<div class="popup-calc">
<div class="container">
<form action="" class="content">
<!-- Контент -->
</form>
</div>
</div>
.popup-calc {
width: 100%;
height: 100%;
background-color: rgba(0,0,0,0.7);
position: fixed;
z-index: 100;
overflow-y: auto;
}
.popup-calc .container {
position: relative;
max-width: 1200px;
margin: 5% auto 0;
}