Waste Allowance: How Much Extra Material Should You Buy?
The area or volume you measure is the net quantity: the amount that ends up in the finished job. The amount you buy must be larger, because planks are cut, tiles break, concrete spills and ground is never perfectly level. The extra is called the waste allowance, and it is usually written as a percentage of the net figure.
Getting it roughly right matters in both directions. Too little and you risk a second delivery or a mismatched batch. Too much and you pay for material you cannot use. This guide explains how to apply the percentage, typical planning ranges, and two details that catch people out.
How to apply an allowance
Measure the net quantity, subtract anything you will not cover, then add the percentage once.
quantity to buy = net quantity × (1 + allowance ÷ 100)A 10% allowance multiplies the net figure by 1.10.
Worked example: an 18 m² floor with a 10% allowance gives 18 × 1.10 = 19.8 m². Suppose the flooring comes in boxes of 2.4 m². Then 19.8 ÷ 2.4 = 8.25, and since boxes cannot be split you buy 9 boxes, or 21.6 m².
Notice what happened: you added 10%, but the real surplus is 21.6 ÷ 18 = 1.20, or 20%. Pack and roll sizes round the order up, so the true allowance is often larger than the percentage you started with. When boxes are big relative to the job, you may be able to use a smaller allowance.
Typical planning ranges
These are common rules of thumb, not requirements. Manufacturers and suppliers sometimes give their own recommendations, and these should take priority.
| Material | Typical allowance |
|---|---|
| Plank flooring, straight lay | 5% to 10% |
| Plank flooring, diagonal or patterned | 10% to 15% |
| Floor and wall tile | 10% to 15% |
| Roof shingles | 10% to 15% |
| Bricks and blocks | 5% to 10% |
| Concrete | 5% to 10% |
| Mulch, gravel and topsoil | 5% to 10% |
| Paint | No standard figure: round up to the next can |
Individual projects are covered in more detail in our guides to flooring, tiles, concrete, roofing and brick walls.
What pushes the number up or down
- Complicated shapes. Many corners, angled walls, curves and obstacles mean more offcuts.
- Layout and pattern. Diagonal and herringbone layouts cut almost every piece at the edges.
- Large units. Big tiles and wide planks leave larger, less reusable offcuts.
- Material behaviour. Brittle materials break, and natural products may need blending around flaws or colour variations.
- Experience. A first-time installer usually wastes more than a professional.
- Uneven bases. Rough ground absorbs extra concrete, gravel or soil.
A simple, square room with large boxes of plank flooring sits at the low end. A small bathroom with a diagonal tile pattern and pipe penetrations sits at the high end.
Choosing a number for your project
Start in the middle of the range, then move toward the top for anything that makes cutting harder and toward the bottom for simple, open jobs. Take a small garden room as an example:
- A plain plank floor of 18 m² in a square room: 5% to 10%, so use 10% to be safe, giving 19.8 m².
- A tiled wall of 6 m² with a window and a niche: 15%, giving 6 × 1.15 = 6.9 m².
- A concrete pad of 1.2 m³ on slightly uneven ground: 10%, giving 1.2 × 1.10 = 1.32 m³.
Then convert each answer to the supplier's unit, such as boxes, tubs or loads, and round up. The final order will be somewhat higher than the percentage alone suggests, as the boxes example above shows.
Two common traps
Adding versus dividing. Some suppliers describe waste as a share of what you purchase, not as an add-on to what you need. If 15% of everything you buy will be lost, you need net ÷ 0.85, not net × 1.15. For 200 m² of net coverage, the first method gives 200 ÷ 0.85 = 235.3 m² and the second 200 × 1.15 = 230 m². The gap grows as the percentage grows, so know which definition a figure uses.
Stacking allowances. If a calculator already adds a waste percentage, do not add another on top of its answer. Likewise, apply the allowance once, to the net area, instead of adding it to the area and again to the number of boxes.
Keep unopened spare units where possible, and check the retailer's return policy before buying extra. A few spare tiles or planks from the same batch make later repairs much easier than trying to match a discontinued product.
Use the percentage calculator to test different allowances, and the tile calculator to see a waste percentage applied to a real layout.
- Percentage Calculator: Pick the question you need answered, enter two numbers and get the result instantly. The guide below shows the formula behind each mode.
- Tile Calculator: Enter the size of the area, the size of one tile, the gap between tiles and a waste allowance to get the number of tiles to order. Add a box size and price to see boxes and cost.
- Square Footage Calculator: Type in the width and length of a room, or of the two sections of an L-shaped one, and get its area in square feet or any other unit. Add a price per square unit to see what flooring, paint or tile would cost.
This article is general information, not professional advice. See our Disclaimer and Editorial Policy.
More articles
Cubic Feet, Cubic Yards and Cubic Metres: Converting Job Volumes
Bags are labelled in cubic feet, trucks deliver cubic yards or cubic metres, and depth is in inches or millimetres. These are the conversions that tie them together.
How Many Bricks or Blocks Do You Need for a Wall?
A wall needs a fixed number of bricks or blocks per unit of area once you include the mortar joint. Work that number out once and the rest is multiplication.
How Much Concrete Do You Need? Slabs, Footings and Post Holes
Concrete is bought by volume, and every shape reduces to length times width times depth. Here is how to get the number, turn it into bags, and avoid ordering short.