How much extra flooring, tile, paint and concrete to buy

Add an allowance to the measured area or volume first, then round up to whole boxes, cans or bags. Published rules of thumb run from 5% for straight-laid solid hardwood to about 10% for click-together planks and at least 10% for tile, with more for diagonal and patterned layouts. The ready-mixed concrete industry association suggests ordering at least 4–10% over the calculated volume. For paint, label coverage and the number of coats matter more than any waste percentage.

Buy tile and flooring, spares included, from one production lot.

Why buy more than you measured?

The measured floor, wall or slab is the minimum. The allowance pays for material that never ends up in place:

• Cuts. Offcuts from walls, doorways and pipes are often too short or the wrong shape to reuse.
• Defects and breakage. Bruce’s solid-hardwood instructions note that industry grading standards permit deficiencies in up to 5% of the flooring, and a tile can crack while being cut or set.
• Repairs later. A spare carton from the same lot lets you replace a cracked tile years from now with one that matches.
• Dimensions that exceed the plan. For concrete, NRMCA lists forms that bow under wet concrete, uneven or settling subgrade, spillage, and slabs thicker than drawn.

Paint has no offcuts; its allowance covers paint left in the tray and roller, patches that absorb more, and a small touch-up reserve.

How much extra flooring and tile for straight, diagonal and herringbone layouts?

For straight rows, about 5% for solid hardwood, 10% for click-together planks and at least 10% for wall tile. Diagonal layouts take 10% for hardwood and 30–35% in all for wall tile. For herringbone hardwood, AHF asks for 5% (10% laid diagonally) and Woodland Reserve for 10%; none of these sources gives a herringbone figure for tile. Each figure is one maker’s or seller’s rule of thumb for its own products, not a standard; the instructions for the product you buy take precedence.

The figures differ partly because they cover different things. Bruce’s 5% is cutting and grading waste only, MSI’s 10% includes tile kept for repairs, and Daltile’s attic stock comes on top of what the job uses. So the familiar “10% rule” is more than a straight hardwood floor needs, about what click planks need, and short of what a diagonal tile layout needs.

The allowance goes up with:

• Diagonal layouts: every piece along a wall is cut at an angle.
• Herringbone and chevron: the zigzag meets every wall at an angle, as a diagonal layout does. How much extra that takes depends on the product: the two herringbone instructions cited here ask for 5–10%.
• Small or broken-up rooms: closets, bays and islands add edges, and so more cut pieces for the same area.
• Large tiles and long planks: each cut discards more, and layout rules limit where offcuts can go. Crossville, for example, recommends offsets of no more than 33% for tiles 15 in or longer.

Should you add waste before or after rounding to boxes?

Before. Apply the allowance to the measured area, divide by the carton’s coverage, and round up last:

$$\text{boxes} = \left\lceil \frac{A \times (1 + w)}{c} \right\rceil$$

$A$ is the measured area, $w$ the allowance as a decimal (10% is 0.10), $c$ the coverage of one box from the carton, and $\lceil\ \rceil$ means round up to a whole number. If the carton lists only pieces, multiply them by the area of one tile.

Worked example: tile for a 10 × 16 ft room

Example inputs: a 10 × 16 ft floor (160 sq ft) and cartons that cover 15.5 sq ft each (use the figure on yours).

At 10%: 160 × 1.10 = 176 sq ft, and 176 ÷ 15.5 = 11.35, so buy 12 boxes (186 sq ft).

Because one box is 9.7% of this floor, the purchase moves in steps: 0% and 5% both buy 11 boxes, and rounding alone leaves 10.5 sq ft (6.6%) spare. The order of operations matters too:

• Round first, then add 10% of the boxes: 11 × 1.10 = 12.1, rounded up to 13, one box more than the allowance calls for.
• Round first, then add one box: 12 here, but only by coincidence; on a 500 sq ft floor one box is just 3.1%.

Why should every box come from the same lot?

Tile and flooring are made in production runs, and runs differ slightly. MSI notes that tiles from one lot match in shade and pattern, but a store shelf can hold more than one lot, and different lots can look lighter or darker side by side. Daltile adds that the dye lot and printing process can change over the years, so spares bought with the original order are what will match a repair. Natural stone varies between quarries and over time.

Size can differ too. Crossville explains that tiles shrink by slightly different amounts in the kiln, so calibrated tile is sorted into two to four size groups, called calibers. When tiles from different calibers meet, as in a two-color pattern, even small size differences can throw off alignment.

1. Before opening any carton, check that the lot (shade) and caliber codes match on all of them, spares included.
2. Buy the quantity, allowance and spares in one purchase.
3. Install from several open cartons at once. Daltile says this for ceramic tile; AHF says to pull planks from at least three cartons to minimize pattern repeats.
4. Keep one or two unopened cartons, labeled with the room and the lot.

Check the return policy for unopened cartons first; MSI warns that tile may not be returnable.

How much extra paint do you need?

Usually only a small allowance, because paint has no offcuts. None of the paint sources here sets a waste percentage, so the 10% below is an example, not a standard:

$$\text{gallons} = \frac{\text{area} \times \text{coats} \times (1 + w)}{\text{coverage per gallon}}$$

Area is the wall area minus doors and windows, and $w$ is the allowance as a decimal.

Coverage is the bigger unknown. PaintCare puts most paints at 300–400 sq ft per gallon and suggests 350 if the label doesn’t say; Sherwin-Williams gives a typical 350–400 sq ft per gallon and about 100 sq ft per quart. PaintCare adds that rough textures can take up to 50% more paint, a switch between very dark and very light colors can take three or four coats, and glossy sheens need more coats than flat ones.

Worked example: two coats on a 10 × 16 ft room

Example inputs: 8 ft walls, one 3 × 7 ft door, one 3 × 5 ft window, two coats, a label coverage of 350 sq ft per gallon and a 10% allowance.

1. Wall area: 2 × (10 + 16) × 8 = 416 sq ft.
2. Openings: 21 + 15 = 36 sq ft, leaving 380 sq ft to paint.
3. Two coats: 380 × 2 = 760 sq ft.
4. Paint: 760 ÷ 350 = 2.17 gal; with the allowance, 2.17 × 1.10 = 2.39 gal.
5. Cans: 2.39 gal is 9.6 quarts, so buy 2 gallons and 2 quarts (2.50 gal).

Moving from the high to the low end of the typical range adds 0.63 gal; the 10% allowance adds 0.19 to 0.25 gal. Texture needs its own budget: 50% more at 350 sq ft per gallon is 3.26 gal before the allowance. To choose between gallons and quarts, compare the price per gallon with the unit price calculator.

A big touch-up reserve does less for paint than for tile. Sherwin-Williams notes that a touch-up can look different even with paint from the same can, because the patch dries differently from the wall around it, and that improved color matching removes the need to overbuy for a later match. Benjamin Moore gives opened, well-sealed paint a shelf life of about two years. Paint you won’t use can often be donated or taken to a household hazardous waste or paint recycling program; oil-based paint counts as hazardous waste.

How much extra concrete should you order?

NRMCA, the ready-mixed concrete industry association, recommends ordering at least 4–10% more than the volume calculated from the forms. Most of its reasons are dimensional (bowing forms, uneven subgrade, a thicker slab), and small errors add up: an extra 1/8 in on a 4 in slab is a 3% shortfall, and hardened concrete may end up about 2% smaller than the fresh volume delivered.

$$V_{\text{ft}^3} = L \times W \times \frac{t_{\text{in}}}{12}, \qquad V_{\text{yd}^3} = \frac{V_{\text{ft}^3}}{27}$$

$L$ and $W$ are in feet and $t_{\text{in}}$ is in inches; a cubic yard is 3 × 3 × 3 = 27 cubic feet.

Worked example: a 10 × 14 ft patio slab, 4 in thick

Example inputs: 10 × 14 ft, 4 in thick, a 10% allowance and 80 lb bags yielding 0.60 ft³ each (the figure QUIKRETE publishes for its 80 lb Concrete Mix; check your bag).

1. Volume: 10 × 14 × 4 ÷ 12 = 46.67 ft³ (1.73 yd³).
2. With the allowance: 10 × 14 × 4 ÷ 12 × 1.10 = 51.33 ft³ (1.90 yd³).
3. Bags: 51.33 ÷ 0.60 = 85.6, so buy 86 bags of 80 lb.

If the ground was dug a little deep and the forms average 4.5 in, the slab needs 10 × 14 × 4.5 ÷ 12 = 52.50 ft³. That is 12.5% more than the 4 in figure, and more than the whole allowance before anything is spilled.

An allowance can’t fix a wrong thickness, so measure the depth at several points across the forms and use the average. With ready-mix, NRMCA also suggests re-checking the remaining volume near the end of the pour and telling the supplier in time, rather than finishing with a small extra load.

Try it

• Flooring and tile: in the flooring and tile calculator, replace the example’s areas with one area 16 ft long and 10 ft wide, then enter a coverage per box of 15.5 sq ft and a waste allowance of 10%. It should show 176 sq ft with 10% waste and 12 boxes (186 sq ft). Change the allowance to 20% to get 13 boxes.
• Paint: in the paint calculator, enter a 16 ft × 10 ft room with 8 ft walls, change the example’s openings to one 3 × 7 ft door and one 3 × 5 ft window (its window count starts at 2), then enter 2 coats, a coverage of 350 sq ft per gallon and a 10% allowance. Expect 380 sq ft to paint, 2.39 gal, and 2 gallons and 2 quarts. Enter 300 and 400 sq ft per gallon, with and without the allowance, to reproduce the table.
• Concrete: in the concrete calculator, make the first element a 14 ft × 10 ft slab 4 in thick and remove the example’s second element, then use a 10% allowance and an 80 lb bag with a yield of 0.60 ft³. Expect 51.33 ft³ (1.90 yd³) and 86 bags. Then set the thickness to 4.5 in and the allowance to 0% for 52.50 ft³ and 88 bags.

Where these rules of thumb stop working

• They assume careful measuring. An allowance covers cuts, defects and breakage, not a mismeasured room or a slab poured thicker than planned. See how to measure a room for square footage and volume.
• Returns change the trade-off. If unopened cartons can go back, a generous allowance costs little. Special orders, discontinued lines and non-returnable tile make running short costly and spare cartons more valuable.
• Structural questions are out of scope. Slab thickness, reinforcement and base depth for load-bearing work are set by local codes and the professionals involved, not by a waste allowance.