woocommerce/templates/single-product/add-to-cart/simple.php

<?php
/**
 * Simple Product Add to Cart
 */

global $woocommerce, $product;

if ( $product->get_price() === '' ) return;
?>

<?php
	// Availability
	$availability = $product->get_availability();

	if ($availability['availability']) :
		echo apply_filters( 'woocommerce_stock_html', '<p class="stock '.$availability['class'].'">'.$availability['availability'].'</p>', $availability['availability'] );
    endif;
?>

<?php if ( $product->is_in_stock() ) : ?>

	<?php do_action('woocommerce_before_add_to_cart_form'); ?>

	<form action="<?php echo esc_url( $product->add_to_cart_url() ); ?>" class="cart" method="post" enctype='multipart/form-data'>

	 	<?php do_action('woocommerce_before_add_to_cart_button'); ?>

	 	<?php
	 		if ( ! $product->is_sold_individually() )
	 			woocommerce_quantity_input( array( 'min_value' => 1, 'max_value' => $product->backorders_allowed() ? '' : $product->get_stock_quantity() ) );
	 	?>

	 	<button type="submit" class="single_add_to_cart_button button alt"><?php echo apply_filters('single_add_to_cart_text', __('Add to cart', 'woocommerce'), $product->product_type); ?></button>

	 	<?php do_action('woocommerce_after_add_to_cart_button'); ?>

	</form>

	<?php do_action('woocommerce_after_add_to_cart_form'); ?>

<?php endif; ?>