老生常谈CSS中的长度单位

前面的话

本文分为绝对长度单位和相对长度单位来介绍CSS中的长度单位的主要知识

绝对长度单位

绝对长度单位代表一个物理测量

像素px(pixels)
 
在web上，像素px是典型的度量单位，很多其他长度单位直接映射成像素。最终，他们被按照像素处理

英寸in(inches)
 
1in = 2.54cm = 96px

厘米cm(centimeters)
 
1cm = 10mm = 96px/2.54 = 37.8px

毫米mm(millimeters)
 
1mm = 0.1cm = 3.78px

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

点pt(points)
 
1pt = 1/72in = =0.0139in = 1/72*2.54cm = 1/72*96px = 1.33px

派卡pc(picas)
 
1pc = 12pt = 1/6in = 1/6*96px = 16px

字体相关相对长度单位

em、ex、ch、rem是字体相关的相对长度单位

em
 
em表示元素的font-size属性的计算值，如果用于font-size属性本身，相对于父元素的font-size；若用于其他属性，相对于本身元素的font-size

CSS Code 复制内容到剪贴板

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

XML/HTML Code 复制内容到剪贴板

<div class="box">
    <div class="in">测试文字div>
div>

rem
 
rem是相对于根元素html的font-size属性的计算值

兼容性: IE8-不支持

CSS Code 复制内容到剪贴板

  </span></span></li> 
    <li><span></span><span class="comment">/* 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 */</span><span>  </span></li> 
    <li class="alt"><span>html{</span><span class="keyword">font-size</span><span>: 2rem;}  </span></li> 
    <li><span></span><span class="comment">/* 2*32=64px */</span><span>  </span></li> 
    <li class="alt"><span>.box{</span><span class="keyword">font-size</span><span>: 2rem;}  </span></li> 
    <li><span>.in{  </span></li> 
    <li class="alt"><span>    </span><span class="comment">/* 1*32=32px */</span><span>  </span></li> 
    <li><span>    </span><span class="keyword">font-size</span><span>: 1rem;  </span></li> 
    <li class="alt"><span>    </span><span class="comment">/* 1*32=32px */</span><span>  </span></li> 
    <li><span>    </span><span class="keyword">border-left</span><span>: 1rem </span><span class="string">solid</span><span> </span><span class="string">black</span><span>;  </span></li> 
    <li class="alt"><span>    </span><span class="comment">/* 4*32=128px */</span><span>  </span></li> 
    <li><span>    </span><span class="keyword">height</span><span>: 4rem;  </span></li> 
    <li class="alt"><span>    </span><span class="comment">/* 6*32=192px */</span><span>  </span></li> 
    <li><span>    </span><span class="keyword">width</span><span>: 6rem;  </span></li> 
    <li class="alt"><span>    </span><span class="keyword">background-color</span><span>: lightblue;  </span></li> 
    <li><span>}  </span></li> 
    <li class="alt"><span>

XML/HTML Code 复制内容到剪贴板

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

ex
 
ex是指所用字体中小写x的高度。但不同字体x的高度可能不同。实际上，很多浏览器取em值一半作为ex值

[注意]ex在实际中常用于微调

CSS Code 复制内容到剪贴板

  </span></span></li> 
    <li><span>.box{</span><span class="keyword">font-size</span><span>: </span><span class="string">20px</span><span>;}  </span></li> 
    <li class="alt"><span>.in{  </span></li> 
    <li><span>    </span><span class="keyword">font-size</span><span>: 1ex;  </span></li> 
    <li class="alt"><span>    </span><span class="keyword">border-left</span><span>: 1ex </span><span class="string">solid</span><span> </span><span class="string">black</span><span>;  </span></li> 
    <li><span>    </span><span class="keyword">height</span><span>: 10ex;  </span></li> 
    <li class="alt"><span>    </span><span class="keyword">width</span><span>: 20ex;  </span></li> 
    <li><span>    </span><span class="keyword">background-color</span><span>: lightblue;  </span></li> 
    <li class="alt"><span>}  </span></li> 
    <li><span>

XML/HTML Code 复制内容到剪贴板

<button>宋体button><button>微软雅黑button><button>arialbutton><button>serifbutton>
<div class="box">
    <div class="in" id="test">测试文字div>
div>

JavaScript Code 复制内容到剪贴板

  </span></span></li> 
    <li><span></span><span class="keyword">var</span><span> aBtns = document.getElementsByTagName(</span><span class="string">'button'</span><span>);  </span></li> 
    <li class="alt"><span></span><span class="keyword">for</span><span>(</span><span class="keyword">var</span><span> i = 0; i < aBtns.length; i++ ){  </span></li> 
    <li><span>    aBtns[i].onclick = </span><span class="keyword">function</span><span>(){  </span></li> 
    <li class="alt"><span>        test.style.fontFamily = </span><span class="keyword">this</span><span>.innerHTML;  </span></li> 
    <li><span>    }  </span></li> 
    <li class="alt"><span>}      </span></li> 
    <li><span>

ch
 
ch与ex类似，被定义为数字0的宽度。当无法确定数字0宽度时，取em值的一半作为ch值

兼容性: IE8-不支持

[注意]ch在实际中主要用于盲文排版

CSS Code 复制内容到剪贴板

  </span></span></li> 
   <li><span>.box{</span><span class="keyword">font-size</span><span>: </span><span class="string">20px</span><span>;}  </span></li> 
   <li class="alt"><span>.in{  </span></li> 
   <li><span>    </span><span class="keyword">font-size</span><span>: 1ch;  </span></li> 
   <li class="alt"><span>    </span><span class="keyword">border-left</span><span>: 1ch </span><span class="string">solid</span><span> </span><span class="string">black</span><span>;  </span></li> 
   <li><span>    </span><span class="keyword">height</span><span>: 10ch;  </span></li> 
   <li class="alt"><span>    </span><span class="keyword">width</span><span>: 20ch;  </span></li> 
   <li><span>    </span><span class="keyword">background-color</span><span>: lightblue;  </span></li> 
   <li class="alt"><span>}  </span></li> 
   <li><span>

XML/HTML Code 复制内容到剪贴板

<button>宋体button><button>微软雅黑button><button>arialbutton><button>serifbutton>
<div class="box">
    <div class="in" id="test">测试文字div>
div>

JavaScript Code 复制内容到剪贴板

  </span></span></li> 
    <li><span></span><span class="keyword">var</span><span> aBtns = document.getElementsByTagName(</span><span class="string">'button'</span><span>);  </span></li> 
    <li class="alt"><span></span><span class="keyword">for</span><span>(</span><span class="keyword">var</span><span> i = 0; i < aBtns.length; i++ ){  </span></li> 
    <li><span>    aBtns[i].onclick = </span><span class="keyword">function</span><span>(){  </span></li> 
    <li class="alt"><span>        test.style.fontFamily = </span><span class="keyword">this</span><span>.innerHTML;  </span></li> 
    <li><span>    }  </span></li> 
    <li class="alt"><span>}      </span></li> 
    <li><span>

视口相关相对长度单位

视口相关的长度值相对于初始包含块的大小。当初始包含块的宽高变化时，他们都会相应地缩放。然而，当根元素的overflow值为auto时，任何滚动条会假定不存在。

关于视口相关的单位有vh、vw、vmin、vmax4个单位

兼容性:IE8-不支持，IOS7.1-不支持，android4.3-不支持(对于vmax，所有IE浏览器都不支持)

[注意]黑莓错误的将其相对于视觉视口来计算；而safari奇怪地相对于html元素来计算，如果html中增加了内容，这两个单位也会发生变化

vh
 
布局视口高度的 1/100

vw
 
布局视口宽度的 1/100

CSS Code 复制内容到剪贴板

  </span></span></li> 
    <li><span>body{</span><span class="keyword">margin</span><span>: 0;}  </span></li> 
    <li class="alt"><span>.box{  </span></li> 
    <li><span>    </span><span class="comment">/* 实现与屏幕等高的效果 */</span><span>  </span></li> 
    <li class="alt"><span>    </span><span class="keyword">height</span><span>: 100vh;  </span></li> 
    <li><span>    </span><span class="keyword">background-color</span><span>: lightblue;  </span></li> 
    <li class="alt"><span>}      </span></li> 
    <li><span>

XML/HTML Code 复制内容到剪贴板

<div class="box">div>

vmin
 
布局视口高度和宽度之间的最小值的 1/100

CSS Code 复制内容到剪贴板

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

vmax

布局视口高度和宽度之间的最大值的 1/100

CSS Code 复制内容到剪贴板

/*类似于cover效果*/
.box{
    height: 100vmax;
    width: 100vmax;
}

以上就是小编为大家带来的老生常谈CSS中的长度单位全部内容了，希望大家多多支持脚本之家~

原文地址：http://www.cnblogs.com/xiaohuochai/p/5485683.html