Trust

Methodology & Sources

Every calculator on this site starts from a published formula, standard, or table, and is checked before it goes live. This page shows what that checking looks like in practice, and lists every source the calculators are built on.

How each calculator is checked

General claims about accuracy are easy to make, so here are specific checks from real calculators. Each one is also written out in that calculator’s “Assumptions & Reference Values” section.

Reproducing published worked examples

Where an authority publishes worked examples, the calculator has to reproduce them before it goes live.

The Stamp Duty Calculator reproduces HMRC’s own worked examples exactly: £275,000 non-residential gives £3,250, and £295,000 residential gives £4,750.
The Mortar Calculator is calibrated to Quikrete’s published yield, where one 70 lb bag plus 240 lb of sand lays about 144 standard bricks or 46 standard blocks.

Rounding so itemized totals add up

Rounding each line separately can make a breakdown visibly fail to add up by a cent. Money figures are rounded in a fixed order so every itemized total reconciles.

The Water Bill Calculator rounds every line to cents before summing, checked across 30,000 random bills with a worst-case gap of $0.00.
The Reverse Sales Tax Calculator rounds the pre-tax price first and derives the tax as the remainder, so $251.58 at 12% splits into figures that sum back to $251.58, not $251.59.
The Time and a Half Calculator uses the unrounded overtime rate: $7.17 × 1.5 is exactly $10.755, and rounding it to $10.76 first would put the pay off by up to $0.20.

Naming the method when there is more than one

Some quantities have several accepted definitions. The calculator states which one it uses, so you can match your textbook, software, or code.

The 5 Number Summary Calculator uses the exclusive quartile method (as on TI-83/84 calculators) and the 1.5 × IQR rule for outliers.
The Price Elasticity of Demand Calculator offers the midpoint method, which gives −1.2222 for a move between $10/100 units and $12/80 units in either direction, where the simple method gives −1.0 one way and −1.5 the other.
The Snow Guard Calculator applies snow load to the roof’s horizontal projection, as ASCE 7 defines it. Using the sloped area instead would overstate the force by 11.8% at a 6/12 pitch.

Exact constants and published tables

Unit conversions use exact defined values, and standard tables are used as published rather than re-derived.

The Well Volume Calculator uses the defined US gallon (exactly 231 cubic inches), so a cubic foot is 7.480519 gallons rather than the rounded 7.48.
The Rebar Calculator takes bar weights straight from the ASTM A615 table; deriving them from area and steel density would be off by up to 0.019 lb/ft.
The Metal Roof Screw Calculator guards against floating-point error when rounding up: 1,600 × 1.10 evaluates to 1760.0000000000002 in a computer and would otherwise round up to 1,761.

Independent cross-checks

Where a formula can be derived two ways, both are computed and compared.

The Pipe Bedding Calculator checks its haunching formula against a numerical integration of the space between the trench floor and the pipe’s springline.
The Hip Roof Calculator computes roof area both as footprint × pitch factor and by splitting the roof into trapezoids and triangles, with zero difference across 50,000 random roofs.
The Roofing Nail Calculator rounds the installed nail count up before adding waste; the extra this can add is at most 2 nails, checked across 200,000 random cases.

Found a result that doesn’t match an official source? Please tell me or email [email protected]. Corrections get priority.

All data sources

367 citations across 169 calculators, from 306 distinct sources: regulators, standards bodies, manufacturers, and published research. This list is built automatically from the same data as each calculator’s sources section, so it is always current. Links open the original source.