Let me be clear about one thing to start...
There is no way to know with any degree of certainty exactly how much you'll need to retire. There are too many factors and variables that can and will change between now and whenever it is that you actually retire to answer that question with any certainty.
What you can do is use the information available to you right now and make educated guesses about your future to come up with some ballpark of what it might take for you to be able to retire comfortably and confidently.
First, you need to figure out what your current lifestyle costs to maintain. Start by adding up all of your fixed monthly expenses. Then add in what you typically spend each month over and above your fixed expenses in order to maintain the lifestyle you currently enjoy. At this point, you can take your non-recurring or one-off expenses (like vacations) from throughout the year, divide them by 12 and then add that number to your monthly total.
As an example, let's say your monthly total comes out to be $10,000. When you multiple that number by 12, you come up with an annual total of $120,000 in living expenses.
While it is highly possible you have expenses in your current total, such as your mortgage, that might go away before retirement, I'd leave them in there as they could be replaced by items in retirement like additional healthcare expenses.
You can now take your annual total expenses of $120,000 and divide by 0.04. You'll come up with $3,000,000. This is a rough estimate of what you'd approximately need to retire and maintain the lifestyle you currently enjoy.
Again, this is NOT an exact figure nor should it be relied on as such. Factors like your level of investment risk, the investment returns you experience, inflation, how early or late you retire, your specific life expectancy and any Social Security benefits or pensions can all change how much you might personally need to retire comfortably.
The best way to tackle this question head on is to figure out your personalized answer as soon as possible. Rules of thumb like the one I provided above work as a guide, but they are not a long-term solution.
If you're interested in a slightly more personalized answer to your retirement picture, I'm happy to provide you with free access to an online projection tool called Advizr Express. (Quick compliance note: Advizr Express is an outside service vendor's software, not my own.)
I hope you find Advizr Express helpful and when you're ready for someone to guide you through creating & maintaining a plan that works best for you and your specific lifestyle, I'd be happy to have a conversation.