Calculate total disk usage by MongoDB. storageSize is the actual used space, totalSize is the amount of disk space allocated.

mongo_disk_usage.js

db = db.getSiblingDB("admin");
dbs = db.runCommand({ "listDatabases": 1 }).databases;

storageSize = 0;
totalSize = 0;

dbs.forEach(function(database) { 
  db = db.getSiblingDB(database.name); 
  stats = db.stats();  
  
  storageSize += stats.storageSize; 
  totalSize += stats.fileSize; 
});