Button center positioning - css

Question

I have a problem with center positioning a button in a row in a table.I used width and margin but it don't work.Can you help me? Here's the code for that button(in css):
.subm {
position:relative;
width:130px;
margin-left:auto;
margin-right:auto;
background-image:url('background.bmp');
border:none;
color:white;
opacity:1;
height:25px;
outline:0 none;
box-shadow:1px 1px 2px black
}
.subm:hover {
background-image:none;
background-color:darkgray;
box-shadow:10px 10px 10px black
}
.subm:active {
color:black ;
font-weight:bold;
width:128px;
height:24px;
background-image:none;
background-color:dimgray;
}

Answer 1

simply try this
button{
height:20px;
width:100px;
position:relative;
top:50%;
left:50%;
}
for just horizontal alignment use either
button{
margin: 0 auto;
}
DEMO

Answer 2

To achieve this, I would use following code:
.subm {
position:absolute;
width:130px;
height:25px;
top: calc(50% - 13px); // 50% - 0.5 * height.
left: calc(50% - 65px); // 50% - 0.5 * width.
}
And for the parent:
position: relative;
You can see a working JSFiddle here

Answer 3

If you used width and margin: auto's and it's still not centering you may just need to add display: block to your .sumb class like so:
.subm {
display:block;
position:relative;
width:130px;
margin-left:auto;
margin-right:auto;
background-image:url('background.bmp');
border:none;
color:white;
opacity:1;
height:25px;
outline:0 none;
box-shadow:1px 1px 2px black
}
Demo: http://jsfiddle.net/re079odr/