Style input element to fill remaining width of its container

Let's say I have an html snippet like this:

<div style="width:300px;">
    <label for="MyInput">label text</label>
    <input type="text" id="MyInput" />
</div>

This isn't my exact code, but the important thing is there's a label and a text input on the same line in a fixed-width container. How can I style the input to fill the remaining width of the container without wrapping and without knowing the size of the label?

Here is a simple and clean solution without using JavaScript or table layout hacks. It is similar to this answer: https://stackoverflow.com/questions/10244927/input-text-auto-width-filling-100-with-other-elements-floating

It is important to wrap the input field with a span which is display:block. Next thing is that the button has to come first and the the input field second.

Then you can float the button to the right and the input field fills the remaining space.

form {
    width: 500px;
    overflow: hidden;
    background-color: yellow;
}
input {
    width: 100%;
}
span {
    display: block;
    overflow: hidden;
    padding-right:10px;
}
button {
    float: right;
}

<form method="post">
     <button>Search</button>
     <span><input type="text" title="Search" /></span>
</form>

A simple fiddle: http://jsfiddle.net/v7YTT/90/

Update 1: If your website is targeted towards modern browsers only, I suggest using flexible boxes. Here you can see the current support.

Update 2: This even works with multiple buttons or other elements that share the full with with the input field. Here is an example.

as much as everyone hates tables for layout, they do help with stuff like this, either using explicit table tags or using display:table-cell

<div style="width:300px; display:table">
    <label for="MyInput" style="display:table-cell; width:1px">label&nbsp;text</label>
    <input type="text" id="MyInput" style="display:table-cell; width:100%" />
</div>

I suggest using Flexbox:

Be sure to add the proper vendor prefixes though!

form {
  width: 400px;
  border: 1px solid black;
  display: flex;
}

input {
  flex: 2;
}

input, label {
  margin: 5px;
}

<form method="post">
  <label for="myInput">Sample label</label>
  <input type="text" id="myInput" placeholder="Sample Input"/>
</form>

Please use flexbox for this. You have a container that is going to flex its children into a row. The first child takes its space as needed. The second one flexes to take all the remaining space:

<div style="display:flex;flex-direction:row">
    <label for="MyInput">label&nbsp;text</label>
    <input type="text" id="MyInput" style="flex:1" />
</div>

Easiest way to achieve this would be :

CSS :

label{ float: left; }

span
{
	display: block;
	overflow: hidden;
	padding-right: 5px;
	padding-left: 10px;
}
	   
span > input{ width: 100%; }

HTML :

<fieldset>
	<label>label</label><span><input type="text" /></span>
	<label>longer label</label><span><input type="text" /></span>
</fieldset>

Looks like : http://jsfiddle.net/JwfRX/

Very easy trick is using a CSS calc formula. All modern browsers, IE9, wide range of mobile browsers should support this.

<div style='white-space:nowrap'>
  <span style='display:inline-block;width:80px;font-weight:bold'>
    <label for='field1'>Field1</label>
  </span>
  <input id='field1' name='field1' type='text' value='Some text' size='30' style='width:calc(100% - 80px)' />
</div>

you can try this :

div#panel {
    border:solid;
    width:500px;
    height:300px;
}
div#content {
	height:90%;
	background-color:#1ea8d1; /*light blue*/
}
div#panel input {
	width:100%;
	height:10%;
	/*make input doesnt overflow inside div*/
	-webkit-box-sizing: border-box;
       -moz-box-sizing: border-box;
            box-sizing: border-box;
	/*make input doesnt overflow inside div*/
}

<div id="panel">
  <div id="content"></div>
  <input type="text" placeholder="write here..."/>
</div>

If you're using Bootstrap 4:

<form class="d-flex">
  <label for="myInput" class="align-items-center">Sample label</label>
  <input type="text" id="myInput" placeholder="Sample Input" class="flex-grow-1"/>
</form>

Better yet, use what's built into Bootstrap:

  <form>
    <div class="input-group">
      <div class="input-group-prepend">
        <label for="myInput" class="input-group-text">Default</label>
      </div>
      <input type="text" class="form-control" id="myInput">
    </div>
  </form>

https://jsfiddle.net/nap1ykbr/