"You don't want to be shocked when you get to retirement and find this out, or discover that Medicare doesn't cover everything," Fronstin said.
Representing an extreme case, a couple with high prescription drug expenses would need to have saved $383,000 to have a 90% chance of having enough to cover their health-care costs.
Advantage Plan coverageThe second coverage scenario analyzed in the study involves a Medicare Advantage Plan, which delivers Parts A and B, and typically Part D, as well extras like dental and vision.
Of Medicare's 64.5 million beneficiaries, 29.1 million are enrolled in Advantage Plans and that number is expected to continue growing.
While many Advantage Plans have no premium, they do have their own deductibles, copays or coinsurance and out-of-pocket maximums that vary from plan to plan.