Guides & help

These guides explain assumptions, common pitfalls, and how to interpret results from our calculators and utilities. If you’re using results for anything important, always verify with an official source (or a professional) and treat estimates as estimates.

Browse guides

Personal finance calculators

Taxes, loans, mortgages, retirement, and how to sanity-check results.

Seller fees (marketplaces & payments)

How platform and payment fees stack (and what to include in your pricing).

Health & nutrition calculators

BMI, TDEE, macros, and how to interpret the numbers safely.

Student calculators & tools

GPA, Pomodoro, unit conversion, writing tools, and study tracking.

Time tools & timesheets

Date math, work hours, overtime rules, and export-friendly records.

Developer utilities

Format, convert, and validate common developer data safely in-browser.

Security tools

Passwords, hashing, HMAC, JWT decoding, and what the outputs mean.

Network tools

IP basics, WHOIS/RDAP, subnet math, and troubleshooting tips.

Builders & DIY

Material estimators, tiling/wallpaper calculators, and measurement tips.

Everyday tools

QR codes, timers, world clock, text tools, and size conversion tips.

Email signatures

Install an HTML signature in Gmail, Outlook, Thunderbird, and Outlook on the web.

How to get the most value from tool sites

A calculator is only as good as its assumptions. Before acting on any result, confirm which inputs are included (for example, tax year, deductions, fees, or measurement units) and rerun your numbers with a conservative scenario.

Smart Web Apps is designed for quick, privacy-focused estimates. Where possible, each tool shows the underlying formula so you can validate the logic or recreate it in a spreadsheet.