The most honest approach would be to look at the components driving net worth individually. Those would be savings rate, expected changes to income and expenses, and rates of return on assets.
With regard to the last one, I expect equities to return 10% before inflation (aka nominal) over a very long time. That implies a doubling every 7 years or so. You ignore inflation at your own peril however. Assuming a 3% inflation rate, the inflation adjusted return (aka real) drops to 7%. At that rate, it takes 10 years to double.
PS... Rule of 72 makes quick and dirty doubling calculations easy (reasonably accurate between 4% and 14% I think). E.g. 72/10%=7 years to double. My dad taught me this in high school. Thanks Dad!
Achieving a doubling in 3 years is awesome and congrats! I think you can look back though and see why this is complicated. Personal capital reports the sp500 rose 40% between Jan 2016 and now. Clearly you were very lucky with returns or you've been saving a good bit.
As your stache grows, the time to double will drift toward the expected rate of return rather than be dominated by savings rates as early on.
Or so I've been telling myself for at least 11 years, probably longer.
I've tracked net worth religiously since about 2003 (and sporadically before that). I did some math a while back starting from when I got married in Jan 1 2008 and we've doubled (nominally) 3 times in 11 years or once every 3.7 years starting from a base then of $200k. And if memory serves correct, I was doubling at about the same pace before getting married.
Every time it happens, I tell myself it can't continue like this. Until very recently, our income growth was around inflation. We had a bout of unemployment in the great recession. A self funded move supporting two properties to land a new job. A multi-year 40% cut to my salary when my business struggled during the great recession. We've never used leverage meaningfully. Our homes have averaged returns of about 4% nominal overall, just barely above inflation. The only things I can point to are increasing frugality (except offset by kiddo), side hustle, and a recent sale of my business which really didn't move the needle that much.
I don't think we could double net worth in 3-4 years again. But I've said that before.
Here's a big picture view of it using our situation. I bet we have an average income since marrying of around $150k gross (3x annual household marriage). Playing with an investment calculator, saving about $72k per year from a base of $200k and with nominal returns of 7%, we hit our target of $1.6m today. I think that sounds like a typical savings rate for us across the period but it's been pretty variable.
In my spreadsheet, I record a monthly narrative synopsis. E.g. sold old car, bought new, market sucked, increased retirement contribs, executed Roth conversion, etc. I think the story matters a lot. I'm sure if I looked at each doubling it'd look like this:
2008-2011 : got married, 1y unemployment for each of us (huge pay cut for me technically), bought new home and left old idle for a year then a cash flow negative renter to wait for real estate market to improve. Looked dire yet frugality saved the day and let us retain options and even a savings rate that was probably 30% throughout this.
2012-2014 : market recovering. Business recovering but profits bought out two partners, increasing my equity from 31% to 50% though I wasn't recording this asset as it was way too illiqud. Savings rate boosted to maybe 40-50%. had son. Renter in old condo throughout. Still cash flow negative.
2015-2018 : first intermittent boosts of income from my business. Wife quits stable job and starts own consulting business but total comp has decreased for her by 20-30% (damn you healthcare insurance). Great market returns. Exited condo but no net worth change event, just liquidation in 2015. First entrance to taxable investing. Bought new home in 2017. Sold old home in 2018. Small networth boost of about 5% because home appreciated slightly faster than expected. Used transaction to borrow back up to 80% to stay in market (first real leverage). Sold business and recorded 15% net worth bump in late 2018. Savings rates probably hovered around 50-60%.
Looking ahead, I just don't see how it'd be possible to do again. Were not selling my wife's business. Our home value is rising but we're not selling for a long time. Our savings rate (as a percent of income after tax) is now around 70%. If we stayed working, we'd be looking at around 6 years to double. I just can't see it occuring in 3-4 yrs.
I also can't see us working at this pace for 5-6 yrs! I'd rather spend more time with our son. So it really, really isn't going to double in 3-4 years. But I said that the last time too.
Sorry for the long post that's pretty focused on our story. I've been thinking about this time to double bit lately. It was amazingly consistent and mind boggling.
Whatever you're doing, Keep it up!