Every-day math — converting units, figuring out a date, splitting a bill, checking a GPA — doesn’t need a spreadsheet or a signup. This guide covers all 12 calculator and converter tools on SysFenix, every one of them running as pure client-side arithmetic with nothing you type ever leaving your browser. That matters more than it might seem: a health calculator, a GPA, or a bill split can be genuinely private information, and none of it is ever transmitted to a server here — the whole calculation happens instantly in JavaScript running on your own device.
Most of these tools solve a problem that’s technically simple to compute by hand, but easy to get subtly wrong — a leap-year edge case in a date calculation, a rounding error compounding across a bill split, or a timezone mistake in a meeting-time conversion. Each tool below is built and tested specifically to get those edge cases right, not just the common case.
Unit and ratio conversion
Conversion errors are one of the most common sources of small everyday mistakes — a recipe measurement off by a factor, a screen resolution that doesn’t match the aspect ratio it’s supposed to. These three tools handle the conversion math precisely so you don’t have to reach for a rough mental approximation.
- Unit Converter converts between metric and imperial units for length, weight, temperature, volume, area and speed — cm to inches, kg to lbs, Celsius to Fahrenheit, liters to gallons, and more — using exact, internationally-defined conversion constants rather than rounded approximations, so a chain of conversions stays accurate.
- Aspect Ratio Calculator simplifies a width/height pixel pair (like 1920x1080) down to its lowest common ratio (like 16:9) using the greatest common divisor, or lets you lock a common target ratio and solve for a missing width or height — genuinely useful when resizing video, planning a print layout, or picking a crop size that matches a target platform.
- Number Base Converter (technically filed under developer tools, but relevant here too) converts between binary, decimal, hexadecimal and octal, plus text-to-binary/hex encoding, for anyone doing low-level or embedded work.
Dates and time
Date arithmetic looks simple until you hit a leap year, a month with a different number of days, or a timezone boundary — these three tools handle all of that correctly so you don’t have to think about it.
- Days Between Dates Calculator computes the exact duration between two dates as a years/months/weeks/days breakdown, plus an optional count of business days only (Monday through Friday). It also works in the opposite direction — enter a starting date and a number of days, and it finds the resulting date, correctly handling every month-length and leap-year edge case along the way.
- Unix Timestamp Converter converts a raw Unix timestamp (the kind you’ll see in server logs, API responses, or a database column) into a readable date in both UTC and your local timezone, automatically detecting whether the number you pasted is in seconds or milliseconds.
- Time Zone Converter is a multi-zone meeting planner: add several cities or time zones to a list, slide one reference hour, and instantly see every zone’s equivalent local time and calendar date — correctly accounting for daylight saving time transitions, which is exactly the detail that trips up a manual mental calculation.
Everyday math
- Percentage Calculator covers the three phrasings people actually search for: “X% of Y” (a tip or discount), “X is what percent of Y” (a grade or completion rate), and the percentage increase or decrease between two values (a price change or year-over-year comparison).
- Fraction Calculator adds, subtracts, multiplies and divides fractions and mixed numbers, and — unlike a plain calculator app — actually shows the step-by-step working: converting to a common denominator, combining the numerators, and simplifying the result, plus a separate decimal-to-fraction converter for the reverse direction.
- Tip Calculator splits a restaurant bill among any number of people, with common tip-percentage presets (15/18/20/25%) or a custom percentage, showing both the total per person and the tip amount per person separately.
Health and study
- BMI Calculator computes Body Mass Index in either metric or imperial units, with the full official WHO weight-category classification — including the three separate Obese subclasses most simpler BMI calculators skip — plus a healthy-weight-range readout for your entered height. Like every calculator here, it’s a screening tool, not a diagnosis; it doesn’t account for muscle mass, bone density, age, or sex, and it isn’t medical advice.
- Sleep Calculator works backward or forward from a bedtime or wake-up time using the commonly-cited ~90-minute sleep-cycle heuristic, suggesting times that land you between complete cycles rather than waking you up mid-cycle, which is the theory behind why some alarm times leave you feeling more rested than others even with the same total sleep duration.
- GPA Calculator computes a proper credit-hour-weighted semester GPA from a list of courses, credit hours, and letter grades — not just a naive average of grades, which would incorrectly treat a 1-credit elective the same as a 4-credit core class — with an optional second step to combine it into a new cumulative GPA if you already know your prior GPA and total credit hours.
- Times Tables Practice is a multiplication-table quiz and printable worksheet generator for kids learning their times tables, covering the 2x through 12x tables with adjustable difficulty and a printable practice sheet for offline work.
Which tool should I use?
| I want to… | Use this tool |
|---|---|
| Convert cm to inches, kg to lbs, or Celsius to Fahrenheit | Unit Converter |
| Simplify a 1920x1080-style ratio, or solve for a missing dimension | Aspect Ratio Calculator |
| Find out how many days until an event | Days Between Dates Calculator |
| Convert a Unix timestamp to a real date | Unix Timestamp Converter |
| Plan a meeting across multiple time zones | Time Zone Converter |
| Calculate a tip and split a bill | Tip Calculator |
| Work out “X% of Y” or a percent increase | Percentage Calculator |
| Add or simplify fractions with the steps shown | Fraction Calculator |
| Check your BMI and healthy weight range | BMI Calculator |
| Figure out the best time to go to bed | Sleep Calculator |
| Calculate a semester or cumulative GPA | GPA Calculator |
| Practice multiplication tables | Times Tables Practice |
All 12 tools are free, unlimited, and run entirely on your own device. Because none of them require a signup or a subscription, they’re genuinely faster to reach for than opening a spreadsheet app or searching for a formula — just open the page, enter your numbers, and get an instant, correctly-rounded answer with no ads interrupting the calculation itself.
A quick note on accuracy: every calculator here is built to handle the edge cases a naive implementation tends to get wrong — leap years and month-length differences in date math, floating-point rounding in bill splitting, and internationally-defined exact conversion constants (rather than rounded approximations) in unit conversion. If you’ve ever been burned by a calculator that quietly rounds a currency total to the wrong cent, these tools are built specifically to avoid that class of mistake.
