What VIX1D measures
Cboe's VIX1D methodology states the objective in one line: the Cboe 1-Day Volatility Index "is designed to measure the 1-day expected volatility of the U.S. equity market, as conveyed by S&P 500 index option prices." It is the same variance-swap construction as the VIX, aggregating the midpoint prices of out-of-the-money SPX puts and calls across a wide range of strikes, but pointed at a single trading day instead of thirty calendar days. Cboe launched it on April 24, 2023, and the methodology lists a first value date of May 2022, so about a year of backfilled history predates the launch.
The reason it exists is the shape of the SPX market. Cboe's full-year 2025 volume release reports that SPX options traded 970.6 million contracts in 2025, an average of 3.9 million a day, and that zero-days-to-expiry contracts averaged a record 2.3 million a day, 59% of all SPX volume. A 30-day index says very little about what the majority of that market is pricing. VIX1D reads expected volatility from the contracts where most of the volume now lives.
It is a different instrument from the VIX rather than a faster copy, and four design choices in the methodology explain almost everything about how it behaves.
Two expiries, not one
The index uses P.M.-settled SPX Weekly (SPXW) options in two buckets. The methodology defines the near-term options as those "where expiry date is equal to current day of calculation," and the next-term options as those "expiring closest to and after" that date. Its own example: on a Tuesday in a normal week, the near-term strip expires that Tuesday and the next-term strip expires Wednesday. Prices are the midpoints of live bid and ask quotes from Cboe's options exchange, and the interest rates come from a cubic spline through the Treasury constant-maturity curve, exactly as in the VIX.
So at any moment VIX1D blends two variances: one from options that stop trading at 4:00 p.m. ET today, one from options that expire at 4:00 p.m. tomorrow. How much of each goes into the number depends on the clock, which is the second design choice.
The clock runs in business minutes
The VIX measures time to expiration in calendar minutes; the worked example in its methodology divides by 525,600, the minutes in a calendar year, and counts every overnight and weekend minute. VIX1D does not. Its methodology says the index "measures time to expiration of a constituent option series, T, in business years and business minutes (vs. calendar years and calendar minutes)." A business year is 252 sessions of 6.75 hours, or 102,060 business minutes, and one session is 405 minutes, from the 9:30 a.m. open to the 4:15 p.m. ET end of SPX trading.
Cboe's sample calculation shows what that does. At 11:00 a.m. ET, the same-day options have 300 business minutes left and the next-day options have 705: the 300 minutes to today's 4:00 p.m. expiry, the final 15 minutes of today's session, and the 390 minutes from tomorrow's open to its own 4:00 p.m. expiry. The gap between today's close and tomorrow's open is zero minutes long on this clock, and that gap is the source of the index's best-known quirk.
The horizon slides through the session
Cboe's Spencer Doar described the weighting plainly in his launch note: "At the beginning of a trading day, the VIX1D Index calculation is nearly fully weighted to the current day's expiring SPX options strip, and as the 405-minute trading day progresses, the weighting gradually shifts to the SPX options expiring on the next day."
The methodology adds a hard rule at the end of the day. When the near-term options have fewer than 60 business minutes left, "at approximately 3:00 p.m. ET," their variance is no longer recalculated; the last valid value is "persisted until the near-term expiration time," and the index becomes an interpolation whose weights "are heavily concentrated on the next-term." From 4:00 p.m. to 4:15 p.m. the expired strip is dropped entirely and VIX1D is computed from the next-day expiry alone. The index is published every 15 seconds from 9:31 a.m. to 4:15 p.m. ET.
The practical meaning: a 10:00 a.m. reading is mostly about the rest of today, a 3:30 p.m. reading is mostly about tomorrow, and comparing the two as if the horizon were fixed is the most common way to misread the index.
Why it drifts up into the close and resets overnight
Put the business-minute clock together with the sliding weights and a pattern falls out. Tomorrow's options are priced for everything that can happen before 4:00 p.m. tomorrow, including the overnight session and, on a Friday, the weekend. The clock assigns that gap no time at all. Spreading the gap's variance over only the business minutes that remain inflates the next-term variance per minute, and as the afternoon shifts weight toward that strip, the inflation leaks into the index. Overnight, the near-term strip rolls to a fresh same-day expiry, and the reading drops back.
Stefan Albers and Lars Kestner document exactly this in Finance Research Letters (2024) under the title "The daily rise and fall of the VIX1D," identifying "a distinct overnight bias" that causes the index to rise during trading hours and fall overnight. They attribute it to the methodology's use of business time and the dynamic weighting of next-term options, which carry the overnight variance risk premium, find the effect larger than the ordinary day-of-the-week effect, and propose either filtering the data or moving to a forward-starting variance to remove it. Albers' 2025 follow-up in the Journal of Futures Markets confirms the daily upward intraday pattern, describes the index as "generally lower and more volatile" than the longer-dated VIX family, and finds that it "overestimates the volatility of the S&P 500," which is what a variance risk premium looks like on a one-day horizon.
Two habits follow. Compare VIX1D readings taken at the same time of day, not a close against the next open. And treat a Friday afternoon reading with extra suspicion, because it carries two overnight sessions and a weekend inside a clock that thinks no time passes.
How it behaves against the VIX
Cboe's Matt Moran published the first year of live and backtested data in May 2023, covering May 13, 2022 through May 10, 2023. Over that period VIX1D ranged from 9.96 to 47.14 while the VIX ranged from 15.78 to 34.02. The largest one-day percentage increase for VIX1D was 153.2%, on January 11, 2023; the largest for the VIX was 22.6%, on June 13, 2022. The average 20-day historical volatility of VIX1D was 372.4 against 84.3 for the VIX. And VIX1D's correlation with the S&P 500 itself was only -0.20, far weaker than the inverse relationship the VIX is known for.
The gap shows up most clearly in stress. Doar's launch note gives the example of the March 2023 bank failures: between March 8 and March 13, 2023, the VIX rose from 19.11 to 26.52, a 38.8% move, while the backtested VIX1D rose from 15.30 to 40.19, or 162.7%. Both indices saw the same options market; the one-day index simply had nowhere to dilute the shock.
Scheduled events produce the mirror image. Moran reports that on the sessions just before CPI and FOMC announcements, VIX1D was up an average of 61.4% while the VIX was up 1.9%, and on the announcement day VIX1D was down an average of 24.8%. A one-day window either contains the event or it does not. The day before, tomorrow's options carry the whole announcement and the afternoon weighting pulls it into the index; once the event has printed, it is gone from the window. None of this is fear rising and falling. It is the horizon.
Turning a reading into an expected move
VIX1D is quoted as an annualized volatility on a 252-session basis, so the conversion to one trading day is the familiar square root of 252, about 15.87, the "rule of 16." A VIX1D of 16 implies a one-day standard deviation of roughly 1.0%; with the S&P 500 at 5,000 that is about 50 index points. A VIX1D of 24 implies about 1.5%.
Three qualifications keep that arithmetic honest. First, it is one standard deviation, not a boundary; under a normal distribution a move outside it is expected about a third of the time. Second, Albers finds the raw index runs above realized volatility on average, the same implied-versus-realized gap that exists at every horizon, so the expected move is a premium-inclusive estimate rather than a forecast. Third, the horizon slides: a morning reading describes the rest of today, an afternoon reading mostly describes tomorrow, and the drift documented above means the afternoon number is biased high.
Where it fits in the volatility toolkit
VIX1D is the shortest point on the family of Cboe volatility indices that continues through the 9-day VIX9D, the 30-day VIX and the 3-month VIX3M, and beyond that into the VIX futures curve covered in our term structure guide. Reading the short end against the long end is where the index earns its keep: a VIX1D well above the VIX says the market is pricing a specific near-term event, while a VIX1D far below the VIX says the coming session looks quiet relative to the month.
There is nothing to trade on it directly. Doar's launch note states: "We currently have no plans to develop derivatives based on the VIX1D Index." Its use is as a gauge for same-day SPX trading, where our zero DTE guide covers the mechanics and the 0DTE SPX strategy guide covers matching a structure to the day. SquawkFlow's VIX term structure page plots the nine monthly VIX futures settlements from Cboe's daily settlement files, which become available the following trading day; it does not carry VIX1D, so for the one-day reading itself use Cboe's index dashboard or a quote vendor under the ticker VIX1D.
What VIX1D cannot tell you
- Direction. A correlation of -0.20 with the S&P 500 in its first year means a rising VIX1D is a weak signal about which way the index moves.
- Whether today is calm. The overnight reset makes the open look quieter than the prior close by construction, before anything has happened.
- Realized volatility. It is an option-implied expectation carrying a risk premium, and the academic evidence says it runs high on average.
- A like-for-like reading across the day. The horizon at 10:00 a.m. and at 3:30 p.m. are different days.
Used with those limits in mind, VIX1D is the cleanest public read on what the busiest part of the options market expects from the next few hours.
Educational content, not financial advice. See our risk disclosure.