# How MaterialCalc Turns a Room Size Into a Shopping List
Most “how much do I need?” answers on the internet are a round number with no working. That is fine until you are standing in a store, holding a carton that covers 15 ft², and you cannot tell whether the website already added waste, rounded up, or used a different box size than the one in your hand.
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MaterialCalc is a small set of free, no-signup calculators for concrete, paint, mulch, gravel, drywall and tile. This note explains how the numbers are produced, so you can check them against a product label instead of trusting a black box.
# The same formula on the page and in the browser
Every tool prints its formula on the page. The same function runs in your browser when you type and in the build that generates the worked examples, so the answer box and the tables cannot disagree. Nothing you enter is sent to a server; the math happens on your device.
The yields are ordinary published figures, not a house brand:
- An 80 lb bag of concrete is treated as 0.60 ft³, a 60 lb bag as 0.45 ft³, and a 40 lb bag as 0.30 ft³.
- Tile boxes are not guessed as “N tiles.” You enter the coverage per box from the carton, add a waste percent, and the tool rounds up to whole boxes.
- Thinset is planned at about 95 ft² per 50 lb bag with a ¼ in notch—the usual catalogue figure, which a ⅜ in notch or an uneven floor will beat.
Those assumptions, and what the cost ranges do and do not include, are spelled out on the About page. If a local price or a bag yield is clearly different, the honest move is to change the input (or check the label), not to fudge the formula.
# A concrete example: tile, not a slogan
Take a 9×12 ft kitchen, 12×24 inch tile, 15 ft² per box, 10% waste.
- Area: 9 × 12 = 108 ft²
- With waste: 108 × 1.10 = 118.8 ft²
- Boxes: 118.8 ÷ 15 = 7.92 → 8 boxes
- Thinset: 118.8 ÷ 95 ≈ 1.25 → 2 bags
Raise waste to 15% for a diagonal layout and the same room becomes 9 boxes. That is the sort of check you can do on paper, or by moving one field in the tile calculator.
The point is not that 8 is a magic number. The point is that you can see why it is 8, and what happens if your carton says 16 ft² instead of 15.
# What the site will not do
It will not quote your local supplier, approve a permit, or promise that a mix or thickness meets code. Cost figures, where they appear, are US averages for budgeting. Always confirm quantities and prices with the product you actually buy.
If a result looks wrong, the About page says how to report it—include the page and the figures you entered. The useful test of a calculator is not that it sounds confident. It is that you can replay the arithmetic yourself.