Last summer, leaders at architecture firm Gensler’s research arm surveyed over 4,000 workers who go into the office at least some of the time, anywhere from all day, every day, to an hour a month. They published their findings in November in their Work, Life and the Workplace report, which opens with a stunning stat: On average, U.S.-based office workers come into their offices for 48% of their typical workweek. But those workers said that in order to be productive, they’d need to bump that split up to 63%. 

“That’s the main takeaway,” Janet Pogue, Gensler’s global director of workplace research, tells Fortune. But it leaves questions. “The office is here to stay, and people need it, so why don’t they come in more often?”

Pogue and her team, in search of a definitive answer, took a “hard look” at workers’ opinions from many angles. Namely, they asked workers whether in-office time was more crucial for individual productivity or for team productivity. But the same answer turned up: 63% of in-office time was needed, ideally, to work at max capacity. 

The finding seems surprising considering that workers have largely wanted to work remotely and insist the greater flexibility doesn’t impact their productivity. But it would hardly come as a surprise to the bosses who have been hammering the idea that working from home wallops productivity. Major bank leaders like James Gorman, David Solomon, and Jamie Dimon have all said as much, alongside Tesla’s Elon Musk (who went so far as to say remote workers just “pretend” to work) and Pershing Square’s Bill Ackman. 

But despite their incendiary remarks, the issue has pressed on for years, with research (sometimes) confirming that office work is undoubtedly more valuable and other times insisting that employee choice is more important for long-term success than work location. 

The fine details of the productivity argument still need ironing out, because many workers aren’t hitting their stride. Two in five respondents told Gensler that the frequency at which they go into the office—whether that’s every day or barely at all—doesn’t fit their needs. Thirty percent of workers say they need to be coming in less, and just 4% of respondents felt that their attendance matches their needs and desires. 

The productivity sweet spot: Two-thirds of the week in-person

While it’s notoriously difficult to pin down a hybrid arrangement that works for everyone, Pogue says the data has pointed to a sweet point: somewhere between 60% and 67% of the week in-person. 

Naturally, unique life factors impact where and how employees work, and what the office provides for them. Those with longer commutes or young kids (or both) are in the office least, but they told Gensler that they actually need to be in-person most, probably owing to distractions at home. 

Those who live within 15 minutes of the office, unsurprisingly, show up most often—though they said they’re there more than they need to be. (The survey respondents were based in New York City, Los Angeles, San Francisco, Houston, Chicago, and Washington, D.C.) 

Nonetheless, it’s critical when considering these percentages to avoid assuming they refer to full days in the office, as opposed to more flexible arrangements. “People are in and out a lot, and always have been, and I think we forget that,” Pogue says. “Partially in-person days are pretty popular.” (Twenty-eight percent of respondents said that would be their ideal.)

When asking workers in different demographics (age, parental status, living arrangement) around the globe which from a list of reasons are the biggest factors in making you want to go into an office, “focusing on work” was consistently in first place. In fact, it’s what all working generations—Gen Z, millennials, Gen X, and baby boomers—agreed on. Also common in the top five were “socializing with colleagues,” “access to technology,” and “to sit with my team.” (As future-of-work expert Annie Dean has pointed out, in-person work is essentially useless unless colleagues are walking distance from one another’s desks.)

Gensler itself practices what its data preaches, Pogue says. “We’re a global firm; we’re often not sitting with our team,” she says. “Our research team is based in five different cities, in five different time zones. We’ve developed how to build those relationships virtually, and when we do come together for work sessions or workshops, it carries over to our virtual work.”

Plus, she said, being in the office leads to undeniable positive bottom-line changes—which is the business case that companies might want to pay attention to next year. Among the top areas of improvement that office work can provide are relationships with colleagues, productivity, work quality, and awareness of critical intel. 

“A sense of presence is not just about building relationships, it has business outcomes too,” Pogue says. “We always knew that great design leads to great business performance, and we’ve been measuring that since 2008.”

But anti-flexibility companies might want to remember that these strong business outcomes still don’t require fully in-person work—or mandates of any kind. Two-thirds of the week in-person, Gensler’s data finds, is more than enough.