A tile shortage announces itself at the worst possible moment: the floor is two-thirds set, the last box is empty, and the supplier says the dye lot you bought is sold out. Ordering too much carries a smaller penalty — a few unopened boxes and a receipt — but it is still money sitting on a pallet. The gap between the two is a takeoff: a tile-by-tile count of the room, adjusted for cuts, breakage, and the boxes tile actually ships in.
This guide walks through that takeoff the way the Tile Layout & Mortar Calculator does it: measure the real floor, count tiles axis by axis with the grout gap included, apply a waste allowance that matches the layout, and round up to whole boxes.
Divide each room dimension by the tile dimension plus the grout gap, round each result up, and multiply: tiles per axis = ceiling(room dimension ÷ (tile dimension + grout gap)). Add 10% waste for straight layouts, 15% for diagonal, herringbone, or large-format tile, then round up to whole boxes. A 10×12 ft room in 12×12-in tile with a 1/8-in grout gap needs 132 tiles — the same number the calculator returns.
Measure the Floor, Not the Plan
Tile is ordered against the room as built, not the room as drawn. Measure length and width at the longest and widest points, in inches, with the tape on the finished floor where the tile will sit. Plan dimensions drift: framing, drywall, and baseboard each take a share, so a "12-foot" bedroom often measures closer to 11 ft 10 in, and older homes add out-of-square walls that cost another inch at one end of a run. An inch of error across one row is nothing; across ten rows it is a full tile.
Irregular rooms break into rectangles. An L-shaped room is two rectangles; a kitchen with a bay window is a main rectangle plus one per angled segment. Measure and count each rectangle separately and add the tile counts — never the dimensions. Measure every alcove, closet, and threshold you intend to tile, and remember that tile usually dies into a transition strip mid-doorway, not at the doorstop.
Square footage still matters as a sanity check, and it drives the materials that genuinely are sold by area — thinset, grout, and membrane. Area is length × width, and this site's convention is 1 sq ft = 0.0929 sq m, so the 10×12 ft room used throughout the tile guides is 120 sq ft, or about 11.2 sq m. If your axis count and your square footage disagree wildly, one of them was measured wrong — settle it before ordering.
Count Tiles, Not Square Feet
The tempting shortcut — room area ÷ tile area — quietly undercounts, because it ignores two facts of a real floor. First, the grout gap steals width on every row: a 12×12-in tile laid with a 1/8-in joint occupies a 12-1/8-in cell. Second, a row that ends in a partial tile still requires a full tile to cut: a 119-in wall takes ten of those cells, not 9.8.
The installer's method counts per axis: tiles per axis = ceiling(room dimension ÷ (tile dimension + grout gap)). Round each axis up separately — a partial tile at the end of a run is a cut tile you must own — then multiply the two axis counts for the base total. That is exactly how this site's Tile Layout & Mortar Calculator thinks: it converts everything to inches, adds the gap to the tile on both axes, rounds each division up, and multiplies.
One measurement detail before the arithmetic: use the tile's actual face size, not its nominal size. Plenty of "12-inch" ceramic measures 11-3/4 in, and "24-inch" planks often run 23-1/2. A quarter inch of error across ten rows is 2.5 in — most of a tile. Pull three tiles from different boxes, measure them, and use the real number.
Worked Example: A 10×12 ft Room
Take a 10×12 ft room — 120 in by 144 in — tiled with 12×12-in tile and a 1/8-in grout gap:
- Cell size: 12 + 0.125 = 12.125 in per tile.
- Short axis: 120 ÷ 12.125 = 9.88 → round up to 10 tiles.
- Long axis: 144 ÷ 12.125 = 11.88 → round up to 12 tiles.
- Base count: 10 × 12 = 120 tiles.
- With 10% waste: 120 × 1.10 = 132 tiles to buy.
That is 132 tiles — the number the live calculator returns for the same inputs. The same run produces the companion materials: 120 sq ft ÷ 75 = 1.6, so two 50-lb bags of thinset at the site's 75-sq-ft-per-bag planning rate, and 120 ÷ 25 = 4.8, so 5 lb of grout at the planning rule of 1 lb per 25 sq ft. The grout figure carries its own caveats — see how much grout you need for why it is a planning estimate rather than a promise.
How Much Waste to Add
Waste is not a fudge factor; it is a budget for specific losses: perimeter cuts, mis-cuts, breakage in the box, and tiles set aside for chipped corners. The percentage follows the layout, because layouts differ in how many cuts they generate and how reusable the offcuts are.
A straight layout burns the least, because every tile in the field goes in uncut and the losses cluster at the walls. Diagonal and herringbone layouts cut constantly, and an angled offcut rarely serves the next opening because its cut edge has the wrong geometry. Large-format tile punishes every error at two-plus square feet per piece. The table below matches allowances to layouts; when a project fits two rows, use the higher number.
| Layout | Typical waste | Why it burns tile |
|---|---|---|
| Straight / grid | 10% | Cuts confined to the perimeter rows |
| Diagonal (45°) | 15–20% | Every perimeter tile is cut twice at an angle |
| Herringbone | 15–20% | Angled cuts throughout the field; offcuts rarely reusable |
| Large-format (any edge 15 in or more) | 15% | One mis-cut or broken tile costs a big piece |
| Small rooms (under 40 sq ft) | Add 5 points | A handful of cut tiles is a large share of the total |
Small rooms deserve the last line. In a 35 sq ft powder room the perimeter is a large fraction of the whole floor, so the same five cut tiles that vanish into the rounding on a 300 sq ft living room push the percentage visibly up. Under about 40 sq ft, treat 15% as the starting allowance even for a straight layout.
Diagonal layouts are the classic under-order, because every edge row is cut twice — once to create the angle, once to fit the wall — and the triangular offcuts pile up fast. For the waste implications of running bond, basketweave, and the Versailles modular set, see tile layout patterns, which covers each layout and the allowance it deserves.
Spares, Dye Lots, and the One Extra Box
Buy one extra box beyond the takeoff and store it. Tile is fired in lots, and shade and calibre shift between production runs; the tile that matches your floor today may not exist at any retailer in three years, because product lines are discontinued and lots sell through. A repair without attic stock becomes a matching hunt through salvage listings — a miserable errand for a two-tile fix.
Store the spares the way tile wants to be stored: flat, dry, indoors, and out of traffic. Stack boxes flat rather than on edge, and label them with the room they belong to. Ten years from now, a cracked tile plus a matching box in the attic is the difference between a ten-minute swap and a floor conversation.
Box Math: From Tiles to Whole Boxes
Tile ships by the box, and the box — not the tile — is what you buy. Coverage is printed on the label in square feet per carton; divide your total by that figure and round up. Never plan on splitting a box: retailers sell sealed cartons, and an opened box is generally unreturnable anyway.
Run the worked example through real packaging: 132 twelve-inch tiles cover roughly 132 sq ft. In cartons marked 10.8 sq ft — a common count for 12×12 tile — that is 132 ÷ 10.8 = 12.2, so 13 boxes, which lands about eight tiles past the takeoff and conveniently close to the spare-box advice above. While the pallet is still in the aisle, check the dye-lot codes printed on the carton ends: every box for one room should carry the same code, or the grid you carefully counted will show shade shifts from run to run.
Plan It With the Tile Layout & Mortar Calculator
The Tile Layout & Mortar Calculator runs this article's math live: room and tile dimensions, grout gap, and waste percentage go in; tile count, thinset bags, grout pounds, and a materials total come out. It also draws the grid visually so you can see where the cut rows land, and it prices the thinset and grout alongside the tile.
Common Mistakes
- Dividing area by tile area and stopping. The shortcut ignores the grout gap and the round-up on every row — on a small floor it can short you a full box.
- Measuring from the plan. Plan dimensions drift from framing and finishes; the takeoff needs the floor as built, in inches, at the widest points.
- Using the nominal tile size. A "12-inch" tile often measures 11-3/4 in; ten rows of that error is 2.5 in of miscounted field.
- Applying one waste number to every layout. Ten percent is a grid number; diagonal and herringbone need 15–20% or the job stops at the last row.
- Skipping attic stock. The waste allowance is for the install; the stored box is for the repair a few years out, when the dye lot is gone.
- Mixing dye lots in one field. Shade and calibre differ between lots; check the carton codes before thinset touches trowel, not after three rows are set.
Frequently Asked Questions
How many 12×12 tiles do I need for 100 sq ft?
About 100 tiles before waste and 110 after. A 100 sq ft floor — say 10×10 ft — counts 10 × 10 = 100 tiles with 12×12-in tile and a 1/8-in gap, and the standard 10% waste allowance brings the purchase to 110. Larger tiles drop the count: 18×18-in tile covers the same floor in roughly 45 tiles before waste.
Do I count the grout gap when calculating tile?
Yes — add the gap to the tile dimension on each axis before dividing. The gap is part of every tile’s cell, so it shifts how many cells fit per row. The effect is small, but on a count that lands near a row boundary it changes the answer by a full tile.
How many extra boxes of tile should I keep?
One box of attic stock beyond the takeoff, stored flat, dry, and labeled with the room. The waste allowance covers cuts and breakage during the install; the stored box covers the cracked tile five years from now, when your dye lot has sold through or the line has been discontinued.
Does a bigger tile change how much area I need?
No — the floor is the same square footage regardless of tile size. What changes is the count (fewer, bigger pieces), the cut waste (large-format tile is budgeted at 15%), and the box math. That is why the takeoff counts tiles per axis instead of buying raw square footage.
How do I calculate tile for an L-shaped room?
Split it into rectangles and run the axis count on each one separately — an L-shaped room is two rectangular rooms that happen to share a corner. Add the two tile counts, then apply the waste allowance once to the combined total so it is not double-counted at the seam.
Related MyHouseLogic Guides
Tile ordering is one link in a chain — layout, joints, and grout all move the numbers. These guides cover the rest of it.
- Tile layout patterns explained — running bond, herringbone, basketweave, and the waste each pattern adds to the order.
- How much grout you need — what tile size and joint width do to consumption, and how to read the coverage chart on the bag.
- Tile spacing and grout joints — choosing a joint width by tile type, and why the gap belongs in the tile count.
The bottom line: measure the real floor, count tiles per axis with the gap included, buy waste that matches the layout, and round up to boxes you can actually purchase — then keep one box for the attic. The takeoff takes ten minutes; the dye-lot hunt it prevents can take weeks.