A couple that saved $1 million to retire at age 43 shares their best money-saving tip

"Mr. and Mrs. 1500" — the pseudonym of husband-wife duo Carl and Mindy — are soon-to-be retirees at age 43.

After getting serious about their savings goals in January 2013, the couple committed to building a portfolio of at least $1 million in 1,500 days in order to retire early. They've since grown their net worth from $568,000 to roughly $1.45 million. You can see their initial financial standing and current net worth on their blog.

While Carl and Mindy made several smart investments, cutting costs was also crucial to achieving their goal.

On an episode of Farnoosh Torabi's So Money podcast, the couple shared their best-money saving tip: "Write down every single expense."

This simple habit of recording daily purchases will allow you to see exactly how much money you're spending — and where there's room to save.

"You'll be surprised," they said. "We started doing this and were like, 'Wow, we spent that much on groceries? What were we thinking?' After you do that, evaluate every one of those line items and see how you can cut those down."

They're not the only early retirees who record all the money they spend.

Jeremy Jacobson and Winnie Tseng, who retired comfortably in their 30s, told Torabi that it's their No. 1 money habit: "We just write out all of our spending and analyze it. … Track your dollars. I guarantee you'll find something that either you didn't know you were spending your money on, or you felt was unnecessary."

If you don't want to track your dollars in a notebook, consider making a spreadsheet on your computer or using an app that will automatically do it for you, such as Mint, You Need a Budget or LearnVest.