S&P Dow Jones Indices said it will add Tesla Inc.’s full weight to the S&P 500 all at once in December, shedding more light on a mammoth addition that has captivated investors across Wall Street.

The index company said Tesla will be added to the broad stock-market gauge before the start of trading Dec. 21, meaning most index-tracking funds that follow the S&P 500 will engage in a flurry of trading the Friday before.

Tesla’s market value has ballooned to about $538 billion, making it the sixth largest company in the U.S. stock market and it would have more than a 1% weighting in the S&P 500. That puts it ahead of Berkshire Hathaway Inc. but behind Facebook Inc.

More than $100 billion will be put into motion in coming weeks as passive fund managers and some actively traded mutual funds all benchmarked to the S&P 500 adjust their portfolios to make room for it, traders and fund managers say.

Weaving Tesla into the S&P 500 means shifting weightings of many other stocks to include the electric-vehicle maker. The addition also comes near the end of a year already marked by huge price swings during the coronavirus pandemic.

S&P still hasn’t announced what stock Tesla will be replacing, saying it will release that decision after the market closes Dec. 11.

Given Tesla’s size, there have been concerns that adding it to the benchmark would stoke volatility in the shares—already notoriously turbulent—as well as the broader market, several investors said.

The index giant had taken the unprecedented step of polling investors on how to proceed with the addition, and the decision comes after asset managers and trading desks across Wall Street deliberated how best to manage an inclusion of Tesla’s size.

Ultimately, S&P opted to put Tesla into the S&P 500 all at once. The company said the decision was based partly on the responses it received from its survey, as well as expectations of ample liquidity on the day most index funds plan to add Tesla shares.

That Friday coincides with a once-quarterly event where options and futures on both indexes and stocks expire simultaneously. Volume is usually heavy on those days and would help boost liquidity on the day of Tesla’s inclusion, several investors said.

“We’re expecting to see a day where volume is much higher,” said Matthew Bartolini, head of SPDR Americas Research at State Street Global Advisors.

Still, several traders and investors had said that adding Tesla on two separate dates rather than at once would have made for a smoother transition. Some even advocated for adding Tesla in two separate tranches, one in December and one in March.

In what appears to be an acknowledgment of that proposal, S&P said some of the suggestions it received from investors were beyond those it had initially proposed.

“There will be less aggregate market impact if it’s broken down into tranches by S&P,” said Patrick Nichols, a partner at trading firm Old Mission Holdings, ahead of the decision.

That problem is compounded by the fact that index funds typically buy and sell shares near the close of the trading session, ensuring they get the last price of the day and to avoid deviations from their indexes, added Greg Sutton, head of portfolio trading at Citadel Securities LLC.

“This name is substantial. Tracking error is a significant concern,” Mr. Sutton said.

Tesla’s size and the billions of dollars put to work around its inclusion have led investors to position for greater gains in its shares, some even anticipating a flurry of demand from index giants. The stock has jumped 39% since its inclusion was announced on Nov. 16 and there has been a frenzy of options trading tied to the shares jumping higher.

Some of the most popular options bets tied to Tesla recently have been bullish calls tied to the shares advancing to $650 or $700, according to data provider Trade Alert, at least a 15% jump from the close of $567.60 Monday.

“It’s kind of the perfect storm for speculators,” said Steve Sosnick, chief strategist at Interactive Brokers.

Tesla’s unusual addition has also highlighted some oddities surrounding the S&P 500 index itself, and how induction into the S&P 500 club isn’t strictly formulaic. Rules govern which companies are eligible for the S&P 500, such as around profitability and the fact that stocks need to trade on a U.S. exchange. But a secretive committee makes the final call on what goes in and what doesn’t.

Tesla’s stock has more than tripled since the start of the year, giving it a market capitalization larger than many behemoths of American industry. But its rise wasn’t necessarily driven by fundamentals. WSJ explains. Illustration: Jacob Reynolds/WSJ[object Object]

Tesla was first eligible for inclusion during the September rebalance after the car maker finally met certain criteria over the summer. Instead, the committee added online marketplace Etsy Inc. and two other stocks that together represented a tenth of Tesla’s market cap. The committee also initially passed on other companies, such as Google Inc. (now known as Alphabet Inc. ), before adding them later.

“It’s a super great case study of what goes on behind the curtain and why the benchmark methodology matters so much,” said Stephanie Hill, head of index at Mellon. “There’s little transparency on what drives the qualitative criteria.”

The S&P 500 isn’t the only index affected. Several other benchmarks will be rejiggered to accommodate the car maker, as well as any deletions from the S&P 500.

Tesla, for example, is the largest weighted stock in S&P’s completion index, a benchmark tracking all U.S. stocks except those in S&P 500, and will have to be removed. Any stock taken out of the S&P 500 will trigger its addition back into the completion index, as well as step downs into S&P’s mid and small-cap benchmarks.

State Street’s Mr. Bartolini said at least five of its exchange-traded funds will have to be rebalanced as a result of Tesla’s addition, including the biggest ETF in the world, the SPDR S&P 500 Trust ETF, and its growth-focused ETF, the SPDR Portfolio S&P 500 Growth ETF.

Tesla’s inclusion “will require a fairly numerous amount of trades,” said Mr. Bartolini. But he said the biggest managers of index funds are well prepared considering the additional lead time given by the index giant ahead of inclusion. He added that fund managers have handled other complex rebalancings before, such as the addition of Facebook in 2013 and the creation of the S&P 500’s communications sector in 2018. Still, Tesla’s size is hard to ignore.

“It’s a glamorous issue,” Mr. Bartolini said. “From our perspective, that’s where the glitz and glamour end.”