Stole this from Battlelog - no credit for me.
SUMMARY/TL:DR VERSION:
Class Points (x2) + Vehicle Points (x2) + Awards (x1) + Awards (x1) = Double XP!
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When a double XP event is active, here is how it is calculated:
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During the course of the match, points are awarded normally. All of the double XP bonuses are awarded at the End of Round (EoR screen), after the scoreboard appears at the EoR screen, the points rundown that appears next breaks it down for you:
1.) CLASSES - First you'll see all of the classes you played, and the points towards each class. You'll notice that the values earned for the classes are actually DOUBLE what you got. So if you earned 2,500 points with Assault, the meter will fill up with a value of 5,000 points. This repeats for all classes you earned points with.
2.) VEHICLES - Identical to classes, the values earned in vehicles will be double what you actually earned. This repeats for all vehicles you earned points with.
3) AWARDS - Then the award processing follows, all awards are given at ORIGINAL VALUE (1X). This is because you cannot really "double" the award of a ribbon or medal, as those have set values. However...
4.) 2X MULTIPLER PERIOD - This will appear after your last award, and the total that shows under this category is equal to the SUM of all of your awards again. This is the double-dip for your awards score. (ex. if you got three, 200 point ribbons earlier, the 2X PERIOD bonus will be 600)
OTHER NOTES:
- If you have earned ADDITIONAL POINTS in the round, the value of them is doubled, and rolled into the "2X MULTIPLIER PERIOD" set.
- Because double XP is awarded at the end of round screen you will need to finish the round in order to receive the double XP bonus. If you quit early or disconnect, you will still earn any points or awards you got, but not the double XP bonus.
- Since the CLASS and VEHICLE XP you are given is simply a 2X value, that means you will indeed move through the CLASS/VEHICLE progressions at a quicker pace. (For unlocks and service stars)
- Ultimately at the end of the day, if you see a final score of 40,000 points, it means that quite simply, if a double XP event was not running, you'd have earned 20,000 points. Everything is doubled.
- Unfortunately, at the moment, MVP ribbons are awarded AFTER double XP has been applied, meaning if premium members are running a double XP event while non-premium members are not, it will give an advantage towards premium members. Hopefully this has been recognized and will be changed soon.
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Subject: Double XP explained 
