Fractional quantities or custom quantities by product category

// Define steps by category
function ccom_get_step_by_product_category( $product_id ) {

	// Categories in priority order
	if( has_term( 'wheels', 'product_cat', $product_id ) ) {
		return 2;
	}
	if( has_term( 'tires', 'product_cat', $product_id ) ) {
		return 2;
	}
	if( has_term( 'fabrics', 'product_cat', $product_id ) ) {
		return 0.25;
	}

	// Default to one
	return 1;
}

// Update min value to the quantity field
add_filter( 'woocommerce_quantity_input_min',
	function( $default, $product ) {

	$product_id = $product->get_id();

	return ccom_get_step_by_product_category( $product_id );

}, 10, 2 );

// Update step value to the quantity field
add_filter( 'woocommerce_quantity_input_step',
	function( $default, $product ) {

	$product_id = $product->get_id();

	return ccom_get_step_by_product_category( $product_id );

}, 10, 2 );

// Replace the WooCommerce filter that is validating the quantity
remove_filter( 'woocommerce_stock_amount', 'intval' );
add_filter( 'woocommerce_stock_amount', 'floatval' );