There’s no getting around the fact that there’s a retirement savings shortfall in America. Perhaps even worse, many Americans aren’t even aware of the state that they are in.

According to the Schroders 2025 U.S. Retirement Survey, a whopping 62% of respondents admit that they have no idea how long their savings will last. To some degree, they can’t be blamed. No one knows with any certainty how long they will live or the exact investment returns they will earn.

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Since these are two important factors in how long retirement savings will last, it can admittedly be hard to come up with a precise determination. But not being able to perfectly forecast the duration of your retirement savings is a far cry from having no idea how long your savings will last.

Here are some of the variables that you can estimate to keep you in the ballpark.

To start your calculation, first account for all of your sources of retirement income. Depending on your personal situation, these might include a pension, 401(k) plan, IRA, personal savings, annuities, or even a part-time job or side gig.

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Determining how much money you’re going to spend is one of the critical variables of retirement planning. Fortunately, it’s also the one you have the most control over.

The general rule is that you should budget between 70% and 80% of your pre-retirement spending after you retire, but this estimate can vary greatly from person to person.

If you plan on living the high life after you retire, more power to you! But be sure to bump up your estimated expenses to get a more accurate reading as to how long your money will last.

Whatever the shape your nest egg takes, you’ll want to invest it if you plan on extending its lifetime.

If you simply keep your retirement savings in a bank account, you could run through that money quite quickly. But if you invest it and earn a 4%, 5%, or even 6% annual return, your money could last you years longer.

Imagine you plan on spending $60,000 per year in retirement and have $20,000 per year coming in from Social Security and pensions. This means your personal savings, including your retirement accounts, will have to make up that $40,000 annual shortfall.

If you have $320,000 in savings, that money will run out in just eight years if you don’t earn a return on it. But if you can generate a 5% return annually, your money will last more than 30 years.

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Something that’s easy to overlook when it comes to financial planning is the effect inflation has on the purchasing power of your money.

If you think you’ll spend $60,000 annually in retirement and that figure will never change, you might notice the quality of your life deteriorating rapidly in only a few years, and certainly after a few decades. With even a 2% rate of inflation, what costs you $60,000 in today’s dollars will require $73,139.67 in 10 years, and just $89,156.84 in 20 years.

If your income can’t keep up with inflation, you’ll have to start trimming your expenses just to get by.

No one knows exactly how long they will live, but for retirement planning purposes, you’ll have to make your best estimate. One good place to start is with the IRS Uniform Lifetime Table, which provides actuarial life expectancy rates based on your age.

You’ll want to tweak this number based on your own personal health situation. If everyone in your family lives past 100 and you take good care of yourself, for example, you might want to plan for a longer lifetime. If you already have chronic health problems and longevity genes don’t run in your family, you might want to adjust that number downwards.

To make your retirement savings last, you’ll have to walk the fine line of withdrawing enough money to meet your needs without depleting your assets prematurely. However, this step becomes easier once you fill in the other variables on this list, from income and expenses to investment return and life expectancy.

Understand that whatever numbers you plug into the formula are just estimates for financial planning purposes, and that you should expect to fine-tune your withdrawal plan along the way.

Planning for retirement is part art, part science. And it’s definitely not a one-and-done scenario. As so many variables can change over time, you’ll need to constantly update them to keep your plan on track.

For example, you might find out after you retire that you don’t spend nearly as much as you thought you might — or vice versa. You’re also likely to experience ups and downs in your investment portfolio that may adjust your withdrawal rate.

But if you begin with a plan that accounts for these variations, you’ll be ahead of the game right from the start. Then, you can confidently say that you have a good idea of how long your retirement savings will last.

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This article originally appeared on GOBankingRates.com: 62% of Americans Have No Idea How Long Retirement Savings Will Last: How To Figure It Out in 2025