How we calculate
Every Homestimate calculator is built on the same idea: take your measurements, apply a standard industry rule of thumb, add a sensible allowance for waste, and round to whole units you can actually buy — then multiply by a price you control. This page explains that process, lists the key assumptions we use, and shows where our numbers come from so you can verify any of them.
Our estimating approach
Behind the simple form on each page, every calculator follows the same five steps:
- Measure the space. We turn your dimensions into the figure that drives the material — an area (paint, flooring, tile, siding), a volume (concrete, gravel, mulch, topsoil), or a length (baseboard, fencing, gutters).
- Apply a coverage rule. We divide by how much one unit covers — a gallon of paint over ~350 sq ft, an 80 lb bag of concrete filling ~0.6 cu ft, a roofing "square" covering 100 sq ft, and so on.
- Add a waste allowance. Real projects have offcuts, breakage, and mistakes, so we add a typical buffer (often 10%) and let you adjust it.
- Round to whole units. You can't buy 2.3 gallons or 7.4 bags, so we round up to the next full unit you'd actually purchase.
- Estimate the cost. We multiply by a typical unit price that is fully editable, so you can match your local store or a quote you already have.
Key assumptions we use
These are the most common rules of thumb across our calculators. They're industry standards, not guarantees — every calculator lets you override the ones that matter (coverage, waste, price) to match your actual product.
| Assumption | Value we use | Based on |
|---|---|---|
| Interior paint coverage | ~350 sq ft per gallon, per coat | Major paint-brand technical data |
| Concrete from bags | 80 lb ≈ 0.60 cu ft · 60 lb ≈ 0.45 · 40 lb ≈ 0.30 | Bag-mix manufacturer yields |
| Roofing "square" | 100 sq ft · 3 bundles of shingles per square | Roofing trade convention |
| Flooring / tile waste | 10% (more for diagonal or patterned layouts) | Standard installer allowance |
| Gravel & aggregate weight | ~1.4–1.5 tons per cubic yard | Typical crushed-stone density |
| Framing / joist spacing | 16″ or 24″ on-center | International Residential Code (IRC) norms |
| Mortar / thinset coverage | By trowel notch size | Tile-setting material data sheets |
Why we add a waste allowance
If you buy the exact area of a room in flooring, you'll come up short — every cut at a wall, every doorway, and every damaged plank eats into coverage. A 10% allowance covers a typical straight-lay job; diagonal, herringbone, or busy-pattern layouts waste more, so we let you raise it. Buying a little extra also means your material comes from the same dye lot or batch, which keeps color and texture consistent and gives you spares for future repairs.
How we estimate cost
Material prices vary by region, brand, store, and season, so we don't pretend to know your exact price. Instead, each calculator starts with a typical unit price as a placeholder and makes it editable. Change it to the price on the shelf or in your quote and the cost estimate updates instantly. Our cost figures are for planning and budgeting — they deliberately exclude things that are too project-specific to guess, like delivery fees, labor, and permits, unless a calculator says otherwise.
Where our numbers come from
Our formulas and coverage figures are drawn from widely accepted, verifiable sources:
- Manufacturer technical data — coverage and yield figures published by product makers (for example, paint spread rates, concrete bag yields, and thinset/grout coverage by trowel size).
- Building codes and trade conventions — established norms such as the International Residential Code for framing and joist spacing, and the roofing "square" (100 sq ft).
- University agricultural extension services — seeding, sod, and fertilizer rates, which vary by grass type and region.
- Standard installer practice — the waste allowances and rules of thumb that contractors use every day.
Where sources disagree, we choose a sensible mid-range value and make it editable so you can dial it to your own product.
Accuracy, limits, and when to call a pro
Our estimates are designed to answer "roughly how much, and roughly what will it cost?" so you can plan and shop with confidence. They are not a substitute for a professional take-off or a structural calculation. For large, load-bearing, or code-regulated work — framing spans, footings, retaining walls over a few feet, electrical, gas, or anything you're unsure about — confirm quantities with your supplier and have the design checked by a qualified professional before you buy or build.
Found a number that looks off?
We review our formulas and assumptions periodically and welcome corrections — if something doesn't match your experience or a manufacturer's spec, please tell us on our contact page. Accuracy is the whole point of this site, and reader feedback is one of the best ways we keep it.