edit 2022 Nowdays, aspect-ratio becomes widely avalaible. set a width or an height then for a square use the 1/1 ratio : aspect-ratio: 1 / 1 ;
ressources:
- tutorial https://css-tricks.com/almanac/properties/a/aspect-ratio/
- description https://developer.mozilla.org/en-US/docs/Web/CSS/aspect-ratio
- browsers that understand it https://caniuse.com/?search=aspect-ratio
snippets becomes (note: no width nor height set, since we are in flex layout and flex-grow is set to 1 flex: 1 0 auto; , doing the sizing job ):
.flex-container {
padding: 0;
margin: 0;
list-style: none;
display: flex;
justify-content: space-around;
}
.flex-item {
background: tomato;
margin: 5px;
color: white;
font-weight: bold;
font-size: 1.5em;
text-align: center;
flex: 1 0 auto;
aspect-ratio: 1 / 1 ;
}
/* Item can be any kind of box from here . flex/grid/block */
.flex-item {display:grid;align-items:center;justify-content:center;}<body>
<div class="flex-container">
<div class="flex-item">1</div>
<div class="flex-item">2</div>
<div class="flex-item">3</div>
<div class="flex-item">4</div>
<div class="flex-item">5</div>
<div class="flex-item">6</div>
<div class="flex-item">7</div>
</div>
<div class="flex-container">
<div class="flex-item">1</div>
<div class="flex-item">2</div>
<div class="flex-item">3</div>
<div class="flex-item">4</div>
<div class="flex-item">5</div>
<div class="flex-item">6</div>
<div class="flex-item">7</div>
</div>
<div class="flex-container">
<div class="flex-item">1</div>
<div class="flex-item">2</div>
<div class="flex-item">3</div>
<div class="flex-item">4</div>
<div class="flex-item">5</div>
<div class="flex-item">6</div>
<div class="flex-item">7</div>
</div>
<div class="flex-container">
<div class="flex-item">1</div>
<div class="flex-item">2</div>
<div class="flex-item">3</div>
<div class="flex-item">4</div>
<div class="flex-item">5</div>
<div class="flex-item">6</div>
<div class="flex-item">7</div>
</div>
<div class="flex-container">
<div class="flex-item">1</div>
<div class="flex-item">2</div>
<div class="flex-item">3</div>
<div class="flex-item">4</div>
<div class="flex-item">5</div>
<div class="flex-item">6</div>
<div class="flex-item">7</div>
</div>
<div class="flex-container">
<div class="flex-item">1</div>
<div class="flex-item">2</div>
<div class="flex-item">3</div>
<div class="flex-item">4</div>
<div class="flex-item">5</div>
<div class="flex-item">6</div>
<div class="flex-item">7</div>
</div>
<div class="flex-container">
<div class="flex-item">1</div>
<div class="flex-item">2</div>
<div class="flex-item">3</div>
<div class="flex-item">4</div>
<div class="flex-item">5</div>
<div class="flex-item">6</div>
<div class="flex-item">7</div>
</div>
</body>original answer
you should not set any size.
you may use an extra element or a pseudo elemnt with vertical padding in %. this will allow you to use width as reference : a snippet to show:
.flex-container {
padding: 0;
margin: 0;
list-style: none;
display: -webkit-box;
display: -moz-box;
display: -ms-flexbox;
display: -webkit-flex;
display: flex;
-webkit-flex-flow: row;
justify-content: space-around;
line-height:30px;
}
.flex-item {
background: tomato;
margin: 5px;
color: white;
font-weight: bold;
font-size: 1.5em;
text-align: center;
flex: 1 0 auto;
height:auto;
}
.flex-item::before {
content:'';
float:left;
padding-top:100%;
}<body>
<div class="flex-container">
<div class="flex-item">1</div>
<div class="flex-item">2</div>
<div class="flex-item">3</div>
<div class="flex-item">4</div>
<div class="flex-item">5</div>
<div class="flex-item">6</div>
<div class="flex-item">7</div>
</div>
<div class="flex-container">
<div class="flex-item">1</div>
<div class="flex-item">2</div>
<div class="flex-item">3</div>
<div class="flex-item">4</div>
<div class="flex-item">5</div>
<div class="flex-item">6</div>
<div class="flex-item">7</div>
</div>
<div class="flex-container">
<div class="flex-item">1</div>
<div class="flex-item">2</div>
<div class="flex-item">3</div>
<div class="flex-item">4</div>
<div class="flex-item">5</div>
<div class="flex-item">6</div>
<div class="flex-item">7</div>
</div>
<div class="flex-container">
<div class="flex-item">1</div>
<div class="flex-item">2</div>
<div class="flex-item">3</div>
<div class="flex-item">4</div>
<div class="flex-item">5</div>
<div class="flex-item">6</div>
<div class="flex-item">7</div>
</div>
<div class="flex-container">
<div class="flex-item">1</div>
<div class="flex-item">2</div>
<div class="flex-item">3</div>
<div class="flex-item">4</div>
<div class="flex-item">5</div>
<div class="flex-item">6</div>
<div class="flex-item">7</div>
</div>
<div class="flex-container">
<div class="flex-item">1</div>
<div class="flex-item">2</div>
<div class="flex-item">3</div>
<div class="flex-item">4</div>
<div class="flex-item">5</div>
<div class="flex-item">6</div>
<div class="flex-item">7</div>
</div>
<div class="flex-container">
<div class="flex-item">1</div>
<div class="flex-item">2</div>
<div class="flex-item">3</div>
<div class="flex-item">4</div>
<div class="flex-item">5</div>
<div class="flex-item">6</div>
<div class="flex-item">7</div>
</div>
</body>an inline-block element could do too, just adapt the display/behavior of the box and it’s content. the magic here comes from padding:50% 0; (100% vertical padding equals width of parent). see w3c about vertical margin and padding
[edit 07/2021]about centering the content inside that square for who ever needs this too (make the square itself a flex boxe too):
.flex-container {
padding: 0;
margin: 0;
list-style: none;
display: -webkit-box;
display: -moz-box;
display: -ms-flexbox;
display: -webkit-flex;
display: flex;
-webkit-flex-flow: row;
justify-content: space-around;
line-height:30px;
}
.flex-item {
background: tomato;
margin: 5px;
color: white;
font-weight: bold;
font-size: 1.5em;
text-align: center;
flex: 1 0 auto;
height:auto;
display:flex;
align-items:center;
justify-content:center;
}
.flex-item::before {
content:'';
padding-top:100%;
}<body>
<div class="flex-container">
<div class="flex-item">1</div>
<div class="flex-item">2</div>
<div class="flex-item">3</div>
<div class="flex-item">4</div>
<div class="flex-item">5</div>
<div class="flex-item">6</div>
<div class="flex-item">7</div>
</div>
<div class="flex-container">
<div class="flex-item">1</div>
<div class="flex-item">2</div>
<div class="flex-item">3</div>
<div class="flex-item">4</div>
<div class="flex-item">5</div>
<div class="flex-item">6</div>
<div class="flex-item">7</div>
</div>
<div class="flex-container">
<div class="flex-item">1</div>
<div class="flex-item">2</div>
<div class="flex-item">3</div>
<div class="flex-item">4</div>
<div class="flex-item">5</div>
<div class="flex-item">6</div>
<div class="flex-item">7</div>
</div>
<div class="flex-container">
<div class="flex-item">1</div>
<div class="flex-item">2</div>
<div class="flex-item">3</div>
<div class="flex-item">4</div>
<div class="flex-item">5</div>
<div class="flex-item">6</div>
<div class="flex-item">7</div>
</div>
<div class="flex-container">
<div class="flex-item">1</div>
<div class="flex-item">2</div>
<div class="flex-item">3</div>
<div class="flex-item">4</div>
<div class="flex-item">5</div>
<div class="flex-item">6</div>
<div class="flex-item">7</div>
</div>
<div class="flex-container">
<div class="flex-item">1</div>
<div class="flex-item">2</div>
<div class="flex-item">3</div>
<div class="flex-item">4</div>
<div class="flex-item">5</div>
<div class="flex-item">6</div>
<div class="flex-item">7</div>
</div>
<div class="flex-container">
<div class="flex-item">1</div>
<div class="flex-item">2</div>
<div class="flex-item">3</div>
<div class="flex-item">4</div>
<div class="flex-item">5</div>
<div class="flex-item">6</div>
<div class="flex-item">7</div>
</div>
</body>You may also think interesting : Responsive grid of squares within a responsive grid of squares & 4×4 grid of squares that scale up to a maximum width if this answer do not fully suits your square needs 😉