Key
Key
Common
+10% chance to open chests for free.

Key Megabonk – Explained

The Key's description says +10% chance to open chests for free. But the real in-game chance is actually closer to 9.1% when you have 1 key.
 
This is because the key uses hyperbolic scaling and does not scale linearly. If it scaled linearly you would have a 100% chance to open a free chest with 10 copies but that is NOT the case as it would be way too overpowered.
 
Instead the 10% you see in the description is used in a hyperbolic scaling formula as the raw input number. But when it has been run through the formula and done all the math, it comes out to approximately 9.1% with 1 key. And if you have 10 keys it would be a 50% chance. You can see more data points in the graph under "How it Stacks"
 
The chests that have a chance to become free are the ones you pay gold for. If you successfully open a chest for free it will show after opening a brown chest:
 Pasted image

How it Stacks

As mentioned, the key scales using a hyperbolic scaling formula. This means every key will contribute in going towards a 100% chance but each key will have diminishing returns. You will never reach a 100% chance to open a free chest regardless of how many keys you have.
 
if n = number of keys, then this formula can be used to describe the chance of getting a free chests from X amount of keys
 
 
$$ \text{chance}(n) = \frac{\text{chance}_\text{raw}(n)}{\text{chance}_\text{raw}(n) + 1} $$
 
 
If you plug this out on a chart, you can see this type of scaling:
 Pasted image 
Example table:
 
Keys (n)Raw x = 0.1nChance (decimal)Chance (%)
10.10.0909099.09%
20.20.16666716.67%
30.30.23076923.08%
50.50.33333333.33%
101.00.50000050.00%
151.50.60000060.00%
202.00.66666766.67%
303.00.75000075.00%
505.00.83333383.33%
10010.00.90909190.91%
20020.00.95238195.24%
You can never reach a 100% chance no matter how many keys you have. You can get very close, but it will require an absurd amount of keys.
 
With 500 keys you have a ~98.03%
With 1000 keys you have a ~99.01%
 
To get a 99.99% chance you need 99 990 keys
 
Even with 1 million keys you don't have a 100% chance but a ~99.999% chance

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