University administrators are awaiting a memo that will lay out how Yale’s $100 million budget gap in the 2009-’10 academic year — a by-product of the University’s shrinking endowment — will trickle down to their budget proposals for next year.

But they already know roughly what to expect: Each Yale department and division will have to come up with 5 percent cuts in its spending on personnel and 5 percent reductions in other spending, originally announced by University President Richard Levin last month. This next memo on the budget, which should come in the next few weeks, will formally kick off the budget planning process that, for the first time in years, will require some belt-tightening.

“We’re really trying to get this right, and we’re taking our time to make sure that we’re giving people the right guidance,” Levin said in a phone interview Wednesday. “We’ve already given the broad guidance in [last month’s] budget letter, so people are already thinking along the right lines. We want people to think in terms of budgeting across revenue sources; we want units to use their budgets more interchangeably and flexibly.”

The budget planning memo is usually issued around Christmas. But this year, Levin sent out a special financial update in which he projected that the value of the University’s endowment fell an estimated 25 percent, roughly $6 billion, between the end of June and December.

Given the circumstances, top administrators needed more time to engineer budget guidelines for the coming fiscal year, Provost Peter Salovey said in an interview last week.

“December was about broad strokes — the University as a whole, the endowment as a whole,” Salovey said. “In January, we’re fine-tuning the general models to communicate unit-by-unit what the impact is locally.”

Planning for the 2010’s $100 million budget gap began in committee, involving top administrators such as Salovey and Vice President for Finance and Administration Shauna King, and led up to the December meeting of the Yale Corporation. At that time, the options were discussed in the University budget committee.

They settled on an approach that took cautious steps at this time rather than moving ahead more aggressively, which might lead them to end up cutting more than necessary, administrators said.

“Markets have been extremely volatile, and the endowment could do better or worse than we are forecasting,” Levin said in his letter last month. “Because of this very high degree of uncertainty, it is important that we not overreact.”

So the administration decided not to close the entire projected budget gap but rather only two-thirds of the current forecasts.

Mild across-the-board cuts are relatively quick and painless, Deputy Provost Charles Long said. Cutting deeper or for several straight years, however, could start to hurt the things that are most important, he added. Hypothetically, as cuts become more severe, he said, they have to become more selective.

The 5 percent cuts are separated into personnel and non-personnel expenses just to make sure that managers distribute the reductions evenly, Deputy Provost Lloyd Suttle said. He said the personnel savings could be achieved through regular turnover and by leaving open vacant positions, while hopefully avoiding layoffs altogether.

Different departments and divisions of various size and relation to the University’s core mission will be faced with different options for cost-cutting. For example, a residential college employing a master, a dean and an administrative assistant for each has few options for making personnel cuts.

But as departments’ individual configurations vary, so may their proposed solutions to trim their spending — details that will be worked out following the upcoming memo.

This degree of budget pruning so far is hardly unprecedented — the University operating budget underwent 5 percent cuts as recently as the 2005 fiscal year, when the aftershock of the technology bubble and Sept. 11 caught up with the two-year lag in the endowment spending rule.

At that time, the University also had 5 percent reductions in personnel and non-personnel expenses; the last time before that was in the early 1990s, according to David Soper, director of operating budget management.

But when making those cuts in 2005, the $1.7 million operating budget faced a deficit of $14.5 million. Now administrators are talking about a $2.7 billion budget with a $100 million deficit.

And, with a projected $300 million shortfall in 2011, the problem is about to get even bigger.

Paul Needham contributed reporting.