Why Is Reddit (RDDT) Stock Up? S&P 500 Add, a 13% Short Float, and the Calls I'm Passing On

Updated August 14, after the close: $178.09, and the pop held into the bell

RDDT closed at $178.09, up 12.63%, a gain of $19.97 on Thursday's $158.12, per stockanalysis.com at 4:00pm ET. The session opened at $175.78, ran to $184.28, and bottomed at $174.00.

The close landed 60 cents under the 10:35am quote this piece was written against, so every row in the trade log stands as struck and none of them need restriking. The stock spent the afternoon giving back 3.4% from its high rather than surrendering the move.

Two numbers move slightly against the close. The August 21 $180 call at $7.65 now needs 5.4% rather than 5.2% to clear $187.65. The September $160 put breaks even at $155.45, 12.7% below the close rather than 13.2% below the intraday mark. Neither changes a stance.

Monday is still the event, and nothing about it has happened yet. The trackers buy at Monday's close and Reddit is in the index before Tuesday's open. What Friday settled is only that the fast money did not sell the pop back to Thursday's level on day one.

TL;DR

• S&P Dow Jones Indices said on August 13 that Reddit joins the S&P 500 before the open on Tuesday, August 18, taking the slot vacated by AvalonBay Communities, which Equity Residential is acquiring.
• RDDT closed Thursday at $158.12, up 3.04%, and closed Friday at $178.09, up 12.63%, after touching $184.28 intraday. Monday's close is the number that decides this.
• Index funds have to own roughly 14.4 million shares by Monday's close, against 19.21 million shares sold short and about 5.83 million shares of average daily volume. The forced buyer and the crowded short arrive in the same three sessions.
• The index add itself has been worth close to nothing for two decades. Greenwood and Sammon put the average abnormal return on S&P 500 addition at 0.3% since 2010, against 7.4% in the 1990s.
• The options market has already picked the squeeze side, and it is charging for it. Next Friday's $190 call costs 2.6 times the $167.50 put that sits closer to the money, and calls traded 3 to 1 over puts. The at-the-money call needs another 5.2% just to break even. I am not paying that.

Why Is Reddit Stock Up Today?

S&P Dow Jones Indices announced after Thursday's close that Reddit will join the S&P 500 effective before trading opens on Tuesday, August 18, replacing AvalonBay Communities. AvalonBay is leaving because Equity Residential is acquiring it; the combined company keeps the index seat under the name Vivmark Residential. Reddit enters in Communication Services, and becomes the second pure-play social media company in the index after Meta.

That AvalonBay merger closes on Monday, with each AVB share converting into 2.793 shares of the combined company. No tracker has to sell AvalonBay to make room, because their shares convert like everyone else's. The forced buying this week is all on Reddit's side.

Nothing about Reddit's business changed on Thursday evening. The whole move is a change in who is obliged to own the shares.

The Board

The forced buying is real. It is also smaller than the short position sitting in front of it.

What the Index Actually Has to Buy

Here is the arithmetic, with the inputs on the table so anyone can rerun it. Reddit's float is 143.71 million shares out of 192.40m outstanding. Something over $7 trillion tracks the S&P 500 against a total index market cap near $70.4 trillion, so index-tracking funds hold on the order of a tenth of any given constituent. Apply that to the float and you get about 14.4 million shares.

Treat that as a floor rather than a point estimate. The index weights companies on float-adjusted value, so the true denominator is below $70.4tn and the true fraction is above 10%. Call it 14 to 17 million shares, most of which prints in the closing auction on Monday, August 17.

Against 5.83m shares of average daily volume, that is two and a half sessions of demand landing in one print.

Two things about that buyer are worth holding onto. It is completely price-insensitive, which is the part everyone quotes. It also stops. Tuesday morning the trackers own their weight and the marginal bid disappears, and what is left is a larger share of the float parked in hands that will never sell it and never trade it again.

What the Index Seat Is Actually Worth

Greenwood and Sammon looked at every S&P 500 change from 1980 to 2020 and found the addition premium collapsing even as index assets exploded: 7.4% average abnormal return in the 1990s, 0.3% in the 2010s. Their paper ran in the Journal of Finance in April 2025. The mechanism most people assume, that trackers bid the stock up because they must buy it, showed almost no explanatory power in the data. Liquidity providers now position for these adds days ahead, which is most of why the effect died.

So I do not believe 12.6 points of Friday move is index demand being priced. The number that fits better is 19.21 million shares sold short, 13.37% of the float at the July 31 settlement, up from 17.00 million a month earlier. Shorts were adding into this, and they now have a hard, calendared date on which a price-insensitive buyer shows up.

One caveat on that percentage, because the denominator moves the conclusion. StockAnalysis puts the float at 143.71m, which makes 19.21m short 13.37%. ChartExchange reports the same July 31 share count against a wider float and gets 11.30%. The share count is the durable number; the percentage depends on whose float you use, and both readings say the same thing qualitatively.

The Options Market Has Already Picked a Side

This is where Friday actually tells me something. Prices below are mid-market on the August 21 weekly, the expiry that spans Monday's auction and Tuesday's index entry, struck against a $178.37 intraday quote around 10:33am ET.

Contract	Distance from spot	Mid price
$190 call	+6.5%	$3.40
$167.50 put	-6.1%	$1.29

The call that sits further from the money costs 2.6 times the put that sits closer to it. Volume says the same thing louder: 19,540 calls against 6,438 puts on that expiry, better than three to one, with nearly 5,900 contracts through the $180 strike alone.

The whole at-the-money straddle runs about 7% of spot, which prices a real move. But the skew inside it means upside is being rationed at a premium and downside is close to free. Everybody wants to own the squeeze, and the chain has repriced so that owning it is expensive.

Is Reddit a Buy Here?

No, and the index seat does not change the answer. Our August 10 decision piece said no at $156 because Reddit's user funnel runs on Google search referrals that AI Overviews are absorbing, and management called that traffic "choppy" with limited visibility. That is a structural problem. It is now 12.6% more expensive and the funnel is exactly where it was on Wednesday.

Index membership changes who holds the register. It does not change how many people arrive. The company that fell 21% on a 61% revenue beat two weeks ago is the same company today, joining the index 29.4% below where it traded a year ago, which is roughly when index committees become able to add a name they passed on at the top.

What would change my mind is unchanged from August 10: a landmark AI-licensing deal priced well above the existing ones, or US daily users growing again in a quarter where management still calls search choppy. Neither prints before Tuesday.

The Options Angle

The question I keep getting is whether to own calls into Monday's auction. My answer is no, and the reason is arithmetic rather than opinion.

• The at-the-money call needs another 5.2%. The August 21 $180 call offered at $7.65 breaks even at $187.65. From Thursday's $158.12 close that is +18.7%, and it requires the stock to add a fifth of Friday's move again inside five sessions, with the central catalyst being an event whose measured average effect is 0.3%.
• I am also not selling the call skew, tempting as $3.40 for a 6.5%-out-of-the-money weekly looks. A 13.37% short float with a calendared forced buyer is the exact setup that runs through a short strike, and the only structural loss in our track record so far was a short-premium range getting breached. Rich premium and a safe short are different claims.
• The trade I do want is the fade, and the skew is paying me to take it. September $160 puts offered at $4.55, 2.5% of spot, break even at $155.45. That is 13.2% below Friday's quote and 1.7% below Thursday's close: it pays if RDDT does nothing more than give back an index pop that the academic record says should be worth 0.3%. A month of time, defined risk, and the cheap side of a chain where everyone crowded the other one.

The obvious way this call is wrong: shorts capitulate into Monday's auction, the squeeze overshoots, and September expiry arrives with RDDT still above $160. I put that at better than a coin flip against me on any given week and still like the payoff, which is why the conviction below is a 6 and not a 9.

Trade log

#	Stance	Structure	Strikes and expiry	Cost or credit	Spot at writing	Implied move	Conviction	Breakeven
1	Pass	Long call into the auction	$180C, Aug 21 weekly	$7.65 ask	$178.37, intraday 10:33am ET Aug 14	~7% (Aug 21 ATM straddle)	7/10	needs $187.65, +5.2% from spot
2	Pass	Long the shares here	n/a	n/a	$178.69, intraday 10:35am ET Aug 14	n/a	8/10	scored on direction to Sep 18
3	Pass	Short the call skew (bear call spread)	Aug 21 $185/$190	~$1.50 credit, $3.50 max loss	$178.37, intraday Aug 14	~7%	5/10	$186.50; breached by any real squeeze
4	Bearish	Long put, the post-inclusion fade	$160P, Sep 18	$4.55 ask, 2.5% of spot	$179.09, intraday 10:40am ET Aug 14	14.4% (Sep 18 ATM straddle)	6/10	$155.45, -13.2% from spot

Every row is logged in the track record ledger and gets graded there, including the passes.

What I Would Watch Instead of the Pop

Monday's close is the event. If RDDT holds most of this gain through Tuesday, the covering was the real driver and the float genuinely tightened. If it fades back toward $158 during Monday's auction, Friday's bid was fast money front-running an effect the academic record says is close to zero, and they will have sold it to the trackers at the print.

The rest of Friday belonged to the macro: July retail sales fell 0.6% at 8:30am ET, and the S&P 500 gave back 0.17% from Thursday's record. The week-ahead hub has the retail earnings calendar that follows Monday's print. Sector breadth is on the heatmap.

The One-Line Read

Reddit's index seat is worth about 0.3% on the record. The other twelve points are 19.21 million short sellers finding out they have a deadline, and the chain is already charging full price for that.