JWT Security Best Practices Every Backend Developer Should Follow
Common JWT pitfalls per RFC 7519 including algorithm confusion, expiry handling, and key rotation.
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Every tool is hand-built with full editorial documentation, worked examples, and FAQ — so you always understand the answer, not just the number.
Finance Calculators
Loans, mortgages, investments, taxes — built with verified formulas from official sources.
16 Tools →Health & Fitness
BMI, BMR, calorie, body composition — using CDC, WHO, and peer-reviewed formulas.
12 Tools →Math Calculators
Algebra, geometry, statistics — with formulas shown and step-by-step examples.
12 Tools →Unit Converters
Length, weight, temperature, data storage, currency — accurate to 8 decimal places.
12 Tools →Text & SEO Tools
Word counters, case converters, readability checkers — privacy-first, runs in browser.
12 Tools →Developer Tools
JSON, Base64, regex, JWT, hashes — per IETF RFCs and NIST standards.
15 Tools →Generators
Password, UUID, QR codes, gradients — using cryptographically secure RNG where it matters.
12 Tools →Time & Date
Age, date difference, time zones, countdowns — handles edge cases like leap years.
8 Tools →Color & Design
HEX, RGB, HSL, WCAG contrast — with the exact W3C relative luminance formula.
7 Tools →Everyday Utilities
Tips, fuel cost, dice, GPA — practical tools for daily life.
12 Tools →Most Popular
Featured Tools This Week
Hand-picked favorites our visitors use every single day.
Loan Calculator
Calculate monthly loan payments, total interest, and the standard amortization formula.
Open tool →Mortgage Calculator
Estimate your full PITI monthly mortgage payment including PMI per the Homeowners Protection Act.
Open tool →Compound Interest Calculator
See how investments grow with the power of compounding — with the standard A = P(1+r/n)^(nt) formula.
Open tool →BMI Calculator
Calculate Body Mass Index with the CDC formula and WHO classification thresholds.
Open tool →Calorie Calculator
Find your daily calorie target for losing, maintaining, or gaining weight.
Open tool →Word Counter
Count words, characters, sentences, paragraphs, and reading time.
Open tool →Readability Checker
Measure Flesch Reading Ease, Flesch-Kincaid Grade, and Gunning Fog index.
Open tool →JSON Formatter & Validator
Beautify, minify, and validate JSON per RFC 8259. Runs entirely in browser.
Open tool →Regex Tester
Test JavaScript regular expressions with flags, match highlighting, and replacement preview.
Open tool →Password Generator
Create strong, random, secure passwords using crypto.getRandomValues — NIST 800-63B compliant.
Open tool →Why Choose Us
Built Different. Built Better.
We obsess over the things most tool sites skip — accuracy, explanation, privacy, and a calm reading experience.
Accurate by Design
Every tool uses formulas verified against authoritative sources — CDC, IRS, IETF RFCs, NIST. Each page cites the source so you can verify.
Privacy First
Most tools run entirely in your browser. Your inputs never touch our servers. No tracking of inputs, no storage, no reselling.
Education, Not Just Output
Every tool includes a real explanation, worked examples, formula transparency, FAQ and limitations. You learn while you calculate.
Instant Results
No page reloads. No signups. No ads inside the tool. Type input, get answer, copy it. The way it should be.
From Our Blog
Latest Articles & Guides
Deep, original explainers that complement our tools — written by humans, edited with care, updated regularly.
Pregnancy Weight Gain: What's Normal and What's Not
Trimester-by-trimester guide based on IOM 2009 recommendations and pre-pregnancy BMI.
Read article →Secure Password Best Practices for End Users and Developers
NIST 800-63B guidance, length vs complexity, entropy, and proper password storage.
Read article →APR vs APY: The Critical Difference That Costs You Money
Why the same interest rate can produce very different real costs or returns — per Truth in Lending Act.
Read article →Body Fat Measurement Methods Compared: Which Is Most Accurate?
Navy method, calipers, DEXA, bioimpedance — accuracy, cost, and practicality compared.
Read article →WCAG Color Contrast Explained: Making the Web Accessible
What the WCAG 2.2 ratios mean per the W3C spec, how to test them, and easy fixes for failing designs.
Read article →Our Promise
Trustworthy Tools, Built With Care
At HT99 Tools, we treat every calculator like a piece of editorial work. Each tool is reviewed by our team for accuracy and clarity. We cite authoritative sources, show you the exact formulas we use, and explain our limitations honestly. If you spot a bug or have a question, our team responds within 1–2 business days at contact@ht99.icu.