Converters
Convert units, dates, colours and numbers — private, in your browser
Convert length, weight, temperature and other units, Unix timestamps and time zones, colour formats and contrast, px to rem, number bases and percentages. 20 free tools that run on your device — no signup, no limits.
All converters
Popular tools
- Unix Timestamp ConverterTurn epoch seconds, milliseconds or nanoseconds into dates, and dates back into timestamps.
- Length ConverterConvert miles, kilometres, feet, inches, metres and more with exact factors.
- Color ConverterPaste any colour and get HEX, RGB, HSL, HSV, HWB, CMYK, LAB, LCH and OKLCH with copyable CSS.
- Percentage CalculatorWork out percentages with the steps shown: X% of Y, percent change, increase and reverse.
Units
9 toolsLength Converter
Convert miles, kilometres, feet, inches, metres and more with exact factors.
Weight Converter
Convert kilograms, pounds, ounces, stone, grams and tonnes with exact factors.
Temperature Converter
Convert Celsius, Fahrenheit, Kelvin and Rankine with the formula shown.
Area Converter
Convert square feet, square metres, acres, hectares, gaj, guntha and cent.
Volume Converter
Convert litres, millilitres, US and UK gallons, cups, pints and spoons.
Speed Converter
Convert km/h, mph, m/s, knots, ft/s and Mach.
Data Storage Converter
Convert bytes, KB, MB, GB, TB and the binary KiB, MiB, GiB with exact factors.
Time Converter
Convert nanoseconds, seconds, minutes, hours, days, weeks, months and years.
Pressure Converter
Convert pascal, kPa, bar, psi, atm, mmHg, torr and inHg with exact definitions.
Date & time
5 toolsUnix Timestamp Converter
Turn epoch seconds, milliseconds or nanoseconds into dates, and dates back into timestamps.
Time Zone Converter
Convert one date and time into many time zones side by side, with a meeting planner.
Date Difference Calculator
Count the years, months, days and business days between two dates.
Age Calculator
Work out exact age, days lived and the next birthday from a date of birth.
Add or Subtract Days from a Date
Add or subtract days, weeks, months, years or business days from any date.
Colour & design
3 toolsColor Converter
Paste any colour and get HEX, RGB, HSL, HSV, HWB, CMYK, LAB, LCH and OKLCH with copyable CSS.
Color Contrast Checker
Measure the WCAG contrast ratio of two colours and get the nearest text colour that passes.
PX to REM Converter
Convert px to rem and em and back with your own base font size, plus a copyable table of common sizes.
Numbers
3 toolsNumber Base Converter
Convert between binary, octal, decimal, hex and any base from 2 to 36.
Percentage Calculator
Work out percentages with the steps shown: X% of Y, percent change, increase and reverse.
Fraction to Decimal Converter
Turn fractions into decimals and percent, and decimals back into simplest fractions.
20 tools shown
Workflows
Common jobs, done in a few steps
Debug a timestamp
Turn an epoch value into a date, then see that moment in other time zones.
Check a colour pair
Convert a colour to the format you need, then test its contrast against the background.
Size units for CSS
Convert pixel sizes to rem for a given base font size.
Convert between bases
Move a number between binary, octal, decimal and hexadecimal.
Why Nelstack
Fast, private converters
Your values never leave your device
Every conversion is calculated in your browser, so nothing you enter is uploaded or stored.
Both directions, live
Type in any field and the others update, with a full table of every unit for the value you entered.
Honest rounding
Results are rounded for display and the page says how many digits are kept, so nothing is silently clamped.
Time zones from your browser
Zone names, offsets and daylight saving rules come from the built-in Intl database, with the offset shown.
No caps or signup
No daily quota, no account and no watermark on anything you convert.
Built for phones too
Inputs come first on small screens, with touch-friendly controls and no sideways scrolling.
Looking for another kind of tool?
These live in their own tool areas and work the same private, in-browser way. Money calculators live in Finance, number-to-words in Text Tools and data formats in Data Tools.
- Finance calculatorsLoans, tax, investments and more
- Roman Numeral ConverterConvert numbers and Roman numerals, in Text Tools
- Number to WordsSpell out numbers, in Text Tools
- JSON FormatterFormat and validate JSON, in Data Tools
- Data ToolsConvert JSON, CSV, Excel, YAML, XML and TOML
- Text ToolsCounters, case converters and text utilities
Converters FAQ
Are these converters really free?
Yes. The unit, date and time, colour and number converters are free with no signup, no daily limit and no watermark. Use them as often as you like.
Do my values leave my device?
No. Every converter runs in your browser, so the numbers, dates and colours you enter are calculated on your device and are never sent to a server or stored.
How accurate are the unit conversions?
Units use standard published definitions, such as the international inch of exactly 25.4 mm. Results are rounded for display, and the page shows a visible message with the number of digits kept, so you can raise the precision when you need more.
Where does the time zone data come from?
Time zone names, offsets and daylight saving rules come from the Intl database built into your browser, which follows the IANA time zone data. A very old or very new browser may know slightly different historical rules, so check important legal or travel times against an official source.
Which contrast standard does the colour checker follow?
The contrast checker uses the WCAG 2.x relative luminance formula and its AA and AAA thresholds. WCAG 3 and APCA use a different method and can give different results for the same pair of colours.
Where are the money calculators, number-to-words and data format tools?
Loan, tax and investment calculators live in Finance, number-to-words and Roman numeral tools live in Text Tools, and JSON, CSV, YAML and XML conversion lives in Data Tools. They all work the same private, in-browser way.