Hourly employees can face a lot of challenges: fluctuations in weekly hours, unpredictable long-term schedules and limited benefits. Those hurdles are not just an inconvenience, they can affect financial stability. Forty percent of hourly workers do not have anything saved up in case of an emergency. And just over 75% report having less than $500 saved, according to Branch, a company that runs a mobile platform for hourly workers. The company recently polled 3,000 people who work as hourly employees in a number of industries, including retail, restaurants and healthcare. Millennials may be particularly vulnerable because, unlike baby boomers, a significant number of these employees entered the workforce and started to build savings following the financial crisis, Branch CEO Atif Siddiqi tells CNBC Make It. "Especially with inflation outpacing wages, what millennial hourly employees earn today is essentially less than what hourly employees earned decades ago," he says. "Because meeting daily expenses is a greater challenge for them, millennials will likely have an even tougher time managing retirement and healthcare costs."

Workers paid by the hour are at a disadvantage

It can be more difficult for hourly employees than their salaried counterparts to save up, Dr. Susan Lambert, associate professor in the School of Social Service Administration at the University of Chicago, tells CNBC Make It. "When paid by the hour, work hours are unstable and unpredictable, causing earnings to be so as well," Lambert says. "A salary, on the other hand, protects workers from unpredictable and unstable earnings, which can make it easier to budget and save." If hourly employees don't work, they typically don't get paid — even if it's their employer's fault. Recent academic research shows that 14% of hourly workers at the 80 largest retailers report their employer cancelled their shift in the past month. Only 39% of these employees say they have regular work schedules. Getting sick may also create financial problems. Currently, the federal government does not require employers to offer paid sick leave. Just under a dozen states have laws requiring companies to provide paid time off, including sick leave, yet the duration and scope of these requirements varies. The majority have left it up to the employer. The Bureau of Labor Statistics reports that, nationwide, 71% of employers offer paid sick days, but again, the rules around who qualifies and how much time is offered varies dramatically. Plus, employers typically do not offer hourly and part-time workers benefits such as robust insurance and retirement plans.

Emergency savings are critical to financial health

After accounting for inflation, today's average wage has about the same purchasing power it did 40 years ago, according to Pew Research. That means it takes more money now to buy the same things. So hourly employees now not only have the challenge of affording day-to-day expenses, but it can be even more difficult when those wages fluctuate, Siddiqi says. "One single emergency could wipe those savings out or cause them to overdraft," he says. "These emergencies can derail finances and make it even more difficult to save." That's where emergency savings come in. Ted Rossman, credit industry expert at CreditCards.com, calls emergency savings a "buffer" between consumers and high-cost debt. If you do have some savings, you'll have an immediate way to pay for an emergency, rather than carrying a credit card balance, racking up overdraft fees, taking out a personal loan or perhaps even relying on a high-cost payday loan.

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