<nav> won't abide to body - html

Question

I'm really not sure that my title is correct english. Hehe. But fortunately I can explain my problem in pictures ;-)
My problem is this:
body {
font: calibri;
background-color: #2d2e29;
margin-left: 150px;
margin-right: 150px;
margin-bottom: 0px;
margin-top: 0px;
}
p {
margin: 0px;
}
nav {
position: fixed;
width: 100%;
height: 80px;
background-color: #FFFFFF;
opacity: 0.75;
top: 0;
left: 0;
margin-left: 150px;
margin-right: 150px;
}
nav ul {
float: right;
list-style-type: none;
padding: 0;
margin-top: 30px;
margin-right: 30%;
}
nav li {
display: inline;
}
nav a {
margin: 5px;
color: black;
}
nav a:hover {
color: #99cccc;
}
#Forside {
background-color: #3f5c93;
height: 800px;
}
#Mig {
background-color: #ccc2a6;
height: 800px;
}
#Faerdigheder {
background-color: #3f5c93;
height: 800px;
}
#Projekter {
background-color: #ccc2a6;
height: 800px;
text-align: center;
margin-left: auto;
margin-right: auto;
}
#Kontakt {
background-color: #3f5c93;
height: 800px;
text-align: center;
margin-left: auto;
margin-right: auto;
}
#logo {
height: 100%;
width: 200px;
background: url("Logo.png");
margin-top: 10px;
margin-left: 50px;
background-repeat: no-repeat;
position: absolute;
background-size: 200px;
}
<div id="all">
<nav>
<!-- navigations-element (menu) -->
<div id="logo"></div>
<ul>
<li>
<!-- internt link: relativ URL -->
<a class="nav" href="#Forside">
Forside
</a>
</li>
<li>
<!-- internt link: relativ URL -->
<a class="nav" href="#Mig">
Mig
</a>
</li>
<li>
<!-- internt link: relativ URL -->
<a class="nav" href="#Faerdigheder">
Færdigheder
</a>
</li>
<li>
<!-- internt link: relativ URL -->
<a class="nav" href="#Projekter">
Projekter
</a>
</li>
<li>
<!-- internt link: relativ URL -->
<a class="nav" href="#Kontakt">
Kontakt
</a>
</li>
</ul>
</nav>
<div id="Forside">
<br>
<br>
<br>
<br>
<br>Forside
</div>
<div id="Mig">
<br>
<br>
<br>
<br>
<br>
<div class="meleft">
<h1>Mig</h1>
<p class="textleft">Lots of words
</p>
</div>
<div class="meright">
<img src="sdp.png" alt="sdp" id="sdp1" />
</div>
</div>
<div id="Faerdigheder">
<br>
<br>
<br>
<br>
<br>
<div class="skillsleft">
<img src="Collage1.png" alt="Collage" id="Collage" />
</div>
<div class="skillsright">
<h1>Færdigheder</h1>
<p class="textright">Lots of words
</p>
</div>
</div>
</div>
I added my code to fiddle, so you can see the problem:
http://jsfiddle.net/9Lj6ck3L/
I hope you can understand the code even though some words is in Danish :-)

Answer 1

Given the HTML and CSS provided, set the width of the nav to calc(100% - 300px);
Demo Fiddle
Otherwise you are telling it to be the full viewport width, offset from the left by 150px which is why it seems to overspill to the right. Using calc you can say, "fine, stretch to the viewport width, but minus the margins"

Answer 2

Working fiddle - http://jsfiddle.net/9Lj6ck3L/11/
Changed width of your header bar.
nav {
position:fixed;
width:63.5%;
height:80px;
background-color:#FFFFFF;
opacity:0.75;
top:0;
left:0;
margin-left:150px;
margin-right:150px;
}
Hope this helps.

Answer 3

Your problem is position:fixed. This element does not inherit anything from it's parent in terms of width.
Your best bet would be to wrap the nav in another <div> and make that position fixed. Then give the <nav> inside it the width that you want.
Move the <nav> outside of the #all div and put your margins on #all instead of the body.
Then give your <nav> css of box-sizing: content-box or box-sizing: padding-box to solve the width issue.