Think you have retirement healthcare cost covered? Think again.

A 65-year-old couple retiring in 2012 is estimated to need $240,000 to cover medical expenses throughout retirement, according to the latest retiree health care costs estimate calculated by Fidelity Investments. This represents a 4% increase from last year, when the estimate was $230,000.

For many Americans, health care likely will be among their largest expenses in retirement. The estimate, calculated by Fidelity's benefits consulting business, does not include any costs associated with nursing-home care and applies to retirees with traditional Medicare insurance coverage.

The estimate has increased an average of 6% annually since Fidelity's initial calculation of $160,000 in 2002, with the exception of 2011 when the estimate declined $20,000. That one and only decrease in the history of the estimate was due to a one-time adjustment driven by Medicare changes that reduced out-of-pocket expenses for prescription drugs for many seniors. This year, health care expenses are rising once again.

"Today's workers must understand that the cost of health care is expected to continue rising significantly in future years," says Brad Kimler, executive vice president of Fidelity's Benefits Consulting business.

"Medical inflation is outpacing salary increases and cost of living adjustments for many people. Until that situation changes, it is critical that individuals include health care costs in their retirement savings strategies today so they can be prepared to pay their medical bills throughout retirement," Kimler says.

Fidelity compared Social Security's average cost of living adjustment (2.3% against an assumed average annual increase of health care costs for retirees nationally (6%). The comparison found that 65-year-old couples retiring this year with a $75,000 household income should expect that 35% of their annual benefit (about $10,476) could be needed for health care expenses today. In 15 years or by 2027, their allocation of Social Security benefits going to health care expenses is likely to almost double to 61% of a $41,205 annual Social Security payment, or about $25,000 a year.

"Retirees relying entirely on Social Security to fund their health care costs will be faced with difficult challenges in the future," says Kimler. "Today's workers should plan to supplement their retirement income to cover their medical expenses. It is never too late to begin utilizing all retirement savings vehicles available, including any 401(k) accounts, IRAs and health savings accounts, to help build a more secure retirement."