What a wrong tile estimate actually costs

A wrong tile estimate rarely costs what the missing tiles cost. Order short and the replacement box comes from a different production batch, which means a visible shade difference in a finished floor. Order long and you are holding stock nobody will take back. The estimate is a small job that quietly decides how the whole project ends.

Most people know this. What surprises them is how small the error has to be before it hurts.

What happens when you order too little?

You go back for two more boxes. Depending on when, one of three things happens.

Same batch still in stock. The good outcome, and the least likely once a few weeks have passed. Cost is a delayed job and an irritated contractor.

Different batch available. The common outcome. Shade and calibre vary between production runs, so the new boxes are close but not identical. Laid in the same room, the difference shows — usually in daylight, usually after the customer has paid. Either the tiler works the new boxes into low-visibility areas, or a section gets relaid.

Design discontinued. On a slow-moving line or an imported range, six months is enough. Now you are matching, and the customer is choosing between a compromise and a much larger bill.

The cost of being two boxes short is almost never two boxes. It is the labour to relay an area, or a customer who tells people the floor does not match.

What happens when you order too much?

Nothing dramatic, which is why over-ordering persists. It is simply money sitting in a garage.

Most dealers will not take back opened boxes, and many will not take back sealed ones from a discontinued lot. On a mid-sized job, a 10% over-estimate on top of the wastage allowance you should have applied is typically two to four boxes — call it several thousand rupees of tile that will never be laid.

Over-ordering is the safer error. It is still an error, and at scale — a dealer estimating for every customer, every week — it is a real number.

Why is batch the thing that actually matters?

Because it turns a small shortfall into a visible defect.

Ceramic and vitrified tiles are fired in batches, and colour is never perfectly consistent between runs — pigment lots, kiln position and firing curve all move it slightly. Manufacturers print a shade or batch code on the box precisely because they expect it to matter. Calibre, the actual dimension within a tolerance band, varies the same way, which is why mixing batches can also produce uneven joints.

None of this is a defect. It is normal, disclosed and unavoidable. But it means the correct time to get the quantity right is the first order, because the second order is a different product in every way that shows.

How much wastage should you actually add?

It depends on the layout, and applying one number to every job is where most estimates go wrong.

Layout	Typical wastage	Why
Straight lay, simple rectangular room	8–10%	Cuts only at two edges
Straight lay, room with many corners or fixtures	10–12%	More cuts, more offcuts unusable
Diagonal lay	15%	Every perimeter tile is cut, and the offcuts rarely reuse
Patterned or herringbone	15–20%	Cuts are directional, so offcuts are not interchangeable
Large format slabs	10–15%	Fewer cuts, but each mistake wastes far more area

Two rules that matter more than the exact percentage.

Apply wastage per room, not to the project total. A 10% allowance across 40 m² spread over five rooms is not the same as 10% in each room, because offcuts do not travel between rooms. Applying it to the total is the single most common cause of a shortfall.

Round up to whole boxes, per room. Tiles are sold by the box, so the real order quantity is always a whole number of boxes and the box count differs by manufacturer and size. A calculation that returns 47.3 m² has not finished the job.

Where does the estimate usually go wrong?

• Floor and wall calculated together. They use different tile sizes and different wastage. Blending them produces one number that is wrong for both.
• Openings not deducted individually. A bathroom with a door and two differently sized windows is not “minus about two square metres”.
• Box count assumed. Dividing area by four, or by whatever the last product came in, rather than using the actual count for that size and manufacturer.
• Skirting forgotten until the tiler asks for it.
• Adhesive and grout treated as an afterthought, then bought on a second trip at whatever the counter has.

What is the practical answer?

Calculate room by room, use the real box count for the specific size, apply wastage per room according to layout, deduct each opening separately, and work out adhesive and grout at the same time rather than later.

Doing all of that by hand for a single bathroom takes a few minutes. Doing it for a house with eleven spaces, three tile sizes and two layouts takes most of an afternoon and will contain at least one error.

We built Tile Calc for exactly that job — floor and wall separately, real box counts from supplier packaging data, adhesive and grout from actual coverage tables, and openings entered one at a time. It is free to use and takes about five minutes for a whole project.

Common questions

Can I mix two batches if the shade codes match?

Matching shade codes makes it far more likely to work, but calibre can still differ between runs, which shows up as uneven joints rather than colour variation. If you have to mix, lay a dry area first and look at it in daylight before fixing anything.

Should I keep spare tiles after the job?

Yes — keep at least one box from the original batch. A cracked tile in three years is a trivial repair if you have matching stock and an expensive one if you do not. This is a good reason not to trim your wastage allowance too finely.

Does wastage change for large format tiles?

The percentage is often similar, but the consequence is larger. One bad cut on an 800×1600 slab wastes more area than several on a 600×600, and slabs are more likely to break in handling. Plan the cutting layout before ordering rather than relying on a percentage.

Who is responsible if the quantity is short — the dealer or the customer?

Commercially it lands on the dealer, whatever the paperwork says. The customer asked how much they needed and was given a number. This is why dealers who estimate accurately keep customers, and why an estimate on the back of a card is a liability rather than a service.

More on this: why showroom staff still estimate on a notepad and five things most tile calculators get wrong. For how we work with this trade, see tiles & ceramics.