Splitting the bills 50/50 works well for some couples. But it can get complicated when one person's income changes and the other person's doesn't.

Consider Diane and Dave. This hypothetical couple had been splitting their household expenses down the middle for years. When they first agreed to it, Diane was making about $90,000 a year and Dave was earning $130,000.

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Dave was making $40,000 more, but he still wanted everything split 50/50. Diane went along with it, even though the arrangement wasn't always easy for her.

Then Dave lost his job. He eventually found another one, but it paid $95,000 a year. Meanwhile, Diane got a promotion that brought her salary up to $140,000.

So now Diane makes $45,000 more than Dave, and Dave wants to change how they split the bills. He thinks Diane should pay more because she earns more.

Diane can't help but see the irony. When Dave was making more, he was fine with 50/50 — but now that Diane is making more, he wants a different arrangement.

For couples in this position, it can be worth taking a fresh look at how the bills are divided when one person's income changes. After all, what worked before may not feel fair anymore.

When 50/50 doesn't feel so fair anymore

There's nothing wrong with splitting the bills 50/50 if it works for both people. The issues start when one person is putting the same dollar amount toward the bills but feeling a much bigger financial squeeze.

That's the situation Diane and Dave could find themselves in now.

Certified financial planner Akeiva Ellis, CFP®, CPA, says couples shouldn't get too hung up on whether the numbers are technically equal.

"Fair is subjective," Ellis told Moneywise. "It comes down to what each couple decides for themselves."

She suggests couples ask whether their arrangement is actually equitable instead of simply equal.

A lower-earning spouse might be perfectly comfortable paying half the bills, Ellis says, while someone else in the same financial position might feel overwhelmed by it. The numbers can be identical, but the experiences can be very different.

That's something Diane and Dave may want to consider. If they each have $2,500 in monthly shared expenses, for example, that $2,500 takes up a larger share of Dave's income now that he earns $95,000 than it did when he was earning $130,000.