Methodology and Editorial Standards

PraxisCalc is published by Zeta Digilux Labs. Every tool on this site is an estimate built from a published formula, and this page sets out which formulas we use, where the rules come from, how the numbers get checked, and what happens when we get something wrong.

We show the formula

A calculator that returns a number without showing its working cannot be checked by the person relying on it. Each tool states the formula it applies, the inputs it expects, and the assumptions baked into the result. If a calculation has a limit past which the estimate stops being useful, the page says so rather than quietly returning a figure.

Which formulas the tools use

We use standard published formulas rather than approximations or proprietary models:

• Loan and mortgage tools use the amortisation formula, M = P x (r(1+r)^n) / ((1+r)^n - 1).
• Growth and savings tools use compound interest and future value, including the annuity form for regular contributions.
• Retirement projections compound an existing balance and a contribution stream separately, then add them, rather than approximating with a single lump sum.
• Pricing tools distinguish markup, which divides gross profit by cost, from margin, which divides the same profit by selling price.

Where the rules come from

Some calculations depend on rules set by an authority, and those rules change. Where that is the case, the page cites the body responsible rather than presenting a figure as timeless:

• Retirement contribution limits and tax figures reference the Internal Revenue Service.
• Lending ratios and borrower protections reference the Consumer Financial Protection Bureau.
• Small business pricing and cost guidance references the Small Business Administration.

Where a calculation varies by country or state, such as property tax or sales tax, the page says so instead of presenting one jurisdiction's rules as universal.

How the numbers are checked

Worked examples on each page are computed, not illustrative. You can reproduce them by hand and get the same answer, and if you cannot, that is a bug worth reporting. Calculator logic is checked against an independent calculation before it is published, and again whenever the formula or its inputs change.

What these tools are not

PraxisCalc produces estimates for planning and education. It is not financial advice, it does not know your circumstances, and it should not be the only input into a decision about a mortgage, a retirement account, or a business price. For decisions that size, use these numbers to arrive at better questions for a qualified professional rather than to replace one. See the full disclaimer.

Corrections

If you find an error in a formula, a worked example, or a stated assumption, report it through the contact page or email support@praxiscalc.com. Accuracy reports go ahead of new features in the queue. When a calculation changes we update the page and its last-modified date.

Who publishes this

PraxisCalc is a project of Zeta Digilux Labs. You can verify the organisation on LinkedIn, X, Facebook and YouTube. Editorial and correction enquiries reach a person at the address above.