
This article has a calculator here.

Absorbative is an Invention perk that gives a 20% (22% on a level 20 item) chance of reducing damage by 5% per rank. It can be created in armour gizmos.
Absorbative effectively provides a 1% damage reduction per rank. However, Absorbative does not reduce hard typeless damage (typeless damage that is unaffected by defensive abilities). Additionally, Absorbative does not work in PvP combat.
Rank

Reduction per hit

Average damage reduction

1 
5% 
1% (1.1%)

2 
10% 
2% (2.2%)

3 
15% 
3% (3.3%)

4 
20% 
4% (4.4%)

 All numbers in parentheses refer to level 20 gear.
Calculations

 Notes
 A calculator for this is at the top of this page.
 Assumption :
 The enemy is dealing damage with anything that is not hard typeless.
 $R$ is the rank of the Absorbative perk.
 $p$ is the proc chance of Absorbative to activate (.2 normally, .22 if the Absorbative perk is on level 20 gear).
 $\textstyle \sum _{n=0}^{\infty }\left(1p\right)^{n}p=1$ when $0\leq p<1$ or $\left1p\right<1$. This represents summing over all possibilities where integer $n\in [0,\infty )$ represents the amount of hits prior to the proc of Absorbative. The probability of Absorbative proccing on hit ${n+1}$ is therefore $\left(1p\right)^{n}p$.
 $d_{n,i}^{(j)}$ is the $i^{th}$ random value uniformly sampled between the enemy's minimum hit and maximum hit. The $j$ describes different sets of hits. The sets are regenerated for every new value of $n$.
 $d_{n,i}^{(0)}$ has $n$ elements with integer $i\in [1,n]$. This is the damage taken before the perk procs.
 Clarification : This does mean that for $n=0$ that there are no elements in this set.
 $d_{n}^{(1)}$ is the damage taken from a single hit when Absorbative procs.
 The set of values in $d_{n}^{(j)}$ for integer $j\in [0,1]$ is the same in both the numerator and denominator of the ratios for any given $n$.
 Damage reduction
 The average ratio of damage taken, $r_{avg}$, from Absorbative to that of without Absorbative is
 $r_{avg}={\frac {\sum \limits _{n=0}^{\infty }\left(1p\right)^{n}p\left[\left(\sum \limits _{i=1}^{n}d_{n,i}^{(0)}\right)+\lfloor {\left(1.05\times R\right)\times d_{n}^{(1)}}\rfloor \right]}{\sum \limits _{n=0}^{\infty }\left(1p\right)^{n}p\left[\left(\sum \limits _{i=1}^{n}d_{n,i}^{(0)}\right)+d_{n}^{(1)}\right]}}$
 The average damage reduction is then, after some rearranging,
 $1r_{avg}={\frac {\sum \limits _{n=0}^{\infty }\left(1p\right)^{n}p\left[d_{n}^{(1)}\lfloor {\left(1.05\times R\right)\times d_{n}^{(1)}}\rfloor \right]}{\sum \limits _{n=0}^{\infty }\left(1p\right)^{n}p\left[\left(\sum \limits _{i=1}^{n}d_{n,i}^{(0)}\right)+d_{n}^{(1)}\right]}}$
 Simplifications
 This can be simplified if the assumption is that every value $d_{n,i}^{(0)}$ $\forall$ integer $i\in [1,n]$, $d_{n}^{(1)}$ $\forall$ integer $n\in [0,\infty )$ is taken to be the same $\left(d_{n,i}^{(0)}=d_{n}^{(1)}\rightarrow d\right)$. In this scenario, there is no random element and the above $r_{avg}$ is then only dependent on the proc chance. This simplification is increasingly accurate in the limit of large $d$. A table is provided at the top of the page.
Using these simplifications, the damage reduction reduces to
 $1r_{avg}={\frac {pR}{20}}$

Gizmo layout 
Possible perks 










The update history project is a workinprogress – not all updates to this topic may be covered below. See here for how to help out!
 patch 22 February 2016 (Update):
 The Absorbative perk no longer uses the 30 seconds of charge.


Abilities 

Damage/Accuracy modifying 

Damage reduction 

Adrenaline 

Prayer 

Additional items 

Experience 

Skilling 

Miscellaneous 

Ancient 
