Store them somewhere dry with the batch number written on the box. These are your insurance policy against future damage.

Use the cut pieces for practice. If you've never cut tiles before, use your offcuts to practice on. Better to break a spare than the last tile you need to finish the job.

Some people use leftover tiles for coasters, trivets, or garden path edging. Others donate them to local community gardens or school art projects. don't throw them away immediately.

Common mistakes to avoid

Measuring in inches

Some older Australian homes have measurements don't convert neatly to metric. A room is "12 foot by 10 foot" sounds simple but 12 feet is 3. 66 metres, not 3. 6. Convert properly or measure in metric on site.

Ignoring grout lines

Grout lines are usually 2-3mm. Over a large area, those millimetres add up. But here's the good news: grout lines work in your favour. They give you a small amount of tolerance for uneven tiles or slightly wonky walls. Don't stress about grout line calculations unless you're doing a precision job like a swimming pool.

Forgetting the skirting

If you're tiling to the floor and putting tile skirting on top, adds height to your wall measurements. A 100mm tile skirting around a 2. 4m by 3m room adds nearly one square metre to your total.

Buying exactly what you calculated

You calculated 21. 3 boxes. The store sells whole boxes. You buy 21 boxes and hope for the best. Don't do this. Round up to 22 boxes. The cost of one extra box is less than the cost of a second trip to the store or a mismatched patch job later.

What it comes down to

Calculating tile quantities isn't hard maths. It's length times width plus a sensible allowance for wastage. The trick is knowing what sensible means for your specific job.

10% works for simple layouts. 15% for diagonals or large format tiles. 20% for complex patterns or if you're new to tiling and expect to break a few.

And always, always keep a couple of spares. Tiles go out of production, batches vary, and accidents happen. Those two tiles sitting in your shed might save you a world of pain in five years when someone drops a hammer.

Keep a few spare tiles after the job. Matching replacements get harder to find over time.