JavaScript function that calculates allocated localStorage memory. Also applicable to get sessionStorage used memory.

utils.getLocalStorageUsedSpace.js

/**
 * Returns localStorage (or its particular key) allocated memory
 * in Megabytes (MB).
 * @param {string=} opt_key inside the storage to calculate
 * used space for.
 * @return {number} with 2 decimal points.
 */
function getLocalStorageUsedSpace(opt_key) {
  var allocatedMemory = 0,
      
      // It's also possible to get window.sessionStorage.
      STORAGE = window.localStorage,
      key;
  
  if (!STORAGE) {
    // Web storage is not supported by the browser,
    // returning 0, therefore.
    return allocatedMemory;
  }
  
  for (key in STORAGE) {
    if (STORAGE.hasOwnProperty(key) && (!opt_key || opt_key === key)) {
      allocatedMemory += (STORAGE[key].length * 2) / 1024 / 1024;
    }
  }
  
  return parseFloat(allocatedMemory.toFixed(2));
}

// Examples:
// Logging how much memory localStorage is using.
console.log( getLocalStorageUsedSpace() + 'MB' );

// Logging how much memory particular key inside the localStorage is using.
console.log( getLocalStorageUsedSpace('YOUR_KEY_NAME') + 'MB' );