How do couples plan for retirement when 1 partner doesn't know how much the other makes?

Couples are worrying more about their retirement savings. Maybe they should start by figuring out how much they have saved in the first place.

When asked how much in savings they collectively have, or how much their partner makes in income, many couples get the answer to basic questions wrong, according to a survey for Fidelity Investments. It's the latest evidence that many couples still aren't on the same page when it comes to planning their financial future, even when the majority says they're worried about outliving their savings in retirement.

More than a third of couples -- 36 percent -- disagreed on how much money they had to invest. An even larger percentage -- 43 percent -- couldn't correctly say how much their partner earns. Of them, 10 percent were off by $25,000 or more.

"Your salary is a very personal number, and it's something we have been taught as a society not to talk about," says John Sweeney, executive vice president of retirement and investing strategies at Fidelity. "But if you can't detail that with your partner or spouse, it's going to show up in your spending and savings patterns being misaligned. And where finances create strains on marriages is when you don't have alignment."

The misunderstandings come even as most couples say they communicate well with each other about finances. One possible reason is that more people are working as freelancers, where incomes aren't as predictable.

Another driver is that in many couples, one person takes charge of how to invest retirement savings, while the other stays less involved. Division of labor is fine, Sweeney says, but it's important for both members of the couple to talk regularly about their finances and ensure they're working together toward the same goal.

That means learning not only how much each person makes and has saved up, but also how much the Social Security Administration's online benefit estimator says they can expect and how much they plan to spend in retirement. That way, either member of the couple can take control of decisions if something were to happen to the other.

The survey for Fidelity by GfK covered 1,051 couples who had household incomes of at least $75,000 or total assets of at least $100,000. The survey covered both heterosexual and same-sex couples, including those who were married and ones who were in long-term committed relationships.