The ARK 21Shares Bitcoin ETF holds the same bitcoin today that it held last week. What changed on August 27 is which exchanges decide what that bitcoin is worth. Coinbase, the biggest dollar-bitcoin venue in the United States, is no longer one of them.

ARKB trades on the Cboe BZX Exchange and reported 101,190,000 shares outstanding on August 3, with net assets just under $2.8 billion late last week. Its most recent quarterly report put the trust's holdings at 32,178.2280 bitcoin at the end of June. Every one of those shares is now marked against a benchmark the fund had never used before Thursday.

What the filing actually says

21Shares US LLC, the fund's sponsor, filed an 8-K on August 26 covering two agreements. On August 20 it signed a benchmark licensing agreement with FTSE International Limited for the FTSE Bitcoin Index. That followed a June 30 notice terminating its license with CF Benchmarks Ltd. for the CME CF Bitcoin Reference Rate — New York Variant, effective August 31. The sponsor and CSC Delaware Trust Company, the trustee, also executed a Second Amended and Restated Trust Agreement to write the new benchmark into the fund's governing document.

A prospectus supplement filed the same week shows the swap as a redline. The old sentence named CF Benchmarks as the index provider. The new one names FTSE. The bitcoin, the 0.21% sponsor fee and the Cboe listing all read the same as before.

Why a bitcoin ETF needs a 4 p.m. price

Bitcoin trades every hour of every day at slightly different prices on every venue. A fund has to publish one number: net asset value per share, once a day. A reference rate is the agreed answer — a single price for one bitcoin at one moment, built from real executed trades on a named list of exchanges, with rules meant to make the number expensive to push around.

Authorized participants create and redeem shares against it. Every performance chart you have seen for a spot bitcoin fund is drawn from it.

Same 4 p.m. fix, different arithmetic

Both benchmarks land on one daily number at 4:00 p.m. Eastern, built from the hour before it. The methods inside that hour differ.

CF Benchmarks cut the window into twelve five-minute partitions, took the volume-weighted median of every qualifying trade in each partition, then averaged the twelve equally. A median throws away how far an outlier sits from the crowd. It only cares which side the outlier is on.

FTSE reads a price every 15 seconds — 240 observations between 3:00:15 and 4:00:00 p.m. — and volume-weights them into a single average. The filtering happens first. Any constituent exchange whose VWAP over the trailing ten minutes sits more than 1.5 standard deviations from the mean across all of them is thrown out. Individual trades more than 2.5 standard deviations from the ten-minute cross-exchange mean are thrown out too. Quotes never count, only executed trades, priced net of transaction costs.

On a calm afternoon the two numbers land within a few dollars of each other. On a violent one they can diverge.

The exchange list is the real change

The CF index priced ARKB off six venues: Coinbase, Bitstamp, itBit, Kraken, Gemini and LMAX Digital.

The FTSE index, per the fund's own disclosure as of July 31, 2026, prices off eight: Kraken, Bitstamp, Bitfinex, LMAX, Gemini, bitFlyer, itBit and Luno.

Coinbase drops out of that list entirely. Bitfinex, a British Virgin Islands trading platform, comes in at 16.194% of pricing volume. bitFlyer and Luno join at the bottom. Kraken, one of six contributors under the old rate, now carries 43.470% of the volume behind the number that sets the fund's NAV.

Share of pricing volume in ARKB's new benchmark

FTSE Bitcoin Index constituent exchanges by share of bitcoin-dollar pricing volume, May 1 to July 31, 2026, as disclosed in the fund's prospectus supplement.

The dollar volumes behind those percentages, over the same three months: Kraken $15.51 billion, Bitstamp $7.68 billion, Bitfinex $5.78 billion, LMAX $4.45 billion, Gemini $1.37 billion, bitFlyer $555 million, itBit $306 million, Luno $27 million. Two exchanges account for roughly two-thirds of it.

Coinbase Custody Trust Company is still one of the fund's bitcoin custodians, alongside BitGo Bank & Trust, Anchorage Digital Bank and BitGo New York Trust. The fund trusts Coinbase to hold the coins. It no longer uses Coinbase to price them.

The fund warns about this itself

The prospectus supplement carries a new risk factor written specifically for the transition, and it is blunter than most. There is "no assurance that the Index will produce the same bitcoin price as the Prior Index at any given time," it says, "and differences between the two may be material." NAV under the new benchmark may come out higher or lower than it would have under the old one. Concentration of pricing volume in one venue, the filing adds, leaves NAV more exposed to a technological failure or a regulatory action at that venue.

The same document flags that the new index has a limited performance history and has carried its current roster of pricing venues only since October 20, 2023. FTSE is authorized as a benchmark administrator by the UK Financial Conduct Authority under the UK Benchmark Regulation, and its digital asset index series is vetted with Digital Asset Research. None of that makes the output identical to what CF Benchmarks published.

What this means if you hold ARKB

BlackRock's IBIT, the largest US spot bitcoin fund, still prices off the CME CF Bitcoin Reference Rate — New York Variant, Coinbase included. Two funds holding the same asset now stamp it at 4 p.m. from different venue sets. Premium and discount figures and end-of-day comparisons between them get noisier from here.

The second change in the 8-K is smaller and easy to skip past. The 0.21% sponsor fee is paid in bitcoin, and its payment schedule moved from weekly in arrears to at least quarterly in arrears. Accrual is still daily, so the drag on your NAV per share works out the same. What shifts is cadence: fewer, larger transfers of bitcoin out of the trust over the course of a year.

My read is that the index provider's name matters far less than the concentration sitting behind it. A single exchange supplying 43% of the pricing volume for a $2.8 billion US-listed fund is the line worth underlining, and it is a bigger story than which London firm gets the licensing fee.

The clause most holders have never read

One sentence in the prospectus deserves a second pass: the sponsor may change either the index or the index provider at its sole discretion, and shareholder approval is not required. Holders find out afterward, through a prospectus supplement or a periodic report. That language is standard across these trusts. It is also the mechanism that produced this week's change, start to finish, with no vote.

A few things worth keeping an eye on:

  • ARKB's premium and discount to NAV over the next few weeks, measured against a fund that stayed with CF Benchmarks. A widening gap on volatile days is methodology, not bitcoin.
  • The constituent exchange list at the next quarterly review. FTSE re-tests venues on volume, licensing, downtime and security history, and the roster can move.
  • Whether other 21Shares products follow. The sponsor runs several US crypto trusts, and benchmark licenses are negotiated at the sponsor level.

The index section of an ETF prospectus is the driest page in the document. It is also the page where the price you are handed at 4 p.m. gets decided. For a fund whose entire job is to hand you one number a day, the list of exchanges that produce that number is the product.