The short version: the math is the base. Constants come from documents we have opened and read, and each one is named on the page next to its number. When a maker does not publish a number, the calculator works it out by rule and says so.
Math first
Every calculator starts from geometry and definitions, not from a lookup table. Cubic yards are length × width × thickness ÷ 27. A standard 8 × 16 block covers 128 square inches of wall, so 112.5 block cover 100 square feet. Rebar laps follow the lap rule for that bar size.
Only when a number cannot be derived — the yield of a bag of mix, the resistance of a wire, a nail spacing — does it come from a published source.
Where the constants come from
Manufacturer data sheets (QUIKRETE, Sakrete, Hilti, Vulcraft, Canam, USG), building codes (ACI 318, TMS 402, the IRC, NEC Chapter 9) and industry references (NCMA/CMHA TEK notes, BIA Technical Notes, ASTM A615). Each calculator lists its own under “Where the numbers come from”, with a link to the document.
We open the document and read the number. We never copy a constant from another calculator or from memory. An earlier in-house version copied a brick table from another source and ran 3 to 8 percent high — that is why this rule exists.
Data sheets are cross-checked
When two makers publish a yield, we compare them and the page says which one it uses and why. QUIKRETE 5000 gives 0.60 cubic feet per 80 lb bag; Sakrete Maximizer gives 0.92 — same bag weight, different mix — so the bag count changes with the product, and the page says so.
When a maker’s number cannot be found, the calculator computes by rule — area × thickness ÷ yield, or the geometry of the piece — and says so next to the number instead of borrowing a figure from somewhere else.
Every calculator has hand-worked tests
Before a calculator ships, an example is worked out by hand — on paper, with the source open — and written into an automated test next to the expected value. The code has to reproduce it, and the test runs again on every change.
Quantities and money use exact decimal arithmetic, never floating point: 6 inches is exactly 152.4 mm, not 152.39999.
Rounding
Bags, sheets, boards, bars and pieces round up to the next whole unit, because that is what you can buy. Volumes show two decimals (11.11 CY).
Waste is never added unless you type a waste percentage — the default is zero. The number you see is the quantity in place, and when you add waste the page shows the with-waste quantity on its own line.
Units
Type lengths the way you say them: 12' 6-1/2", 150" or 3.81 m. Inside, every length is stored in exact millimetres using the defined factors — 1 inch = 25.4 mm, 1 foot = 0.3048 m, 1 pound = 0.45359237 kg — so switching between ft-in and metric never changes the quantity, only how it is written.
The drawings
Each page draws the job as a PLAN, SECTION or ELEVATION with dimension strings, so you can check what you typed against the plan before you order.
They are sketches to help you order material and price the work. They are not engineered or shop drawings. Always build from your approved project drawings.
Spanish
Every page has a Spanish version with the same math, the same sources and US units. An automatic test fails if any formula or source text is missing its Spanish. If a term differs where you work, let us know.
Corrections and reviews
These are community calculators: we check every number against its source, but mistakes are possible. Each calculator shows a “Last reviewed” date under its title. To report a wrong number, use the “Report a problem or suggest something” box under the calculator — it can attach what you typed. We check it against the source, fix the engine, add a test for that case and update the date.
Tell your PM: which calculator, what you typed and what you expected.
Sources we use
- QUIKRETE and Sakrete product data sheets — bag yields for concrete, mortar, grout, stucco and veneer mortar
- NCMA / CMHA TEK notes — block and mortar per 100 square feet, joint reinforcement, bearing
- ACI 318-19 — rebar lap splices and development lengths; standard hook shape (Table 25.3.1)
- CRSI Manual of Standard Practice — rebar shop dimensions out to out; recommended end hook sizes
- TMS 402 — masonry lap lengths, veneer ties and bearing
- IRC — fastening schedules, sheathing, roofing underlayment, EIFS drainage
- NEC Chapter 9 Table 8 and 310.16 — conductor resistance and ampacity
- BIA Technical Note 10 — brick and mortar quantities
- ASTM A615 — rebar sizes and nominal weights
- Manufacturer data — Hilti (adhesive anchors), Vulcraft and Canam (metal deck), USG (joint compound and tape), APA (panels)
The full list for each calculator, with links, is under “Where the numbers come from” on its page.