The vast majority of American retirees get benefits from Social Security, and many of them depend on the program to provide them with the majority of their regular income in retirement. One of the best features of Social Security is that monthly payments are tied to the rate of inflation, so most of the time, recipients get benefit increases every January that match up with rising prices.

Despite the program's provision for cost of living adjustments, Social Security hasn't been as kind to retirees in recent years as many would like. 2018's increase of 2% was actually the largest since 2011. Now, things are shaping up for Social Security's COLA to be even bigger -- but the root cause will end up making many seniors feel less than excited about the incremental bump they'll see in their monthly checks.

How to estimate Social Security's expected 2019 rise

Calculating the cost of living adjustment isn't as hard as it might sound. In order to come up with the appropriate increase each year, the Social Security Administration looks at a particular measure of inflation that the Bureau of Labor Statistics provides every month. The Consumer Price Index for urban wage earners, or CPI-W, provides the readings that you need to come up with each year's COLA.

In order to be ready to make an increase every January, the SSA looks at the inflation readings for the CPI-W during the summer months of July, August, and September. It compares the average CPI-W figure for those three months with the corresponding average for those same three months a year earlier. If the figure is lower -- indicating deflation -- then no adjustment is made. That's happened several times over the past decade, including most recently the 2015 calculations that left 2016 benefits unchanged.

If the figure goes up, then you take the percentage increase and round it to the nearest tenth of a percent. You can then take whatever your gross Social Security benefits are before any deductions for Medicare payments or other charges and then increase it by the COLA percentage to find out what to expect when 2019 rolls around.

This bad news is sending COLA expectations higher

We know from last year's calculations that the average figure for the CPI-W from July to September in 2017 was 239.668. Obviously, it's too early to know how the corresponding figures this year will turn out. Yet the Bureau of Labor Statistics just released April figures that gave a CPI-W reading of 244.607. Even if that number doesn't move between now and September, that would produce a potential increase of 2.1%. Year over year, the CPI-W has risen 2.6% since April 2017. Either one of those rates would lead to the largest boost for benefits in seven years.

Many retirees who like to travel won't be happy with the cause for their Social Security checks going up. During April, gasoline prices jumped by more than 6%, bringing the year-over-year gain for prices at the pump to more than 13%. For those who need to fill up with a recreational vehicle or large-sized car or truck, a COLA of between 2% and 3% won't go very far to handle rising gasoline prices. With the typical monthly benefit for retirees in 2018 weighing in at $1,404, COLAs of roughly $28 to $42 per month won't even cover a full tank of gas for many retirees.

The bittersweet impact of larger Social Security increases

Because Social Security uses price levels to calculate cost of living adjustments, increases always match up to higher costs for something. Those retirees who don't travel as much might end up hanging onto more of their COLA, as those who don't work have fewer needs to buy gas for commuting to work every weekday. Even so, the stress of dealing with inflationary pressures on a fixed income is hard.

It'll be several months before we know for sure what the final 2019 Social Security increase will be. Even if it ends up being the biggest boost in years, you'll want to watch closely to see if you'll be able to keep up in terms of the purchasing power of what you get from Social Security.