You've saved it, but how do you spend it?

Millions of Americans are well-versed in how to save in their 401(k) plans – everything from contributing regularly to securing company matches – but they have no real plan for turning those savings into income that will last, a survey of more than 2,100 workers by Nuveen and the TIAA Institute showed. Workers correctly answered only about one-quarter of survey questions on retirement withdrawals, on average, and nearly half couldn't answer a single question correctly on that topic. Only 19% of near-retirees have even thought much about how to withdraw retirement income from their 401(k) savings.

Withdrawal decisions are crucial during retirement. Without a plan, retirees could run out of money or end up budgeting so tightly they leave too much on the table, not enjoying retirement to the fullest, experts said.

"With more than $8 trillion in assets spread across 725,000 plans serving 80 million active employees, 401(k) plans are the primary employer-provided retirement savings vehicle in the private sector," said Brendan McCarthy, head of Nuveen Retirement Investing. "Yet despite that scale, too many Americans arrive at retirement without a clear strategy for turning their savings into income that will last. And that gap has real consequences."

How can Americans calculate what to spend?

There are many general rules and tools people can use to help ensure their retirement savings last. Some include:

• 4% rule: Withdraw 4% of your total retirement portfolio in the first year and adjust that amount each subsequent year to account for inflation. Historically, under this rule, a portfolio of stocks and bonds with steady returns lasts about 30 years assuming stable expenses.

• 4.7% rule: The updated version of the 4% rule, which some think has become outdated and too conservative. Critics of the 4% rule said it didn't account for more diversified portfolios that include more than stocks and bonds and for the recently booming stock market.

• Healthspan and lifespan: A person's lifespan and healthspan, or the number of healthy years, can be used to determine what you can spend. With a signed HIPAA (Health Insurance Portability and Accountability Act) release from clients, Abacus Life can tap your medical records, find odds on people who match your health condition and medical history and provide you with the most probable lifespan and plan around that. People wary of signing over access to medical records can choose free online health tests like those at the American Academy of Actuaries or Abacus to get an idea of how long they might live.