How much money do you need to retire? And when should you start to save?

Many people look forward to retirement. You can do all the things you never had time to do because of work and other obligations.

Retirement is not as easy as sitting by the pool or taking a stroll along the beach. It can take years of preparation and saving to make sure you can have a comfortable and secure post-retirement lifestyle. It's never too early to start putting away money, even a little. You'll thank yourself later.

If it seems overwhelming, don't worry. The best way to start is to ask yourself some basic questions about your retirement goals and timing. We'll get you started below.

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How much money do you realistically need to retire?

There is no exact, set-in-stone number anyone should have to retire; it all depends on the individual.

When you're planning for retirement, there are several questions you have to ask to know how much is enough for you, according to Shweta Lawande, lead adviser at Francis Financial in New York City, which offers financial planning, wealth management and other services.

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When do you plan to retire?

These include: When do you plan to retire? What are your expenses now compared with what you want them to be in retirement? What are you currently doing to save for retirement?

"For anyone who's contemplating (how much you need to retire), especially the ones who are nearing that retirement age, I cannot recommend enough working with a financial adviser to create a custom plan for saving and spending in retirement," Lawande said.

Determining how much you'll need to retire

But if you want a more immediate and concrete idea, there are a few good estimates of how much you may need, such as assuming you'll need about 80% of your pre-retirement income in retirement, Lawande said. Pre-retirement is considered the period when you decide you want to retire and pick your retirement date.

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Things that will determine what you need to retire

Since the amount of retirement savings you'll need depends on the individual, having an understanding of your current personal finances in relation to what you want to accomplish in retirement is important, Lawande said.

Additionally, you need to consider finances you'll need to support loved ones and family, as well as for medical expenses.

Especially when it comes to medical expenses, life can be unpredictable, so it is key to consider the costs of procedures or medications, even if you are in good health.

If you plan to travel, entertain or pursue an expensive hobby, you may want to add extra savings for "more flexible, discretionary expenses," according to Merrill.

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One guideline to keep in mind is the 4% rule, which allows a retiree to have a withdrawal rate of 4% from their financial portfolio each year for 30 years, adjusting for inflation over time.

Using this rule, people can decide if their investments are reasonable for them and if the number can support them in any given year in retirement, Lawande said.

When should I start to save for retirement?

To best save for retirement, you should start as soon as possible. If you have a full-time job, start thinking about savings.

Lawande said if you're earning an income, start to think about retirement by saving 10%, and work your way up from there. On average, Fidelity estimates you should aim to save about 15% of your pre-tax income each year, which includes any employer match and assumes you're saving while you're between 25 and 67 years old.

"It's hard because retirement feels so far away to people who are that young," said Lawande. "When you have those number of years on your side for your money to grow in that way, it's just completely invaluable."

What is the best way to invest for retirement?

While saving for retirement is important, investing for retirement is just as important, added Lawande.

Investing allows your money to grow, whether it's through interest or appreciating stocks and dividends, over time. You'll be able to contribute less cash to achieve your goal since early investing allows you to benefit from compound interest.

Compound interest, which Albert Einstein is said to have called "the most powerful force in the universe," is when you earn interest on interest. For example, if you have $100 that earns 5% interest annually, you will have $105 the first year and $110.25 the second. It seems like a small difference but it adds up, which is why if someone offers you $1 million or a penny doubled every day for 30 days, you should take the penny. By day 30, that penny will be worth several million dollars.

"Consider the investments," Lawande said. "Let that work in your favor, especially when you have time on your side, before you retire and even beyond."

This article originally appeared on USA TODAY: How much do I need to retire? And when should you start to save?