Code:
<form>
<input type="text" name="source" value="0">
<input type="text" name="copy" value="0">
</form>

<script type='text/javascript'> 
var source = document.getElementsByName('source')[0];
source.onkeyup = function () {
    var copy = this.form.elements.copy;
    copy.value = this.value;
}

</script>
The code above works like the follow.
If I enter "someNumber" in the box "source",
The "someNumber" will be simultaneously shown in the box "copy".
It works fine.

I like to make it like the follow.
If I enter "someNumber" in the box "meter",
The "someNumber*0.9144" will be simultaneously shown in the box "yard".
The trial code below doesn't work correctly, but it will show what I want.
Code:
trial code

<form>
<input type="text" name="meter" value="1">
<input type="text" name="yard" value="0.9144">
</form>

<script type='text/javascript'> 
var source = document.getElementsByName('meter')[0];
source.onkeyup = function () {
    var yard = this.form.elements.yard*0.9144;
    yard.value = this.value;
}
</script>