Greyscale to black-opacity conversion

I have discovered

color: rgb(51, 51, 51);

and

color: #333333;

and

color: black; opacity: 0.8;

all produce equivalent shades of grey.

How can I generalize this?

In other words, given an arbitrary RGB or HEX color/shade of grey, how can I determine the equivalent value of opacity when color: black;?

Here is the Codepen Demo.

<style>
  div {
    font-family: sans-serif;
    font-size: 48px;
    margin: 20px;
    padding: 60px;
  }
  .rgb {
    color: rgb(51, 51, 51); /* given this value... */
  }
  .hex {
    color: #333333; /* ...or given this value */
  }
  .opacity {
    color: black;
    opacity: 0.8; /* how can I determine this value? */
  }
<style>

<body>
  <div class="rgb">rgb(51, 51, 51)</div>
  <div class="hex">hex #333333</div>
  <div class="opacity">opacity 0.8</div>
</body>

Solution

You could use this Hextool site too. Use the RGBA. Fourth value is the opacity of that color.

Eg:-

rgba(51, 51, 51, 0.87)

87% Opacity

Hex Opacity Values

100% — FF
95% — F2
90% — E6
85% — D9
80% — CC
75% — BF
70% — B3
65% — A6
60% — 99
55% — 8C
50% — 80
45% — 73
40% — 66
35% — 59
30% — 4D
25% — 40
20% — 33
15% — 26
10% — 1A
5% — 0D
0% — 00