Two ways of writing the same color
RGB and hex are just two different notations for the exact same red/green/blue values - RGB spells out each channel as a plain number from 0-255, while hex packs each channel into a two-digit base-16 pair. Which one you need usually comes down to the tool or codebase you're working in: CSS and most design tools accept both formats interchangeably, so the choice is often just a matter of which one you have on hand versus which one a specific field expects.
Converting in either direction
Enter RGB values (0-255 for each of red, green, and blue) and convert to hex, or type a hex code directly and convert back to its RGB values - both directions update the live color swatch instantly so you can see exactly what color you're working with before copying the result.
Understanding 3-digit shorthand hex codes
CSS supports a shorthand 3-digit hex format where each digit is simply doubled to expand it - #0E7 expands to the full 6-digit #00EE77. This shorthand only works when each of the three channel values happens to be a repeated digit pair; a color like #0E7C7B can't be shortened this way since its digit pairs aren't doubled, so not every color has a valid 3-digit shorthand.
Why this matters for web and design work
Design handoff between a designer and a developer frequently crosses between these two notations - a design tool might display a color as RGB while the CSS that implements it needs hex, or vice versa. Having a fast, reliable way to convert between them removes a small but recurring friction point in that handoff.