Fix a css menu for width

Difficult to make recommendations without having access to the page.

Guessing I would do this

#cssmenu ul,
#cssmenu li {
  list-style: none;
  margin: 0;
  padding: 0;
  box-sizing: border-box; /* <=== CHANGE THIS FROM content-box TO border-box */
}
#cssmenu ul li {
  float: left;
  min-height: 1px;
  vertical-align: middle;
  width: 12.5%;    /* <=== ADD THIS */
  text-align: center;    /* <=== ADD THIS */
}

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Changing the box-sizing means that you can set a width without having the padding / border affect how that fits on the page. So a width of 12.5% means that it will only take up 12.5% of the screen irrespective of borders and padding. This is useful as it allows us to set a size on the container and still specify a border and padding and not have to worry about those interfering with the width.

There are a few things you need to do to fix this.

1. Change the margin on the li to padding

#cssmenu > ul > li {
  padding: 0 10px;   /* <=== CHANGE TO THIS */
}

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2. Remove the height of the ul

#cssmenu > ul {
  border-bottom: 1px solid #252A30;
  border-top: 1px solid #252A30;
  -moz-box-shadow: inset 0 1px 0 #8799a9, 0 1px 1px rgba(0, 0, 0, 0.5);
  -webkit-box-shadow: inset 0 1px 0 #8799a9, 0 1px 1px rgba(0, 0, 0, 0.5);
  box-shadow: inset 0 1px 0 #8799a9, 0 1px 1px rgba(0, 0, 0, 0.5);
  background-image: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAABNCAIAAADo7ZnJAAAAA3NCSVQICAjb4U/gAAAAUUlEQVQYlXWPyRGAMAwDd1wwHVADJS+POERk4OVD1mGO8yq1wFIKLXHsJLDGH8wSou8q0bfGxplYcpaHRerG/J/zS/edLTnrjvDo7PHv1Nhy3lZMnHg0MO2JAAAAAElFTkSuQmCC);
  background-color: #566171;
  background-image: -webkit-gradient(linear, left top, left bottom, color-stop(0, #6e7d8f), color-stop(1, #404854));
  background-image: -webkit-linear-gradient(top, #6e7d8f, #404854);
  background-image: -moz-linear-gradient(top, #6e7d8f, #404854);
  background-image: -o-linear-gradient(top, #6e7d8f, #404854);
  background-image: linear-gradient(#6e7d8f, #404854);
  height: 27px; /* <=== REMOVE THIS === */
  padding: 15px 15px 15px 5px;
}

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3. change the height back to 12.5%

#cssmenu ul li {
	float: left;
	min-height: 1px;
	vertical-align: middle;
	width: 12.5%; /* <=== CHANGE TO THIS */
	text-align: center;
}

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4. Change box-sizing on ul and li to border box

#cssmenu ul, #cssmenu li {
	list-style: none;
	margin: 0;
	padding: 0;
	box-sizing: border-box; /* <=== CHANGE TO THIS */
}

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The problem with using 10.99% is that it won't always work. 12.5% is the right value - you just need to massage the rest of the CSS to make the 12.5% work.

The key things were that the margin on your li was outside the box so increasing the space used - changing to padding and the box-sizing to border-box put the padding inside the container.

You need to take into account the padding

#cssmenu ul ul {
	visibility: hidden;
	position: absolute;
	top: 100%;
	left: 0;
	z-index: 598;
	right: 0; /* <=== ADD THIS */
/*    width: 100%;  REMOVE THIS */
}

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Then the border-radius has one too many curved bits

#cssmenu ul .has-sub ul {
	-moz-border-radius: 0 3px 3px 3px;
	-webkit-border-radius: 0 3px 3px 3px;
	border-radius: 0 0 3px 3px;  /* <=== REMOVE THE TOP RIGHT (2nd) VALUE MAKE IT 0 */
        ...
}

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And give the <ul><ul> a margin to bring it in to match  the button

#cssmenu ul ul {
/*	margin-top: 0; REMOVE THIS */
	margin: 0 10px;
}

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Remove the width: 170px

#cssmenu ul ul {
	width: 170px;
}

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You didn't do the last one - add the margin: 0 10px;

#cssmenu ul ul {
	visibility: hidden;
	position: absolute;
	top: 100%;
	left: 0;
	z-index: 598;
	right: 0;
	margin: 0 10px; /* <== ADD THIS */
}

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Looks fine to me - try doing a cache refresh Shift + F5

You are welcome.