Funding rate arbitrage, and what it really costs

Funding exists to keep a perp near spot. When the perp trades above the index, longs pay shorts; when below, shorts pay longs. In a market where leveraged demand is persistently long, funding is persistently positive, and somebody who is short the perp is paid for standing on the unpopular side. Pairing that short with a spot holding of the same size removes the price risk and leaves the payment.

Suppose you hold ten thousand dollars of BTC spot and short ten thousand dollars of BTC perp. Funding is running at one basis point every eight hours, so three basis points a day, which is three dollars a day on ten thousand of notional. Annualised on that notional it is about ten point nine five percent. But the capital committed is not ten thousand: it is the ten thousand of spot plus whatever margin the short needs. At four times leverage that is another two thousand five hundred, so twelve thousand five hundred is tied up and the gross return on capital is closer to eight point eight percent. Every honest version of this calculation divides by the capital, not by the notional.

You do not strictly need spot. If one venue is paying positive funding on a market and another is paying negative on the same market, going long where you are paid and short where you are paid is delta neutral across the pair and collects both legs. The catch is that you now have margin at two venues, two liquidation levels, and basis risk between two books that can diverge. The capital efficiency is better and the operational risk is meaningfully worse.

The gross number above is a ceiling that very few positions actually reach.

• Funding is not a fixed rate: it is reset every period and can flip sign without warning
• Fees on four fills, since both legs have to be opened and closed
• The spot leg usually cannot be posted as margin for the short, so it sits idle on another venue
• Bridging, gas and the time the capital spends in transit
• Venue, contract and oracle risk on both sides, which is the tail that makes this not an arbitrage

The characteristic failure is not funding turning negative, it is the short leg being liquidated in a rally. Take the same position with four times leverage on the short: it is closed somewhere around a twenty four percent rise. Your spot leg has gained twenty four percent, so on paper you are level, but the spot gain is unrealised and sitting somewhere else while the short has been realised as a loss and the hedge no longer exists. You are now long the market at the top of a rally, with no position to offset it, at exactly the moment you had intended to have no view at all.

Margin the short leg far below the venue maximum, so that the liquidation level sits outside any move you consider plausible. Keep spare collateral where it can reach the short venue quickly. Decide in advance the funding level at which the trade is no longer worth the operational risk and close it there rather than waiting to be persuaded. And size the whole thing as an allocation to a specific venue, because that is what you are underwriting.