I build and maintain the calculators on Toolsque, and I write the arithmetic inside them. That is the whole of my claim. I am a developer, not a contractor, not an accountant, and not a tax adviser. I have never framed a roof or filed anyone else's return, and I would rather say that in the first line than let a vague job title do work the code should be doing.

How a calculator gets built here

Each tool starts with a document. Before writing a line I read the thing the tool is meant to reflect: the IRC span tables, Approved Document K for stair rise and going in the UK, IRS guidance on self-employment tax, ISO/IEC 18004 for how a QR symbol carries its payload. Then the formula, then the edge cases, then input validation, in that order. The formula itself goes on the page in plain text so you can check my working against your own.

The second input is search. I read the queries people arrive on, and the phrasing usually gives away the real question. Someone typing "how many bundles for a 1,500 square foot roof" almost always has the building footprint rather than the roof surface, and those are two different numbers. So the roofing calculator asks for footprint and pitch, applies the multiplier, and states what it did.

Three examples of the working

Roof area. The multiplier for a sloped plane is the square root of one plus the square of rise over run. At 6:12 that is 1.118, so a 1,600 square foot footprint covers 1,789 square feet of roof, or 17.9 squares before any waste factor is added. Skip the multiplier and you order roughly two squares short.

Markup against margin. A 40 percent markup on an $80 cost gives a $112 price and $32 of gross profit, which is a 28.6 percent margin. To actually keep 40 percent you price at 80 divided by 0.6, or $133.33, which is a 66.7 percent markup. The contractor markup calculator prints both figures together because the two words get swapped constantly in pricing advice and the gap between them is eleven points of profit.

Self-employment tax. It applies at 15.3 percent to 92.35 percent of net profit, so $60,000 of net profit produces $8,477.73, before income tax and before the deduction for half of that amount. The 1099 tax calculator shows the 92.35 percent line on screen instead of jumping straight to a total, since that intermediate figure is where most people spot an input they mistyped.

Where the tools stop

A calculator only knows what it is handed. The joist span calculator reads published tables for species, grade, spacing and load, and it has no idea about your snow load, a notched member, a local amendment, or the point load from a filled bathtub. Treat a building result as the figure you take into a conversation with whoever signs the permit. Treat a tax result as arithmetic, not a filed return. Where a default is baked in, such as a waste percentage or a rounding direction, the page says so and lets you change it.

If a number looks wrong

Tell me. A useful report has four parts: which tool, the exact inputs you entered, the figure it returned, and the figure you expected together with the source you took it from. Send it through the contact page. I reproduce it before touching anything, because reports often turn out to be a units mismatch rather than a bad formula, and a units mismatch still counts as my fault since it means a field label is unclear. Real errors get fixed in the code and the change is dated on the page. When the disagreement is about an assumption rather than the maths, I write the assumption into the page in plain words and make it an editable input wherever the format allows.