A HEX color is three pairs of hexadecimal digits: red, green and blue, each from 00 to FF (0 to 255). #3b82f6 splits into 3b, 82 and f6, which is 59, 130 and 246, so it equals rgb(59, 130, 246). Three-digit shorthand such as #0af doubles each digit: #00aaff. HSL describes the same color as hue (0-360 degrees), saturation and lightness in percent.
Type in either the HEX box or the RGB box (for example 255, 99, 71) and the other formats update. Colors are converted with the standard sRGB formulas.
Convert each pair from base 16 to base 10. Example: #ff8800 gives ff = 255, 88 = 136, 00 = 0, so rgb(255, 136, 0). The binary to decimal converter explains the same idea for base 2.
The last two digits are the alpha (opacity). This tool reads the first six digits and ignores alpha.
It makes it easy to build shades: keep the hue and change lightness to get lighter or darker variants of the same color.
See also: case converter · aspect ratio calculator · PPI calculator