Accuracy & Methodology.
A calculator is only as trustworthy as the formula behind it and the honesty of what it claims. This page sets out where our formulas come from, which published standards we follow, and — just as importantly — what our tools cannot tell you.
How a calculator gets built
Every tool starts from a published formula, not from our own invention. For financial tools that usually means the standard amortization equation; for health tools, a named and peer-reviewed equation such as Mifflin–St Jeor; for unit conversion, the defined SI relationships. Where a calculator uses a specific published equation, we name it on the page so you can check our working against the source rather than taking our word for it.
We then test each tool against worked examples whose answers are already known — a textbook problem, an official example, or a figure published by the organisation that owns the standard. A calculator that cannot reproduce a known answer does not go live.
Standards we follow
These are published standards our tools are written against. We are not certified or audited by any of these bodies, and we do not claim to be — they are simply the specifications the maths follows.
Conversions use the defined SI relationships for mass, length, volume and temperature, so a conversion is exact by definition rather than approximated.
The JSON validator follows the ECMA-404 interchange standard, which is why it rejects trailing commas and single-quoted keys that some editors accept.
Spine width and royalty tools use the PPI constants and printing costs Amazon publishes. KDP changes these periodically; see the note on updates below.
Some tools calculate in your browser; others compute on our server and discard the input immediately. Nothing you enter is logged, stored or shared either way.
What our calculators cannot do
This matters more than any accuracy claim. Our tools produce estimates for planning and understanding. They are not financial, medical, legal or engineering advice, and they should not be the only basis for a decision that carries real consequences.
- Financial tools cannot know your credit profile, a lender's rounding conventions, arrangement fees, or how a variable rate will move. Expect a small difference from a real quote.
- Tax and payroll tools apply published bracket tables to the figures you enter. They do not know your allowances, credits, benefits or filing situation, and tax rules change during the year.
- Health tools use population-level equations. They describe averages, not you — body composition, medical conditions and medication all change the answer. Speak to a clinician before acting on a number here.
- Publishing tools reflect the printing specifications in force when we last checked them. Always confirm against your printer's current template before sending a cover to production.
Where a calculator carries a limitation worth knowing before you use the result, we state it on that calculator's own page rather than burying it here.
Keeping figures current
Some tools depend on numbers that change: tax brackets, contribution limits, printing costs. Where a calculator uses a figure tied to a specific year, that year is shown on the page so you can judge whether it is still current. We update these when the issuing body publishes a change, and we would rather show you a dated figure honestly than an undated one that looks authoritative.
Found something wrong?
Tell us and we will fix it. The most useful report includes the calculator name, the exact values you entered, the result you got and the result you expected. Write to workshop@usecalcforge.com or use the contact form. We correct errors rather than removing pages, and we are glad to be told when we are wrong.