About

What this site is

Zayvora Calculators is a small collection of free calculators, each built to answer one practical question. Sellers can work out marketplace fees, markup and margin, and break-even advertising returns. Homeowners can estimate roof pitch, concrete slabs and solar costs. Makers and pet owners can estimate the cost of a 3D print or a dog’s age in human years.

Next to the tools you will find guides that explain the ideas behind the numbers, so you can understand a result and not only copy it.

How the tools work

Every calculator runs in your own browser. When you type a number into a tool, the calculation happens on your device. Your inputs are not sent to our server and they are not stored. You do not need an account, and you do not need to install anything.

If you close the page, the numbers you typed are gone. That is by design.

How each calculator is built and checked

Each calculator page explains its formula in words, so you can see exactly how a result is worked out. The formulas are written out step by step, and each page includes a worked example with real numbers.

Before a tool is published, it is checked against hand-worked examples. We enter known values, work the same problem out by hand, and compare the two answers. If they do not match, the tool is fixed before it goes live. Results are rounded for display only, and the full-precision numbers are used in the middle of every calculation.

Where rates and figures come from

Some calculators need rates, such as marketplace fees, electricity prices or cost per watt. Every one of these is an editable box with a starting value. The starting values are placeholders, not promises. They can be out of date, and they differ by country, category and account.

Where a page mentions a published source, it says so on that page. Please replace the defaults with your own numbers whenever you have them, and check official sources before you make a decision.

Corrections

We want the tools to be right. If you think a result is wrong, or a formula or a page is unclear, please use the Contact form and choose “Report an error”. Tell us which calculator you used and what you entered, and we will look into it.

When an error is confirmed, it is fixed, and the “Last updated” date on that page is changed so you can see that something was corrected.

Free to use, and possible ads later

The site is free to use. It may show advertising in the future to help cover its costs. If that happens, the Privacy Policy will be updated before any ads go live.

Important notes

The calculators give estimates for information only. They are not financial, tax, legal, engineering or other professional advice. Please read the Disclaimer and the Terms and Conditions. For common questions, see the FAQ page.