It can be hard to remember what work at the office was like before the pandemic forced millions of Americans to start working from home. That shift was monumental and seemingly implausible, until it happened. But people soon adapted to saying “sorry, you’re on mute” on Zoom calls and wearing sweatpants all day.

This spring, workers are finally heading back to the office en masse and into another untested and ambitious experiment in work life: hybrid working.

“This is a brave new world – we’re doing something we’ve never done before, which is we’re going to go, en masse, hybrid,” said Nicholas Bloom, an economics professor at Stanford.

Many companies that are bringing their workers back to the office are doing so on this basis, meaning they are allowing employees to do a mix of in-person and virtual work during the week. Some workers will be expected to work a set number of days in the office; others will get to choose which days they want to come in.

The great experiment is already under way. During the week of 9 March, the average office occupancy rate across 10 large US metro areas was 40.5% – the highest percentage since December, according to Kastle Systems.

Companies that once told employees that they would be working from home indefinitely have set dates to return to the office. Microsoft was the first big tech company to announce that it wanted its employees back in the office, by the end of February. Other companies, including Google and Apple, soon followed suit. Twitter, which at the beginning of the pandemic told employees they could work from home “for ever”, said it would reopen its offices in mid-March.

But these companies are telling staff that the era of the five-day office week is over. Some of employees’ time can be spent working from home.

Whether this new structure of office work will last is unclear, largely because this is the first wide-scale adoption of hybrid work.

Bloom, who has studied remote work for nearly two decades, has been consulting with hundreds of companies and managers on their return to office plans. Managers, it seems, want their workers back in the office. A survey from Slack found that executives were three times more likely than non-executive employees to want to return to the office full-time.

Employees, on the other hand, want to keep working remotely – at least some of the work week. With today’s tight labor market, Bloom said, the worst thing a company can do is expect workers to come in five days a week. Multiple surveys have shown that workers have liked the perks of working from home so much that many are willing to switch jobs to keep that option.

“That is the fastest way to destroy your company,” Bloom said. “Hybrid work-from-home, it’s become completely a standard.”

Bloom points out that before the pandemic, many people were already doing some type of hybrid work.

“Most of us probably did some work on the weekend, some work in the evenings. So it isn’t actually such a radical shift to say ‘we’re going to formalize this’,” he said.

The best hybrid mix right now, according to Bloom, is the “boring and predictable” hybrid model of three days in the office and two days working from home. The number of days in this model are not random. Research has shown that people want to work from home on average 2.5 days a week, making the three-two model the closest compromise.

The plans of companies like Citigroup, Apple and Google will allow most workers to follow this model. Other companies, such as Twitter, are leaving it up to employees to work out with their teams how often they will work in the office.

Some companies that have been testing hybrid models have noticed that employees seem to appreciate this flexibility.

Joanne Wright, vice-president of enterprise operations and services at IBM, said the company had been working on opening its 100 office locations scattered across the country for employees and teams to use as much as they want.

“We know that there’s going to be days when they’re going to work from home and continue to be productive, and there’s days where it makes sense for them to be with a team or clients, collaborating and innovating,” Wright said.

“We were flexible before the pandemic, and that’s what we’ve been trying to drive in this new world, is maybe we need to even think more flexibility than before.”

Management experts stress that return-to-office plans should work toward accommodating balance between employees’ work and non-work lives, and employers need to communicate to their employees that hybrid models are still experimental. Surveys and discussions with employees will be crucial in understanding how return-to-office plans are affecting their lives.

“We’ve never actually been in this place before,” said Heidi Brooks, a lecturer at the Yale School of Management. She said employers needed to be especially aware of how the nature of productivity has changed during the pandemic, when people built their work lives outside the office. “The question that’s now up is: can we transition back and hold on to what works … and all of the gains that were possible when people didn’t have to commute and transition [to the office]?”

Brooks emphasizes the importance of fairness in return-to-office plans and says executives must consider the pandemic’s “differential impacts”, particularly around race and gender. For example, women with young children have to figure out childcare to go into work.

Johnny C Taylor, president and chief executive of the Society for Human Resource Management, said that employers would need to see how return-to-office plans affect morale and a company’s culture, not just worker productivity.

“[A plan] may prove to be incredibly effective, but you may not be creating an environment where people enjoy their work and are engaged,” Taylor said.

Employers also have to be clear about how return-to-office plans balance the needs of employees and the company.

“We have to bring more balance into the discussion that this is good for you, but it also has to be good for us too,” Taylor said, adding that employers needed to be transparent about how they are still figuring out what will work best for the company.

“It is important for employers to be honest about the fact that we don’t really know how this is all going to work out.”