WHAT DOES it take to make a jazz club work?

Considering the spate of jazz club closings across the country, a lot of jazz entrepreneurs would like to know the answer.

The average life span of a jazz room in most cities is less than five years under the best circumstances. Club owners find that most fans aren't consistent in their loyalties, and jazz performers now yearn to earn the fees of pop musicians. Add the backdrop of this country's current economic inflation and you've got big problems.

Lately the situation seems to worsening. The showboat Lounge in Silver Spring closed last week after a little less than two years operation. La Bastille in Houston closed recently. The Jazz Workshop and Paul's Mall, companion rooms in Boston's downtown, are scheduled to close April 9.

Joe Segal, owner of the Jazz Showcase in Chicago, says he's trying to hang on for a few more weeks until what he hopes will be a spring customer boomlet. And there've been other club closings in Miami, Salt Lake City, Philadelphia, San Francisco, Raleigh, Seattle Denver, Kansas City, Cincinnati and Santa Fe.

Paradoxically, cities such as Los Angeles and New Orleans are experiencing an upsurge in jazz club activity. And the New York jazz scene is the best in the country. New clubs - all over Manhattan and adjoining boroughs - featuring avant-garde jazz are doing well alongside older rooms programming established musicians.

But the Showboat closed primarily, according to booking agent Pete Lambros, because too many performers had radically increased their fees - in the last 18 months. Lambros claims that the fee of one musician, whom he declines to identify, shot from $4,500 to $7,500.

It's the same in Boston. "It's very difficult to run a jazz club successfully unless you have a great many marquee names," says Fred Taylor, co-owner of the Jazz Workshop, "and these marquee names cost too much now. Many of them, like Herbie Hancock, Weather Report and Freddie Hubbard have gone on to enormous popularity. They don't want to and don't have to play clubs anymore."

Some artists (or their booking agents) pit clubs against each other in bidding competition, with the losing club left dangling. That's the case in Chicago. Segal says his club has lost many commercially viable p erformers to Cafe American, which is operated by HOliday Inn in a downtown motel.

"They pay more than I do," explains Segal, "and musicians get to stay at the motel. They've also taken my older, established fans."

He encounters additional competition from Park West, a theater that books performers for one-night engagements. "They had Freddie Hubbard last Friday and he just knocked out Milt Jackson, who was here for a week," moans Segal.

Segal, who's been promoting jazz for 30 years, contents that most entrepreneurs pushing jazz today aren't dedicated to the music - only booking what will sell.

But it's hard to ignore economic realities. Says Taylor. "The inflation of the last year has caught up with everything. It's difficult to maintain a pure jazz club. Since the oil embargo in 1974, costs have doubled for groups to travel or to maintain utilities in a club. It's even hard to bring in groups from New York. Costs have doubled for everyone, but popularity didn't increase for everyone."

Moreover, groups are using - and transporting - instruments because of electronics. And some musicians insists on traveling with their own sound systems. Both practices put additional burdens on club expenses.

Taylor tried to solve his problems by moving to larger quarters. He wanted to convert an old theater to a cabaret format seating 650. But a city building code prevented him from converting to more than 350 seats. Meanwhile, the lease expired on the first room, which held only 200. So now he'll focus on promoting concerts.

Lambros estimates that a club would need at least 300 seats to make a regular profit. He figures that on the basis of generally charging a $5 cover and a two-drink minimum.

Sandy's Jazz Revival, in suburban Boston, takes a unusual approach to stay in business - it closes during the winter months when business is slow.

But many club owners think jazz rooms will have to take the route of the most successful pop rooms like the Cellar Door, Amazingrace in Evanston, the Great American Music Hall in san Francisco or the Bottom Line in New York - book a variety of jazz, folk and pop, and bring in groups f or no more than three nights at a time.

The pattern has been set in these rooms and it's hard to argue with their success. "Jazz rooms will have to be larger and expand their booking base," argues Lambros. "You've got to book some marginal acts like Noel Pointer, who doesn't play pure jazz, but he's popular. If you book an act that has youth-appeal, you'll get a good turnout. But if you book an older artist, not many people will come out."

Lambros has two other areas of complaints - fans and the media.

"I have a quarrel with the fans," he said. "At times you wonder where they are."

Sometimes fans fail to turn out for important artists. After an absence of 30 years from the Washington night club scene, Dexter Gordon drew only a spares audience to his opening night performance last year at the Showboat. A few weeks ago Lee Konitz, who hadn't played in Washington in 20 years, attracted only a handful of listeners of Harold's Rogue and Jar. True, it's not easy for a couple in these times of economic stress to spend $25 to $30 to hear one set by a jazz musician. But fans of pop music do it regularly.

Lambros also complaints that most of the local media don't have much feeling for jazz artists. "A major jazz musician can come to town and be completely ignored, mostly by radio and television," he said.

Clubs will continue opening and musicians will continue working. But the question is under what circumstances. Jack Whittemore, who manages McCoy Tyner, Stan Getz, Ron Carter and others, thinks the picture isn't as dark as it seems.

Maybe it isn't, but everything points to club owners having to book a variety of artists, including established performers who have slipped in popularity but retain their artistry. And everyone, for the moment at least, will have to tighten his belt.