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

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

Here is how to calculate tile quantities properly so you never get caught short again.

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 is 7.2 square metres of floor.

Account for the stuff in the way

You do not need to tile under the vanity, the toilet, or the bathtub if they are already installed. But you should tile under them if this is a new build or complete renovation.

Here is the thing though: even if you are 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 discover the old one was covering mismatched tiles.

Tile sizes and coverage

Tile coverage is not just about square metres. It is 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 is where it gets tricky. You cannot buy 2.8 tiles. You have to buy whole tiles. And that is where wastage calculations come in.

Why 10% wastage is not always enough

The standard advice is to add 10% for cuts and breakages. That works for simple layouts in small rooms. It does not 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 that is it. A brick pattern (also called running bond or subway pattern) is nearly as efficient. You still have straight cuts, just 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 is the most wasteful. You are cutting every tile at 45 degrees and the geometry means you cannot always use the offcuts elsewhere.

Pro tip: If you are set on a complex pattern, buy one extra box of tiles and keep the receipt. If you do not 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 could be subtle or it could be glaring under your bathroom lighting.

Always check that all your tiles have the same batch number before you leave the store. If you are 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 is the thing: if those boxes are heavy and you are 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 is a judgement call based on your tolerance for risk and your access to transport.

When to break the rules

Sometimes the standard calculations do not fit your situation.

Feature walls

If you are doing a feature wall with mosaic tiles or a different colour, calculate that 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 that 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, that vertical face needs tiles too. Do not 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 that cannot be used elsewhere and possibly a few breakages. Here is 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 have 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. Just do not throw them away immediately.

Common mistakes to avoid

Measuring in inches

Some older Australian homes have measurements that do not convert neatly to metric. A room that 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 is the good news: grout lines work in your favour. They give you a small amount of tolerance for uneven tiles or slightly wonky walls. Do not stress about grout line calculations unless you are doing a precision job like a swimming pool.

Forgetting the skirting

If you are tiling to the floor and putting tile skirting on top, that 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. Do not 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.

The bottom line

Calculating tile quantities is not hard maths. It is just 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 are 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.

Happy tiling. May your cuts be clean and your grout lines straight.