This couple retired at 43 to travel Europe full time on $18,000 a year - here's the spreadsheet that helped them get there

Jason and Julie Buckley have diligently tracked their pennies for years.

At first, it was part of their strategy to minimize everyday costs, pay off their mortgage, and invest in passive index funds. But eventually, the pair discovered they could afford to take up to two years off work to travel.

In 2011, the Buckleys set off to tour Europe and North Africa in their motor home, earning passive income from renting out their residential properties at home in Nottingham, England.

When their funds ran dry after two years, they returned to work - Jason as an IT project manager and Julie as a marketing manager - and quickly devised a plan to take their lives on the road full time.

Shepherded by meticulous spreadsheet estimates, they retired - both aged 43 - in September 2015 to travel indefinitely. They had about £30,000 (~ $36,800) in cash savings with the remainder of their net worth invested in rented-out residential property, private pensions, and investments including ETFs and bonds, Jason told Business Insider in an email.

"We developed a strategy to avoid living off savings. In theory, and in practice so far, our savings will increase over time," he said. Currently, the couple lives off of cash flow from investments - rental income, dividends, and interest - as well as advertising and book sales on their travel blog, which they spend just two hours a day maintaining.

Thanks to the low costs of living in a motor home, which enables them to visit expensive countries "for a fraction of the cost," their budget is a modest £15,000 (~ $18,400) a year. "We've tracked our spending for so many years it's almost become second nature, knowing how much we can spend, but still we track where the money's going to the nearest cent," Jason said.

The Buckleys have been traveling nonstop for 10 months now, though they keep a room in one of their homes open for intermittent returns.

"Our latest tour ... took us high into the French and Italian Alps for winter, down to the beaches of Croatia, up through the old ex-Soviet countries to Finland, further North to the top of mainland Europe at the North Cape in Norway, down the fjords and over to Sweden then Denmark," Jason said. "We're currently heading home for a short time before turning South for Spain and Morocco for the winter."

Jason said the couple occasionally discusses taking on full-time work, but hopes to continue traveling well into the future.

"The sensation of freedom is intense, once attained, and giving it up again isn't easy!" he said.

Below, Jason and Julie share the spreadsheet models they used for reaching financial independence and planning for their years in retirement. These spreadsheets do not include the Buckley's exact figures, but instead represent the formula they used to achieve early retirement and how they plan to sustain it for decades to come.

Access a test copy of the Buckleys' spreadsheet to follow along, or to make a copy of your own.