Get current user level without using while loops?

In my messaging app, I have a level system that track’s a user’s activeness with XP. It doesn’t store the actual level, only the amount of XP they have. Here’s the function for calculating the level:

function calculateLevel(num) {
  var lv = 0;
  do {
    lv++;
  } while (num >= Math.floor(Math.pow(30, Math.sqrt(Math.sqrt(lv)))));
  var x = num-Math.floor(Math.pow(30, Math.sqrt(Math.sqrt(lv-1))));
  var left = Math.floor(Math.pow(30, Math.sqrt(Math.sqrt(lv))))-num;
  var thislv = x + left;
  return {
    level: lv,
    xp: x,
    left: left,
    next: Math.floor(Math.pow(30, Math.sqrt(Math.sqrt(lv)))),
    prev: Math.floor(Math.pow(30, Math.sqrt(Math.sqrt(lv-1)))),
    thislv: thislv+1
  };
}

The formula for calculating how much XP each level requires is 30^{√(√(level))}. However, I don’t want to use a while loop, because the app starts to slow down a bit after level 60. Is there a way to get the current level without having to use loops?