Form button not centering

<form id="myform" class="contact-form" action="contact.php" method="post" >
                <div class="row">
                    
                    <div class="mymail">
                        <input type="text" name="name" value="" maxlength="100" placeholder="Name"/>
                    </div>
                    
                    <div class="mymail">
                        <input type="text" name="postal_code" value="" maxlength="10" placeholder="Postal Code"/>
                    </div>
                    <div class="mymail">
                        <input type="text" name="email_address" value="" maxlength="100" placeholder="E-Mail"/>
                    </div>
                    <div class="divider" style="height: 50px"></div>
                    <div class="col-md-12">                        
                        <input class="sub-btn p-btn-medium" type="submit"></input>
                    </div>
                </div>
            </form>

.sub-btn,
.p-btn {
    margin-right: auto;
    margin-left: auto;
	display: inline-block;
    font-size: 15px;
    line-height: 25px;
    letter-spacing: 0.5px;
    font-weight: 300;
    padding: 6px 25px;
    border: 1px solid #3c3c3c;
}