Tom Clancy's The Division

As was customary in almost all games that came out earlier, each level requires you to earn more and more experience to unlock the next one.
In total, to reach level 30 we will need to earn 4,023,750 experience units.

Story missions make the greatest contribution to leveling up experience in PvE mode. However, each mission has its own level of difficulty and its own cost of experience units.
Entry-level missions provide much less experience than final-level missions.

To make the most of the experience gained from these missions, you need to remember the following rule:
1. Each mission has its own recommended difficulty level. It can be viewed on the map when a particular mission becomes available.
2. The level is indicated as minimum and maximum recommended. We need to complete the next mission as soon as we get a level one unit higher than the minimum recommended. This way it will be quite comfortable to complete the mission and we will not spend an insane amount of time on it, and the experience gained for completing it will raise our level by at least half of the pumping scale.
Why should you start immediately after achieving the level I recommend?
The thing is that boxes with loot on missions come across in large quantities. These boxes drop weapons and equipment of your level or one rank higher. If we start completing a mission when our level is more than half full, then upon completion of the mission the drops from the boxes will partially lose their relevance.

To reach the level recommended for missions, it is necessary to complete side tasks in the location in which this mission is located, or in locations with a lower level.
An exception may be the final locations, which do not (possibly yet) have their own missions. There you can calmly prepare for the final missions. However, if you did a complete clearing of each of the locations as you progressed. This won't be required.

At the time of completion of the story, provided that all underlying locations are 100% cleared and boosts for pumping are timely unlocked at the operational base, we get level 29 upon completion of the last story mission and we have three zones left that are not affected by us at all.

When choosing the order in which to open a base, be sure to look at the talents for those perks that provide an increased bonus to leveling up experience. We are trying to open them as early as possible.

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A small addition that is very important to remember.
Regardless of the level of the zone in which we are located, the level of the mobs is adjusted to the level of the most pumped player from your party, if the level of the zone is lower than his level.
This applies to both free play on the map and missions.
Let's say we go through one of the first missions that recommends a 4th level player. The mobs in it are level 4-5. When a player of say level 20 joins this mission, mobs for all players will begin to have a level of 20 or higher!
Playing against mobs that have a level higher than yours by more than 2 units is very difficult and most players will not like it.

It is for this reason that there is no point in asking your comrades to “pull” you through the last mission in order to get, say, level 20 right away. All mobs and enemies will simply become your level and the pumping speed will not increase in any way. Vice versa. It will even become a little slower. Because one level is not enough. The level itself does not give anything. It simply allows you to equip items of the appropriate level, but you will need to find them yourself, because... drop - random.
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How to calculate how much experience the next level will require?
Everything is simple according to the formula below!

1500+(300x(current level))+(450x(current level) squared)

Example. How much experience do you need to earn to get level 11 from level 10?
1500+(300x10)+(450x10x10)=49, 500