Dyanmic resize a CSS3 loading spinner by pixels

Question

I've been asked to take a pure CSS3 loading spinner and make it dynamically resizable by pixels to use in different places in a program.

My current code is: (Which apparently doesn't run well in SO's snippets)

.loader {
  animation:spin 1s infinite linear;
  border:solid 2vmin transparent;
  border-radius:50%;
  border-right-color:#71c491;
  border-top-color:#f7941d;
  box-sizing:border-box;
  height:20vmin;
  left:calc(50% - 10vmin);
  position:fixed;
  top:calc(50% - 10vmin);
  width:20vmin;
  z-index:1;
  &:before {
    animation:spin 2s infinite linear;
    border:solid 2vmin transparent;
    border-radius:50%;
    border-right-color:#21409a;
    border-top-color:#92278f;
    box-sizing:border-box;
    content:"";
    height:16vmin;
    left:0;
    position:absolute;
    top:0;
    width:16vmin;
  }
  &:after {
    animation:spin 3s infinite linear;
    border:solid 2vmin transparent;
    border-radius:50%;
    border-right-color:#13b0e6;
    border-top-color:#18244c;
    box-sizing:border-box;
    content:"";
    height:12vmin;
    left:2vmin;
    position:absolute;
    top:2vmin;
    width:12vmin;
  }
}

@keyframes spin {
  100% {
    transform:rotate(360deg);
  }
}

<div class="loader"></div>

I googled and tried transform:scale() but as far as I can tell that only takes specific input and increases/decreases the size by percentage. (2 = 200% size)

I'm thinking I need some sort of wrapper, but I'm not too familiar with advanced CSS to get the effect. When I tried to create my own, only the top border of the spinner would be resized into a weird shape and not the inner borders. I'm just stumped. If you could point me in the right direction, I'd be appreciative. Thank you.

Answer 1

You could try a mix of CSS var() / calc() / clamp() / grid ... and relative/absolute positionning to lay the loader over the parent where you need it , if that inspire you :

demo with a few loader within a div sized and the possibility to set an average size to start from, % size based on the width of the parent.

value to reset in the demo is --size ; you may also tune the other --MyVarCss values to your needs.

* {
  box-sizing: border-box;
}

:root { /* init for the var() values */
  --size: 20;/* value used to set the loader's width and adjust border's width */
  --width: calc(var(--size) * 1%);
  --widthBorder: calc( clamp(20px, 6vw, 80px) * var(--size) * 0.005);
}

.a,/* for the demo , just a bunch of containers */
.b,
.c,
.d,
.d,
.e {
  position: relative;
  /* what the parent loader needs to be (absolute/fixed/sticky works too, static not) */
  float: left;
  border: solid;
  margin: 1em;
}

div.a {
  --size: 50; /* reset the value used to set the loader's width */
  width: 50%;
  padding-top: 50%;
}

.b {
  --size: 10;/* reset the value used to set the loader's width */
  width: 600px;
  height: 200px;
}

.c {
  --size: 15;/* reset the value used to set the loader's width */
  width: 25%;
  padding-top: 20%;
}

.d {
  --size: 30;/* reset the value used to set the loader's width */
  width: 800px;
  height: 400px;
}

.e {
  --size: 14;/* reset the value used to set the loader's width */
  width: 90%;
  min-height: 20vh;
}

div {
  width: 20%;
  padding-top: 20%;
}
/* loader styles */
.loader {
  position: absolute;
  top: 0;
  left: 0;
  display: flex;
  width: 100%;
  height: 100%;
  margin: 0;
  padding: 0;
}

.loader b {
  display: grid;
  animation: rotate 3s -1s infinite linear;
  border: solid var(--widthBorder) transparent;
  padding: calc(var(--widthBorder) / 2);
  border-radius: 50%;
  border-right-color: #71c491;
  border-top-color: #f7941d;
  grid-row: 1;
  grid-column: 1;
  margin: 0;
}

.loader>b {
  margin: auto;
  width: var(--width);
}

.loader>b:before {
  content: "";
  padding-top: 100%;
  grid-row: 1;
  grid-column: 1;
}

.loader b b {
  border-right-color: #21409a;
  border-top-color: #92278f;
}

.loader b b b {
  border-right-color: #13b0e6;
  border-top-color: #18244c;
  padding: 0;
}

@keyframes rotate {
  100% {
    transform: rotate(360deg);
  }
}

<div class=a>
  <div class="loader"><b><b><b></b></b>
    </b>
  </div>
</div>
<div class=b>
  <div class="loader"><b><b><b></b></b>
    </b>
  </div>
</div>
<div class=c>
  <div class="loader"><b><b><b></b></b>
    </b>
  </div>
</div>
<div class=d>
  <div class="loader"><b><b><b></b></b>
    </b>
  </div>
</div>
<div class=e>
  <div class="loader"><b><b><b></b></b>
    </b>
  </div>
</div>