Free online tools that just work
90 calculators, converters and developer tools. Fast, private and free, with no sign-up. Everything runs in your browser.
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See all 90 →Add Days to Date
Find the date that falls a given number of days, working days, weeks, months or years before or after any date.
Age Calculator
Enter a date of birth to get an exact age in years, months and days, plus the total in weeks and days and a countdown to the next birthday.
Amortization Calculator
See every payment of a loan laid out, with the interest, principal and remaining balance, by month or by year.
APR Calculator
Turn an interest rate plus upfront fees into one comparable number, the annual percentage rate, the way US lenders calculate it.
Auto Loan Payoff Calculator
Get today's payoff figure for your car loan and see exactly how much sooner an extra payment each month clears it.
Auto Loan Refinance Calculator
Find out whether refinancing your car loan actually saves money once fees and the new term are taken into account.
Base64 Encode & Decode
Convert text to Base64 and back, with full Unicode support and an option for the URL-safe alphabet used in JWTs and web addresses.
BMI Calculator
Work out your body mass index from your height and weight, see your category, and find the healthy weight range for your height.
BMR Calculator
Estimate how many calories your body burns at complete rest, using the three most widely used BMR formulas side by side.
Body Fat Calculator
Measure your neck, waist and (for women) hips to estimate body fat percentage, fat mass and lean mass.
Calorie Calculator
Estimate your daily calorie needs (TDEE) and get simple targets for keeping, losing or gaining weight.
Car Affordability Calculator
Start from the payment you can manage and work backwards to the most car you can buy, including tax, fees and your trade-in.
Browse by category
Developer Tools
Format, validate, encode and decode data without pasting it into a server. 14 tools
Text Tools
Count, clean, convert and rearrange text in a single click. 14 tools
Finance Calculators
Loans, savings, interest and investments, worked out in seconds. 35 tools
Health & Fitness Calculators
BMI, calories, body fat and other everyday health numbers. 6 tools
Math Calculators
Percentages, fractions, averages and the formulas you forgot from school. 2 tools
Unit Converters
Length, weight, temperature, data and more, with every common unit. 10 tools
Generators
Passwords, UUIDs, placeholder text and random numbers on demand. 4 tools
Image & Color Tools
Compress and resize images and convert colors, all on your device. 1 tools
Date & Time Tools
Ages, date differences, timestamps and time zones made simple. 4 tools
Guides
All guides →How to Calculate a Finance Charge
The three ways card issuers compute interest, a side-by-side example, and how the finance charge on a loan is disclosed.
How to Calculate a Loan Amount
Work out how much you can borrow three different ways, and why the smallest of the three answers is the one that counts.
How to Calculate Interest on a Car Loan
See exactly how much interest a car loan charges each day, each month and in total, and how the rate and term change it.
How to Calculate Compound Interest (Formula and Excel)
Learn the compound interest formula, see how compounding frequency and monthly deposits change the result, and do it all in Excel.
How to Calculate GAP Insurance
Find your own gap between what you owe and what the car is worth, see what GAP covers, and decide whether you need it.
How to Calculate Line of Credit Interest
Daily interest, variable rates, interest-only payments and the jump at repayment, explained with a worked HELOC example.
Why use ZTool?
Most online tools get in your way. They show pop-ups, ask for an email address, or quietly upload your files to a server you know nothing about. ZTool works differently. Every tool is a lightweight page that does its work on your own device, so your data stays with you and results appear as you type.
- Private by design. Your text, numbers and images are never sent anywhere. Disconnect from the internet after the page loads and most tools still work.
- Built for speed. Pages are pre-built static HTML with almost no JavaScript, so they open instantly even on slow mobile connections.
- Explained, not just calculated. Each tool comes with a plain-English guide that covers the formula, worked examples and common mistakes.
- Free for good. No accounts, no trials and no limits on how often you use a tool.