Why does the experience-to-next-level not change between some levels?

At every level from 1st to 11th, the experience needed to reach the next level (from the current level) increases every level. However, this pattern gets broken for most of the later levels:

• Going from 10th to 11th takes 21,000xp (= 85k - 64k), but 11th to 12th takes only 15,000xp! (= 100k - 85k).
• Both 13th and 14th level only take extra 20k experience each, i.e. the amount needed doesn't increase between those two levels.
• Levels 16 and 17 similarly only need 30k experience each, and 18 and 19 only need 40k experience each.

Why does the last half of the experience chart have some of these levels maintain the same experience costs, and even briefly decrease the experience cost for 12th level?

First, I don't believe that "why" is something that this community can answer; this was a decision of the designers and their reasons, to the extent that they have any, are a mystery.

Notwithstanding, your question is why the XP per level looks like this:

Steady growth until 11th level, then a sharp drop and not reaching the 10->11 level again until 14->15.

However, the XP values are only one side of the equation; the other is how much XP is gained per encounter (p.82 DMG). Ignoring modifiers, by combining these you get this chart:

Easy, Hard and Deadly encounters are approximately 2/3, 1.5 and 3 times a Medium encounter (presumably because of rounding off). Focusing just on the "Medium" encounters (which should be the bulk of encounters) it can be seen that you need 6 to reach levels 2 and 3, 12 to reach 4, approximately 15 for levels 5 through 10, 17 for level 11 and then about 9 to 10 for levels 12 to 20.

However, due to the strange way that XP budgets do not equate with XP awarded, you will only have this number of encounters if every encounter is with a single monster. If your encounters are typically with 3-6 creatures (most of mine are) then you will need twice as many encounters to get the same number of XP.

In this context, the jump at level 11 is only about 10% and then it falls to a much lower and approximately constant value.

If I were to speculate, and I will, I would guess that the design intent is to:

• Provide relatively rapid advancement through the early levels.
• Slow down this advancement in the mid-levels (4-11) to an approximately constant level of about 15 medium/10 hard encounters (noting that difficulty factors will make the actual number of encounters 2-3 times this).
• Provide more rapid advancement (about 1/3 quicker) for the levels 12-20.

This accelerates the PCs through the fragile early stages and provides rapid gratification, provides a long period of play in the mid-levels, suitable for the dungeon-grind and then move more quickly through the levels where nation and world shaking events may be happening.

• If you want to dig, you'll be able to find a tweet or article that has a d&d designer explain that the xp ammounts were based on a survey of campaign lengths. The drop from 10 to 11 is to help games get past level 10. – GMNoob May 21 '15 at 15:23
• @GMNoob Here's the tweet: "Level 10 - 11 XP: It's by design. Data shows campaigns stop at 10, we're trying to speed up 10+ a bit so groups can reach 20 in a campaign" (Mike Mearls) So the the dip and the flattening both serve the purpose of accelerating the final climb to 20. – SevenSidedDie May 21 '15 at 18:44
• @GMNoob I think if Dale's not going to incorporate your suggestion, it'd be great for the site to have the "word of developers" answer as an option to vote on. I know highly-upvoted comments like yours and SSD's follow-on aren't likely to hit the trashbin, but I'd love to see your info in a "real" answer rather than just in comments. – nitsua60 Nov 22 '16 at 5:13
• Dale, these graphs might be useful in answering this recent question if you're interested: Is there a mathematical formula to determine how much XP is needed per level? – V2Blast Sep 2 '18 at 4:15

According to the game designer Mike Mearls the change in xp needed to the next level after 10 dips on purpose. The goal is that since according to their research most previous campaigns tended to stop at level 10, they are tried to make the hump of level 10 easier to get over.

Level 10 - 11 XP: It's by design. Data shows campaigns stop at 10, we're trying to speed up 10+ a bit so groups can reach 20 in a campaign" (Mike Mearls)

The simplest way to answer this is to look at the total XP, not the XP to next level.

From each level, the XP required to reach the next level looks like this:

(this graph borrowed from Dale M's answer)

This doesn't make a lot of sense, which is the source of the confusion. However, if you look at a character's total XP as they progress through the levels, it looks like this:

This nice smooth curve makes it much more obvious. The slight deviations from the curve are because each level has been rounded to the nearest multiple of 1000 (or 5000 at higher levels). That rounding is likely the source of the strange progression in XP-to-next-level, and the fact that the curve flattens tends to a rough asymptote explains the flattening off of the increases in XP-to-next-level. From this graph, the XP required to level up is simply (next - current), and at that point the slight wobbles in the curve are made much more obvious.

I'd like to take a different direction from the other answers here - what happens at level 11 that might cause the designers to make this level more difficult to attain than the others (comparatively)?

At level 11, almost every class gets a transformative power. Level 11 is the level that casters get level 6 spells, which involve things like instantaneously teleporting the entire party around the campaign world. In my experience it's a really big change to the type of preparation that the DM has to perform. At level 11, the caster's cantrips get a real jump in effectiveness (their last jump was at level 6).

In 4th edition I believe levels 11+ were called the Paragon levels, when the character transcends normal mortal limits and becomes a legendary hero.

So (potentially) the designers made the jump from 10-11 be expensive XP-wise to encourage the DMs to do something interesting or epic when progressing the players through this stage, and also to attempt to keep the party together at level 10 for longer until they and the DM are ready for them to hit 11.

• I'm not really sure why you've got -1, but I'm hesitant to upvote because it seems more like speculation. On top of that, if your hypothesis is right, why isn't there a similar type of difference in level experience at levels 5 or 17th level? – Premier Bromanov Apr 13 '16 at 15:04