What is a scrap factor and how do I calculate material waste?

A scrap factor, also called a waste factor, is the extra material you add to an estimate to cover what will be cut off, broken, spilled or left over. To calculate it, compare what you bought with what you installed on past jobs: (purchased − installed) ÷ installed. Then multiply the measured quantity of a new job by one plus that percentage, and round up to the size the material is sold in.

Why it matters

Material you could not install still has to be paid for. Bid 1,000 square feet of tile at exactly 1,000 square feet and you will buy more in the middle of the job, at whatever price and lead time you can get, with a crew waiting. On a job where materials are half the cost, leaving out a 10 percent waste factor takes five points off the margin.

The formula

Quantity to order = measured quantity × (1 + waste percent)

For 1,000 square feet at 10 percent waste, order 1,100.

One trap: waste added on top is not the same as yield. If 10 percent of what you buy ends up in the dumpster, you need 1,000 ÷ 0.90 = 1,111, not 1,100. Decide which way your company measures it and use it the same way every time.

What drives waste up

• Cuts and layout. Diagonal patterns, small rooms, many corners and penetrations.
• Fixed sizes. Sheets, lengths and rolls leave offcuts that fit nowhere else.
• Breakage and handling. Brittle material, long carries, several moves around the site.
• Batch materials. Concrete, mortar, grout, coatings: what is mixed and not placed is gone.
• Crew experience. A new installer wastes more than a veteran.
• Theft and weather. Material stored on site for weeks does not all survive.

Typical ranges, and why to distrust them

Rules of thumb usually fall between 5 and 15 percent, lower for simple runs of pipe or wire and higher for tile, roofing, drywall and anything laid in a pattern. Treat these as a starting point only. Your crews, your suppliers and the kind of jobs you take decide the real number.

Round up to how it is sold

Waste is only half of the gap between measured and ordered. Materials come in bundles, boxes, bags, pallets and truckloads. If the job needs 1,100 square feet and a box covers 48, you are buying 23 boxes, 1,104 square feet. Ready-mix is ordered by the yard, and a short load costs more than the extra half yard would have. Price the quantity you will actually buy.

Build your own numbers

1. At the end of each job, record what was purchased and what was installed, for the main materials only.
2. Work out the percentage for each material.
3. Keep it by material and by kind of work. Tile in a bathroom and tile in an open lobby are different numbers.
4. Update the factor when three or four jobs point the same way.

Returns count. If a supplier takes back unopened boxes, the waste is what stayed, not what was delivered.

Do not hide waste inside markup

Markup is for overhead and profit. Waste is a cost of the material, and it changes with the job. Keep it in the material quantity where you can see it, compare it with what really happened, and adjust it.

Where ConstructStream fits

In ConstructStream a product carries its materials as a recipe: each material has a formula for the quantity, a waste percent, and the step it is bought in. The estimate works out the quantity with waste, rounds it up to that step and costs it, so the margin you see already includes the material you will not install.