We have 2 simple tools to calculate what is the maximum muscle you can put on while naturally.
FFMI - Fat Free Mass Index
First, calculate your rough body fat. You can use this link: US Navy Body Fat Calculator. We find this is generally accurate for most people.
Then calculate your FFMI using this calculator: FFMI Calculator
What is the FFMI? A simple description would be that a study was done of elite level athletes who were self declared as natural or on steroids. What they found that consistently, those who were claimed to be natural hits a plateau at FFMI of 25, while those who self-declared as on steroids can go as high as 30 to even 40 on the FFMI. You can see more information here: FFMI of Natural / Steroids Bodybuilders
Based on your numbers, you can see how much more you can gain towards a natural FFMI. Of course, this does not mean those with FFMI below 25 is not on steroids, it just says that those with FFMI of above 26 is highly likely to be on steroids or have a good genetic disposition. Those who are above 29 are almost definitely on steroids.
We do not think that people who take steroids are bad. That is their body and their personal choice.
However, we think it is unfair if fitness personalities claim to be natural, and endorse supplements which would mislead the general public that with effort and supplements, their physique is achievable. Hence, you can use this calculation method to see if your fitness idols are on juice, or probably natural.
Steve Reeves FFMI = 25.7
What is your genetic potential and maximum body weight? The simplest formula for your weight would be (Your height in centimetres - 100) which would be your ideal bodyweight provided you are at 5-6% body fat shredded state. So if your height is 170cm, you should be able to get 70kg bodyweight with 5-6% body fat. If you are at 10% body fat, you can minus 96 or 98 instead of 100 as you have slightly more body fat on. For more information, you should see this, which is written by Martin Berkhan: Maximum Muscular Potential of Drug-Free Athletes