Difficulty Levels & Awards Update (January 2026)

[euler]

During the summer break last year we began exploring a new system to replace the difficulty metrics, which we felt suffered from two problems: heavy reliance on the fastest solvers to obtain the measure of a problem's difficulty, which meant the metric was skewed by more experienced problem solvers, and its static nature meant that it was unable to adapt to the evolution of programming languages; that is, not all X% difficulty problems are equal.

After arriving at a system which we felt had the capability of properly representing the entire Project Euler community, and running the system against test data, we feel that we have been able to develop a system that is going to provide a far more meaningful measure of problem difficulty. But please note, as the new system tracks encounters between members and the problems they solve to help calibrate the difficulty metric for each problem, our privacy policy has been updated to reflect these changes.

Instead of using difficulty percentages, the problems are now given a difficulty level, similar to your own level of progress. Essentially, the entire problem set is ranked by this new metric and then grouped into blocks of twenty-five to create the difficulty levels from level 0 upwards. The Archives table will colour code the levels from green to a light red to show you which problems are most suitable for you to tackle at your current level. This is not something that will work for everyone, but we hope it helps to guide most members on which problems to tackle and which ones to currently avoid.

It is worth noting that, due to the dynamic nature of the new system, which continues to learn the difficulty of each problem, the difficulty level for some problems may change slightly. The good news is that observing the data during this test period indicates that problems quickly find their place and many are now very well established. The challenge will be problem visibility. That is, those problems "in the middle" that do not receive as much attention as others. Over time as they are tackled they may rise or fall, pushing other more established problems up or down respectively. But we felt that these shifts would be limited and it was a small price to pay for a system that was continuously providing a measure with increasing accuracy.

To assist differentiating those with a stable rating versus those still in a state of less certainty, you will see a small coloured bar below the difficulty level for each problem. Green indicates a high confidence in the difficulty level, orange indicates that it is probably in the right place but may shift slightly, and grey indicates less certainty.

In addition to the new difficulty system, the awards have undergone some changes. Upon review we decided that a primary objective is that each award should be reasonably achievable. So you will notice that a number of awards have been removed, like Gold Medal. In the case of this award, we felt that earning the top spot in any fastest solver table was reward in itself. We know that some of these decisions might frustrate members who have worked hard to earn awards that have now been removed, but we hope you agree that the ultimate purpose of awards is to support and encourage all of our members as they work through the problems.

However, with that potentially disappointing news, there is some really good news. All awards are now permanent. To accommodate this, you will see this has been reflected in the slight changes to criteria for some of the existing awards. For example, Master of Archives now requires you to solve fifty problems in ten grids. Still challenging, but achievable.

There are a few other changes you will notice. The Problems tab on your Progress page now uses a more vibrant coloured heat map to show the levels. We have even provided a switch that lets you turn on/off the entire heat map.

As you can imagine, these many changes will undoubtedly lead to bugs. So if you notice any issues, please report them here.

We really hope that you appreciate the considerable time and effort that the team have put into these changes to allow Project Euler to continue providing an enjoyable place for everyone to solve a diverse collection of high quality, interesting problems.

[SAG145]

At first glance, the change looks good, and the hard work put into it is greatly appreciated! Regarding bugs/fixes:
1. The link for each award is broken.
2. If I understand correctly, for the "Dating Game" award you need to solve a problem on each of the dates of the month (1-31). I would recommend changing the description because I think it's a bit confusing.

[Swistakk]

Dear euler and the rest of the team, I appreciate the hard work that you put into maintaining and developing Project Euler, but I had to admit that I really dislike pretty much everything about this update. I hope what I will write is a constructive criticism showing a different view on the features

1) Awards update
While I am aware that solving problems is the main purpose of solving problems, I really liked the awards gamification that came with it. I found it very nice that awards showcased a big spectrum of difficulty catering to various types of solvers and meshed some silly ones with actual challenges. Now basically all meaningful awards were removed and I don't really see the rationale behind it. The awards for the more casual solvers are still there and what removing harder awards does is that it's easier for casual solvers to max them out or get a higher completion percentage, which I don't think is a particularly important property, while at the same time being a serious hit to the more dedicated part of your userbase. Sure, the harder awards were achieved by a small percentage of users, but these are the most dedicated users for which these awards mean the most and which worked the most for them (and their percentage becomes much bigger when you don't compare it to >10^6 users that solved #1, but to the number of users that even look at the recent problems). Similarly, only a tiny fraction of people would be able to participate in the Olympic Games, but that doesn't ruin the purpose of organizing them. Achieving progressively harder goals is a serious motivator and that mechanism can be seen not only on Project Euler, but also for example on Steam achievements, where I have achieved multiple gaming achievements, which sounded very much out of reach initially, and a similar thing was happening here. Last 10/25 problems awards were certainly not "inachievable" and there are multiple users which admit that they motivated them to push for harder problems which they wouldn't have solved otherwise. It seems that the Gold Medal award has some opponents (though I personally hate to see it go), but Perfection, D for Dedication, last 10/25 problems solved, TOP10, Mount Euler were all very good awards (Mount Euler still exists, but became pretty much meaningless)

2) Difficulty color coding
The color coding was already there, but it wasn't that extreme, so it's mostly just a visual change rather than something substantial. "Progress" was probably my most visited subpage of PE, because I found it satisfying to look at, both as it was aesthetically pleasing and also as an indicator of my progress. But now it became really hideous. When grouped by difficulty (i.e the big 39x25 grid below) it looks fine, but when effectively randomly shuffled with so many contrasting colors being close to each other, it really hurts to look at it. If you don't want to fully revert back to the old color scheme, please at least make it possible as an option. The new scheme may be more informative, but at least for me it's a way too big price to pay for the dramatic aesthetics loss.
Also, the chosen gradient is somewhat questionable. Imho red is the color that screams "the hardest" and it's weird to see yellow as the choice for the top of the difficulty. Rainbow gradient seems more intuitive.
And the pinned problems (displayed in red) are now blending in as ~28 lvl problems (the font is technically white instead of black, but it's hardly noticeable)

3) Difficulty recalibration
To be honest, I thought that the old difficulty algorithm was very good and didn't see a reason for changing it. For sure, there were many problems where it didn't perfectly align with my personal rating, but then it was more of a personal intricacy and the actual data and community consensus were more in line with the assigned difficulty. I didn't stare at the new difficulties enough to confidently claim if they are more or less accurate, but the biggest issue with the old ones were the inflated difficulties of old problems (200s and especially 100s) and that update definitely haven't solved that issue seeing how 177 shot up to the second highest difficulty bracket, even though in my opinion it was definitely the most overrated problem on the whole website already at 80%. People are naturally resistant to changes in things they are used to and completely reworking difficulties is a very invasive update, which I don't see the logic behind and whatever the gain is, in my opinion is not worth paying the high price of getting rid what people got used to. And tbh I liked the feature that difficulties are finalized at some point and it's a specific measure one might refer to and associate with a problem. Now, even when no real recalibration will happen, the levels will inevitably change as more and more problems are released as the levels are theoretically uncapped.
Also, as I understand it, there are some fallacies in the logic and Amoebas in a 3D Grid is a good example of that. It was significantly dragged down in the difficulty measure (was the hardest problem on the site before the update, but now is rated as 15th). It received many solves during the years not because it is easy, but because it became famous and was attracting a lot of attention.
As another curious example, universalset mentioned #700 (Eulercoin) as an example of a problem that is rated more accurately now as it was underrated at 5%, but is rated higher in the new system (Level 20), but to me it seems that the correction severely overshot the right place. Right now it has 4141 solvers and stands in the difficulty grid between 656 and 736 that are at 279 and 260 solvers respectively. I get that, similarly as Amoebas it might have attracted more attention for being at the other end of the difficulty spectrum, but I wouldn't expect that effect to be that big. For some comparisons, 686 was 5%, got 4132 solves -> Level 3, 692 was 10%, 1607 solves -> Level 2, 751 was 5%, got 2741 solves -> Level 3. I get that there may be some other factors that are not directly visible through these numbers, but Level 20 for 700 seems very questionable.

4) Archives page
Could you please bring back the view of all unsolved problems sorted by difficulty? Other than Progress, that was my other most visited subpage, but in the current view, what I was seeing on one page, is now split over 15 different subpages, making it much less convenient for me to use.

[i707107]

If the awards are permanent, does it apply to the "Ultimous Decimator"? I have seen it come and go, Now, I have the award, but if I don't solve an entire row that recently entered the archive, does the reward go away or stay?

[hacatu]

This is a meme I posted in the discord in May 2024, right before getting "State of the Art" for the first time

put it back please

I really like the new difficulty system and the visual design. There are some problems which I found easier or harder, but that's subjective. 177 is definitely an issue though...

The new awards icons are very nice, and I really do like the new progress page. The progress bars for each grid are nice

Minor Issues:
Options to change/disable the color scheme would be nice

Sort by difficulty should still show 50 problems per page

The new awards "Dating Game", "Daily Puzzle", "Two halves make a whole" and the changed award "Master of Archives" (renamed and modified "Archivist") do not appear on Statistics > Awards, and none of the Statistics > Awards pages actually work, although this might be intentional from what I hear

"Show only active members" is missing on the statistics page

It's a bit difficult to parse out pinned problems and comparison on a friend's page (this UI could do with a change to highlight differences in solves better)

However, I am sad about the removal of some awards.
Good (or at least understandable) Award Removals:

• "Master of Archives" (old version), "Perfection", and "High Flier": these removals are good because these were redundant. Every level is already an achievement, and Perfection is the biggest accomplishment of all that almost no one can reach
• "Baby Steps": fair enough, this was quite easy
• "Three Strikes", "Perfect Game", "Ternary Triumph", "Lucky Luke", "Square Triangle": these removals are mostly good/understandable because they just required solving specific problems. "Ternary Triumph" in particular required 729, which is harder than some level 38 problems to me, and makes what looks like a beginner friendly achievement very tough. I was actually trying to get "Perfect Game" as a long term goal because it's surprisingly tough (for perspective, if I had focused only on solving diagonal problems, I would be the lowest solve count user with the award)
• "Gold Medal": I cheered when I saw this was removed

Removals I don't like

• "D for Dedication": this was already made easier when it was changed from "solve the first 5 grids" to "solve any 5 grids". Obviously, this is an extremely difficult achievement, but it gives users with 400+ solves something to aim for. I solved the first 100 problems in antiquity, the first 200 with 163 in August 2022, and the first 300 with 262 in July 2025. The rest are much harder, but I'm sure I'll get the 800s and 300s/600s eventually
• (be a top 10 fastest solver): My fastest solve so far was 13th on 966 (also my record for most wrong answers submitted ...). To me, Project Euler is about improving myself, so I don't focus on leaderboards. This award might not be in the spirit, and it might be too difficult, but having no leaderboard award above top 100 is a bit disappointing
• "Ten out of ten" and "State of the Art": PUT IT BACK PLEASE!! These awards, along with the addition of bonus problems, increased my motivation SO MUCH. 25/25 is HARD, but you have months to do it, and so it is a great long term goal. I've gotten 25/25 twice. With 866-890, I learned a lot and would probably not have solved 870, 876, or 880 otherwise. 913-937 was easier, but I still struggled and learned a lot on 923, 930, and 936. I think if there had been like "solve the most recent 100 problems", that would be too hard and too unfair with increasing difficulty over time. But 25 is still small enough that runs with only 2-3 hard problems come up and it's a good goal to work towards. I really hope these awards come back

Also, I think I'm still forgetting one award (probably either "solve (some) square numbered problems" or "solve (some small number of total/consecutive) problems"), and "Conquering Difficulties" and "Mount Euler" were not properly adjusted for the new difficulty system.

Finally, I've always thought that one of the biggest issues for Project Euler is being too intimidating for new users. Does removing hard awards help make the site more welcoming? Perhaps, but it's definitely not the way to do so. Ideally imo, there should be awards at various difficulty levels to motivate people at different levels of progress. It makes sense for there to be more easy awards, since once you've solved over 2 or 3 hundred problems, obviously you're inherently motivated by the journey. But it's still nice to have markers along the trail

[avirukt]

Long time solver, first time poster. Thanks so much to the team, <3 PE!

Must agree that the new cmap for the grid is a bit loud. Made a tampermonkey script to approximate the old one at https://gist.github.com/avirukt/2370881 ... 8181e8dea9 (at least until the next update)

[Vinny360]

At first glance, the change looks good, and the hard work put into it is greatly appreciated! Regarding bugs/fixes:
1. The link for each award is broken.
2. If I understand correctly, for the "Dating Game" award you need to solve a problem on each of the dates of the month (1-31). I would recommend changing the description because I think it's a bit confusing.

The link for the levels are broken too.

[SAG145]

Tag search doesn't work:

[euler]

Sadly people in the UK and most of Europe can't see imgur images.

Tag searching seems to be working for me. Can you describe what is happening? Are you sure you've not turned on any filters?

[TitanCode]

Eulercoin (problem 700) moved all the way from 5% to level 20. Thank you. I feel better now

[TitanCode]

Re Mount Euler:

Has it become too easy?

I suggest making a “Foothill Euler” and “Mount Euler”. The Foothill-version could be the current or maybe moved a step down. And the really challenging version could be similar but moved a few steps up.

Or Maybe “Mount Fermat” and “Mount Euler”?

I have solved 180 problems and went from only having finished on step on Mount Euler to more than two-thirds of the way finished. Seems too easy now.

[SAG145]

Tag searching seems to be working for me. Can you describe what is happening? Are you sure you've not turned on any filters?

Now everything seems to be working for me too. In the image you can see a search for the tag "combinatorics" and it says that only one problem was found.
I'm sure I didn't add any filters to the search, but I may have caused it in some other way.
Has this happened to anyone else?

[flobacus]

On net I like the update! I agree with others that the harder awards were a big motivator and it's sad to see them go, though I'm lucky in that there are still a few that will take me a while to achieve. I like the new difficulty system--I haven't looked closely, but so far it seems to align with my judgments. I especially felt like the old difficulties carried little meaning among the hardest problems, e.g. some 100% problems seemed way easier than others. Arithmetic Derivative and Sum of Sum of Divisors were substantially easier for me than Titanic Sets, but now the former two are ranked a bit lower, which is nice. I'll also echo the sentiment that it's sad to see Mount Euler made easier; I was having a lot of fun chipping away at it and was nowhere near done, but with the update I got it automatically. I'd love to see a harder version of it if possible.

[hacatu]

I think the highest (and maybe lowest) levels should be be merged, similar to how the max level is always one behind (number of problems)/25. Otherwise, there will be funny situations when only 1 problem is at the max level. Maybe you have already accounted for this and a new problem level only gets added when it would have at least say 12 problems?

[euler]

Now everything seems to be working for me too. In the image you can see a search for the tag "combinatorics" and it says that only one problem was found.
I'm sure I didn't add any filters to the search, but I may have caused it in some other way.
Has this happened to anyone else?

That's quite amusing. I search for "combinatorics" too. I think I know what had happened. Before I made some additional changes this morning, the new Level sorting was not only showing the levels in order of difficulty, but it was separating the levels on to separate pages: 0, 1, 2, and so on. In other words, it was actually filtering too. So if you were sorting by level and you used the tags, there may have been a limited number or no tags at that level. This morning, I changed the Level sort into what members would expect: sorting in order of difficulty with 50 problems per page, and added a new filter to do the job of separating levels onto separate pages.

[Vinny360]

I don't know why, but when I go to Statistics then Levels, the labels are partly sticking out of the boxes.

[SergioMartinezCid]

Hello, I detected a minor bug. After clicking on the ⓘ button in the top right corner of a math field, a popup with "Exploring expressions in Windows using NVDA or JAWS" appears. The bug is that after clicking this button, the links on the menus and on the bottom change color to pink (fixed after refreshing). I found these button on problems 65 and 66.

[euler]

That was a rare discovery! Nothing to do with our recent site update though, it's an issue with the latest version of MathJax (4.1.0), which we use. They introduced some amazing accessibility support tools that integrate with Windows. The issue will be fixed with the next release (4.1.1), which is currently being finalised, and we will update to it as soon as it becomes available.
https://github.com/mathjax/mathjax/issues/3503

[TitanCode]

Award wording not consistent: Hovering over Conquering Difficulties I see “at least one problem at twenty five difficulty levels” and Progress: 24 / 30

[hacatu]

Thank you for adding the color scheme options.

There is a minor bug where if you view a friend's progress page, it is locked to their color scheme.

Additionally, Bonus Problem Heegner is missing from progress > bonus and if I try to access its url directly. I also can't go to the forum via progress > posts on my page or griff's page.