Many people know how to earn money, but that's only a small part of the equation. You also have to know how to multiply it over time, and when you should — and should not — spend on something.

It took me 20 years of trial and error before I achieved a multimillion-dollar net worth. I had to exercise tremendous discipline and invest as much money as possible into income-generating assets.

Now, I draw income from the 18 companies I started, and the 12,000 apartment units I own that make passive income.

Here are the four unpopular money and spending rules I followed, at a young age, that helped me get rich:

1. Don't make big purchases unless you have 2x its price saved up.

As soon as some of my friends started earning more money, they would reward themselves with fancy cars, boats and trips to Paris.

But I set a rule for myself: In order to buy an expensive watch or even a house, I had to have twice its price saved up. This kept me from overspending on something, while also giving me time to consider whether I really needed it.

Rather than dropping cash on big one-time purchases that had limited effects on my overall quality of life, I focused on putting my earnings towards improving my businesses.

2. Don't buy anything you can lease.

While working my way to becoming a millionaire, the only big-ticket items I would buy were ones that could increase my cash flow, such as commercial properties I could rent out.

I never bought anything I could lease — like a primary residence or car — so that I could avoid the higher monthly payments and upkeep costs associated with owning.

In 2012, for example, I sold my home and lived in rental houses for almost 10 years. This freed up more money for me to invest into money-generating real estate. Once I saved up a hefty amount, I bought a house with cash.

To this day, I still lease my cars. However, I never extend a lease for more than 24 months, even if it makes the bill more affordable. A lot can change in two years, and I try to avoid being stuck with a car that doesn't serve my needs.

3. Don't spend to impress others.

My goal has always been to create a legacy of generational wealth for my family. That was more important to me than buying things I didn't need.

So even when I could afford something, I didn't buy it just to impress my peers. Instead, I invested at a higher rate and built my wealth in private.

Even though I now have money to buy expensive things, I still consider myself as fairly frugal. I don't care about being flashy, and I try to live below my means.

4. Only spend your passive income.

If I earned $100,000 a year from my job and $20,000 a year in passive income, I would try to only spend $20,000 on things beyond my basic living expenses.

All these rules require a great amount of discipline, and they might not be for everyone. Occasionally, I would wonder why I was working so hard but not able to enjoy the fruits of my labor.

But because I abided by these rules for two decades, I've successfully weathered several major recessions and a global pandemic.

Grant Cardone is the CEO of Cardone Capital, bestselling author of "The 10X Rule," and founder of The 10X Movement and The 10X Growth Conference. He owns and operates seven privately held companies and a $3.6 billion portfolio of multifamily projects. Follow him on Twitter @GrantCardone.

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