Sum and count of custom field values

Thanks for your help guys. They didn't quit work exactly, but I managed to get the result I needed by doing the following

$reviewsNum = 0;
$reviewsCount = 0;  
$argsSchema = array( 'post_type' => 'reviews', 'posts_per_page' => -1 );
$loopSchema = new WP_Query( $argsSchema );
$count_reviews = wp_count_posts( 'reviews' )->publish;
while ( $loopSchema->have_posts() ) : $loopSchema->the_post();
    $reviewrating       =   get_post_meta(get_the_ID(), "_reviewrating", true);
    if (!empty($reviewrating)){
        $reviewsNum += $reviewrating;
    }
endwhile;
$averagerating = $reviewsNum / $count_reviews;

I did something similar where I added a custom field for the weight of a product. Here's my code:

function epcpt_total_tonnage(){


    $query_args = array(
            'post_type' => 'projects',
            'post_status' => 'publish',
            'order' => 'DESC'
        );


        $total_tonnage = 0;
        $query = new WP_Query($query_args);
        if ( $query->have_posts() ) {
            while( $query->have_posts() ) {
            $query->the_post();
            // do the processing for each post

            $total_tonnage = $total_tonnage + get_field( "tonnage", $post->ID, 123 );
            // $total_tonnage = $total_tonnage + get_post_meta( $post->ID, 'tonnage', true );
            }
        }
        echo '<p>Total output tonnage to date: ' . number_format_i18n( $total_tonnage ) . ' Tonnes</p>';
    }

You can change your post_type to 'reviews' and then amend your get_field() values to suit.

Something along the lines of the following should work. I used the get_field function because I noticed you tagged this post with "advanced-custom-fields".

function get_average_rating( $post_id ) {
    $rating_sum = 0;

    $reviews_of_post = get_posts( array(
        'post_type' => 'NAME_OF_YOUR_REVIEW_POST_TYPE',
        'posts_per_page' => -1,
        'meta_query' => array(
            array(
                'key' => 'NAME_OF_A_CUSTOM_FIELD_ON_THE_POST_TYPE_CONTAINING_RELATIONSHIP_TO_POST',
                'value' => $post_id,
                'compare' => '=',
            ),
        ),
    ) );

    if ( empty( $reviews_of_post ) ) {
        return 0; // If there are no reviews, we return 0.
    }

    foreach ( $reviews_of_post as $review ) {
        $rating_sum += get_field( 'NAME_OF_CUSTOM_FIELD_CONTAINING_RATING', 'post_' . $review->ID);
    }

    return $rating_sum / count( $reviews_of_post );
}