Setting font-size based on height of flexbox container

Question

I am wondering if it possible to set the font-size using the height of flexbox items. I have a flexbox container set using viewport units and the height of the items are determined by flex-grow property. What I am looking do is set the font-size to the height of these items, and to retain these relationships as the viewport changes.

I have a basic idea that is somewhat working but I am not exactly sure how to isolate only the letter (baseline to cap height) and scale that to the item container.

https://codepen.io/NewbCake/pen/JvwNJq (resize window vertically to set font-size)

I am open to any suggestions on how to approach this or any pitfalls that this may encounter.

HTML

<section class="center">
  <div class="container">
    <div class="item1">H</div>
    <div class="item2">H</div>
  </div>
</section>

CSS

section {
  display:flex;
  flex-direction:row;
  height:95vh;
  width:100%;
  border:1px solid red;
}

.container {
  display:flex;
  flex-direction:column;
  height:80vh;
  width:80vh;
  border:1px solid blue;
}

.container_wide {
  display:flex;
  flex-direction:column;
  height:80vh;
  width:80vh;
  border:1px solid blue;
}

.center {
  justify-content:center;
  align-items:center;
}

.item1 {
  flex-grow:1;
  flex-shrink:0;
  flex-basis:auto;
  border:1px solid green;
  line-height:.75;
}

.item2 {
  flex-grow:3;
  flex-shrink:0;
  flex-basis:auto;
  border:1px solid green;
  line-height:.75;
}

JS

var resizeTimer;

$(window).on('resize', function(e) {

  clearTimeout(resizeTimer);
  resizeTimer = setTimeout(function() {

    // Run code here, resizing has "stopped"
    $(".item1").css("font-size", $(".item1").css("height"));
    $(".item2").css("font-size", $(".item2").css("height"));            
  }, 250);
});

Any help is appreciated!

Answer 1

If you have the control of the flex-grow you can do some calculation to get the font-size based on the height on the container. So if you have a 1 + 2 as flex-grow, it means that the second one will be twice the first one so we can define a H height as H+2*H = height of container = 80vh so H = calc(80vh / 3).

So the first item will have font-size:H and the second one will have font-size:2*H.

You may also consider CSS variable to better handle this.

body {
   margin:0;
  padding:0;
  font-family: sans-serif;
}

header {
  display:flex;
  height:5vh;
}
section {
  display:flex;
  flex-direction:row;
  height:95vh;
  width:100%;
  border:1px solid red;
}

.container {
  display:flex;
  flex-direction:column;
  --h:80vh;
  height:var(--h);
  width:var(--h);
  border:1px solid blue;
}

.gauge {
  display:flex;
  flex-direction:column;
  height:80vh;
  width:10vh;
  border:1px solid blue;
}

.center {
  justify-content:center;
  align-items:center;
}

.item1 {
  flex-grow:1;
  font-size:calc((var(--h) / 3));
  flex-shrink:0;
  flex-basis:auto;
  border:1px solid green;
  line-height:1;
}

.item2 {
  font-size:calc((var(--h) / 3) * 2);
  flex-grow:2;
  flex-shrink:0;
  flex-basis:auto;
  border:1px solid green;
  line-height:1;
}

<header class="center">resize window vertically</header>
<section class="center">
  <div class="gauge">
    <div class="item1"></div>
    <div class="item2"></div>
  </div>
  <div class="container">
    <div class="item1">Haj</div>
    <div class="item2">Hlp</div>
  </div>
</section>