Retirement is unpredictable. In order to ensure you'll have enough money to last your entire life, you need a plan that can flex and change with your needs. Here are three reasons you should consider reevaluating your retirement plan at least once a year or after you experience a life-changing event, like a job change or major health problem.

1. Your needs may change over time.

A significant health issue could derail your retirement plan. If it's a chronic condition, you can expect higher medical bills to continue into retirement. You can account for this by increasing your monthly savings to cover your higher estimated bills and by adding a long-term care policy if necessary. You could also reduce your planned spending on travel or other discretionary purchases.

Retirement savings plan with pen, calculator and glasses

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You may change your mind over time about travel or downsizing a home, and this can also affect how much you need to save for retirement. Living expenses may rise more quickly than you anticipated, or you may get married or divorced. The only way to accurately plan for these changing expenses is to draft a new retirement plan with a new savings goal.

2. Your risk tolerance will change.

When you're young, you can afford to invest your retirement savings more heavily in stocks because your assets have plenty of time to recover if they suffer a loss before you need to begin drawing upon them. But as you near retirement, your risk tolerance diminishes because you don't have the same luxury of time. That usually means moving more of your money from riskier, potentially more valuable stocks to more stable investments like bonds.

You must reevaluate your portfolio periodically and reallocate your money, if necessary, or else you could leave yourself exposed to too much risk as you age. If you're not sure how to properly reallocate your funds, consider bringing in a financial adviser who can suggest the best investments for your needs and goals. Just make sure you choose a fee-only financial adviser, instead of advisers who may earn commissions on some of their recommendations. This can create a conflict of interest.

Another option is to invest your money in a target-date fund. These funds are designed for individuals planning to retire in a certain year; the assets in the fund become more conservative over time so you don't need to worry about making these changes yourself.

3. Your investments may not grow at the rate you anticipated.

When you create a retirement plan, you include an estimate for how quickly your investments will grow. This is used to determine how much money you need to save each month to reach your goal. But your investments may grow more quickly or more slowly than you anticipated, affecting how much you need to save each month.

By rerunning the numbers once per year, you can make slight adjustments to your savings rate to account for the variable growth, so you don't risk saving too little and ending up without enough money in retirement.

When to update your retirement plan

Recalculate how much you'll need for retirement at least once per year. Use your current retirement savings as your starting point and update your estimated retirement expenses, if need be. The results may be the same as your previous plan, in which case you won't need to change anything you're doing. But if your monthly savings goal changes, adjust your budget accordingly. If you cannot set aside as much as you need to right now, save as much as you can and look for opportunities to boost your income or cut spending so you can hit your target.

You should also reevaluate your retirement plan after a major life change, like a marriage, divorce, or job loss. These events can all significantly affect how much you need to or are able to save for retirement.

Your retirement plan is fluid, not something that you make up once and never look at again. There's always some risk that you won't save enough, but by updating your plan periodically, you can reduce this risk and increase your chances of having a comfortable retirement.