add-acf-data.php

//Get the Custom Post Type
// -1 means we get all the posts type.
<?php 

$things = get_posts(['post_type' => 'CPT','posts_per_page' => -1]);
//Start with 0 calories
$total = 0;
//Then Loop through all the posts in your CPT
Foreach($LotsofPosts as $singlePost): 
    //well grab the calories from each post
    $post_data = get_field('field_name',$singlePost->ID);
    $total = $total + $post_data
endforeach;

?>