Detailed description of length units in CSS

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This article is divided into absolute length units and relative length units to introduce the main knowledge of length units in CSS

Absolute length unit

Absolute length unit represents a physical measurement

Pixel px (pixels)

On the web , the pixel px is a typical unit of measurement, and many other length units map directly to pixels. Finally, they are processed in pixels

inches

1cm = 10mm = 96px/2.54 = 37.8px

millimeters

1mm = 0.1cm = 3.78px

1/4mmq(quarter-millimeters) 1q = 1/4mm = 0.945px

Points(points)

1pt = 1/72in = =0.0139in = 1/72*2.54cm = 1/72*96px = 1.33px

PICAS(picas)

1pc = 12pt = 1/6in = 1/6*96px = 16px

Font-related relative length units

em, ex, ch, rem are font-related The relative length unit

em

em represents the calculated value of the font-size attribute of the element. If used for the font-size attribute itself, relative Relative to the font-size of the parent element; if used for other attributes, relative to the font-size of the own element

<style>  
.box{font-size: 20px;}  
.in{  
    /* 相对于父元素，所以2*2px=40px */
    font-size: 2em;  
    /* 相对于本身元素，所以5*40px=200px */
    height: 5em;  
    /* 10*40px=400px */
    width: 10em;  
    background-color: lightblue;  
}  
</style>

<p class="box">
    <p class="in">测试文字</p>
</p>

##rem

rem is the calculated value relative to the font-size attribute of the root element html

Compatibility: IE8-not supported

<style>  
/* 浏览器默认字体大小为16px，则2*16=32px，所以根元素字体大小为32px */
html{font-size: 2rem;}  
/* 2*32=64px */
.box{font-size: 2rem;}  
.in{  
    /* 1*32=32px */
    font-size: 1rem;  
    /* 1*32=32px */
    border-left: 1rem solid black;  
    /* 4*32=128px */
    height: 4rem;  
    /* 6*32=192px */
    width: 6rem;  
    background-color: lightblue;  
}  
</style>

<p class="box">
    <p class="in" id="test">测试文字</p>
</p>

ex

ex refers to the font used The height of the lowercase x. But the height of x may be different for different fonts. In fact, many browsers use half of the em value as the ex value

[Note]ex is often used for fine-tuning in practice

<style>  
.box{font-size: 20px;}  
.in{  
    font-size: 1ex;  
    border-left: 1ex solid black;  
    height: 10ex;  
    width: 20ex;  
    background-color: lightblue;  
}  
</style>

宋体微软雅黑arialserif
<p class="box">
    <p class="in" id="test">测试文字</p>
</p>

<script>  
var aBtns = document.getElementsByTagName(&#39;button&#39;);  
for(var i = 0; i < aBtns.length; i++ ){  
    aBtns[i].onclick = function(){  
        test.style.fontFamily = this.innerHTML;  
    }  
}      
</script>

ch

ch is similar to ex and is defined as the width of the number 0. When the width of the number 0 cannot be determined, half of the em value is used as the ch value

Compatibility: IE8-not supported[Note]ch is mainly used for Braille typesetting in practice

<style>  
.box{font-size: 20px;}  
.in{  
    font-size: 1ch;  
    border-left: 1ch solid black;  
    height: 10ch;  
    width: 20ch;  
    background-color: lightblue;  
}  
</style>

宋体微软雅黑arialserif
<p class="box">
    <p class="in" id="test">测试文字</p>
</p>

<script>  
var aBtns = document.getElementsByTagName(&#39;button&#39;);  
for(var i = 0; i < aBtns.length; i++ ){  
    aBtns[i].onclick = function(){  
        test.style.fontFamily = this.innerHTML;  
    }  
}      
</script>

##Viewport related relative length unit

The viewport-related length value is relative to the size of the initial containing block. When the width and height of the initial containing block change, they will scale accordingly. However, when the root element's overflow value is auto, any scrollbars are assumed not to exist.

Regarding the viewport-related units, there are four units: vh, vw, vmin, and vmax. Compatibility: IE8-not supported, IOS7.1-not supported, android4.3-not supported ( For vmax, all IE browsers do not support it)[Note] BlackBerry incorrectly calculates it relative to the visual viewport; and Safari strangely calculates it relative to the html element, if content is added to the html , these two units will also change

vh

1/100 of the layout viewport height

vw

1/100 of the layout viewport width

<style>  
body{margin: 0;}  
.box{  
    /* 实现与屏幕等高的效果 */
    height: 100vh;  
    background-color: lightblue;  
}      
</style>

rrree

##vmin

1/100 of the minimum value between layout viewport height and width

<p class="box"></p>

vmax

1/100 of the maximum value between layout viewport height and width

/*类似于contain效果*/
.box{  
    height: 100vmin;  
    width: 100vmin;  
}

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