What Is One-Leg Arbitrage?
One-leg arbitrage — also known as single-leg or one-sided arbitrage — is a strategy where you use the price difference between two exchanges as your entry and exit signal, but you only place a real order on one of them. The second exchange is never traded. It exists purely as a reference: its orderbook tells you when the spread is wide enough to enter, and when it has mean-reverted enough to exit.
Think of classic arbitrage as two orders tied together by a rubber band — one buys cheap, one sells expensive, they cancel each other's price exposure, and you profit purely from the spread narrowing. One-leg cuts the rubber band. Only one order is real. You still use the spread signal from the pair, but you are taking a directional bet on the single exchange you picked.
This makes one-leg a hybrid between pure arbitrage and a directional trade. You keep the statistical edge of spread signals — entries still require a meaningful pricing dislocation between two venues — but you accept full directional exposure on the active leg in exchange for higher capital efficiency, fewer fees, and the upside of being right about market direction on top of the spread capture.
The strategy goes by several names. Russian-speaking traders coined the term "одноногий арбитраж" — literally "one-legged arbitrage" — where "leg" refers to one side of a paired trade. English-speaking communities use "single-leg arbitrage", "one-sided arbitrage", "asymmetric arbitrage", or "half-hedged arbitrage" to describe the same mechanic. The "arbitrage" label is technically loose — without a second real leg, there is no perfect hedge — but the term has stuck because the entry and exit logic comes from cross-exchange spread analysis, not from a directional chart pattern. Whatever the label, the core remains identical: two exchanges, one real position, one synthetic reference.
Classic vs. One-Leg: Side by Side
Classic arbitrage is delta-neutral. When a signal fires, Arbitron simultaneously buys on one exchange and sells on the other, in equal quantities. The long and short legs offset each other: if the underlying asset jumps 5%, your long leg gains 5% and your short leg loses 5%, netting zero price impact. You profit only from the spread narrowing. This is the textbook market-neutral strategy — boring by design, which is exactly why it is safe.
One-leg removes half of that equation. When the same signal fires, Arbitron places a real market order on only the exchange you configured and leaves the other side untouched. Your position is no longer hedged. A 5% jump in the underlying asset directly moves your PnL by 5% of notional, in the direction of your active leg. The capital committed is roughly half of what classic arbitrage requires, but so is your safety margin.
Both modes share the same signal engine. Spread thresholds, depth multiplier validation, signal strength, cooldowns, auto-close, scale-in — everything works identically. The only thing that changes is execution: two real orders become one real order plus a reference price snapshot from the counter-leg. Funding tracking, PnL display, and position management all continue to work the same way.
The capital efficiency difference is concrete. Consider a $10,000 trading account. A classic card running 5× leverage on $1,000 of notional ties up about $200 margin on each exchange — $400 total — and the position can take a 5% adverse move before margin pressure becomes serious. The same $1,000 notional in one-leg mode ties up only $200 margin on one venue, freeing $200 of capital for another trade. Across ten concurrent cards, the difference compounds: $4,000 of locked margin classic versus $2,000 one-leg. That extra $2,000 can fund additional cards or stay as a buffer. The trade-off is direct: each one-leg card now bears full directional risk. A 5% adverse move on a $1,000 long is a $50 unrealized loss, regardless of what the spread did. Capital efficiency is real, but it is paid for in directional volatility.
How It Works Inside Arbitron
When a one-leg card arms and waits for a signal, Arbitron monitors both exchanges with real-time orderbook feeds just as in classic mode. Spreads, signal strength, depth multiplier checks — nothing about signal generation changes. The counter exchange is not turned off; it is actively providing the reference that makes the signal meaningful.
The moment the spread crosses your Open threshold, Arbitron captures a snapshot of the best bid and ask from both venues. It places a market order on the active leg you configured and records the counter-leg's BBA as a synthetic reference fill. This synthetic fill is never sent to any exchange — it exists only inside the state machine, as a frozen price used for PnL context and dashboard visualization.
When the position closes, the same logic applies in reverse: a real closing order goes to the active leg, and a fresh reference is captured from the counter-leg. Your actual profit or loss comes entirely from the real leg's open and close prices, minus the fees on that one exchange. The synthetic reference is not part of cash flow — it only shows the spread context you traded against, so you can judge whether the signal worked as intended.
Choosing Which Leg to Execute
Every one-leg card asks you two questions at creation time. The first is which leg to execute on the Upper spread direction — when Exchange A is cheaper than Exchange B (classically "Buy A, Sell B"). Your options are Buy on A (long position on A) or Sell on B (short position on B). Both capture the same spread, just on different venues.
The second question is which leg to execute on the Lower spread direction — when A is more expensive than B (classically "Sell A, Buy B"). Your options are Sell on A or Buy on B. This is independent of the Upper choice, giving you four possible combinations: always on A, always on B, A-on-upper plus B-on-lower, or the reverse. Both mode oscillates between them automatically; Long and Short modes use only one.
The choice depends on where you prefer to hold risk: liquidity, fee structure, funding rates, and your own confidence in the venue. If Exchange B has tighter order books and lower taker fees, trade there. If Exchange A has consistently positive funding on shorts, use it for the short direction. Your selection can be changed at any time while the card is closed — it is a configuration value, not a commitment.
How Profit and Loss Are Calculated
Classic PnL is straightforward: profit equals the spread captured at open minus the spread captured at close, multiplied by order quantity, minus fees on both exchanges. Price movement cancels between the two legs, so the math depends only on how the spread itself behaved between entry and exit.
One-leg PnL is directional. There is no second real fill to cancel the first, so profit is the straight difference between the real leg's open and close prices, multiplied by order quantity, minus fees on the one exchange you traded. If you opened a long at $152.00 with 10 units and closed at $152.50, you earned $5.00 minus about $0.15 in taker fees. If the close was at $151.50 instead, you lost $5.00 plus fees — even if the spread signal said the trade worked.
A concrete example: a signal fires when Exchange A trades at $152.00 and Exchange B at $152.30 (a 0.20% spread). You configured "Buy on A" for the upper direction. Arbitron opens a 10 SOL long on A at $152.00, committing $1,520 of notional. The reference on B is $152.30 but it does not touch cash flow. Later the spread mean-reverts: A is at $152.25, B is at $152.28. Your real close at $152.25 gives $2.50 gross, minus about $1.50 in round-trip taker fees, for roughly $1 net. Had A instead drifted down to $151.50 while the spread still converged, you would be down $5 real despite the "good" spread. That is the directional exposure you signed up for.
The Risks You Must Understand
Directional exposure is not a bug — it is the whole point of the strategy. But it is also the primary risk. A classic card can survive a 10% flash move in the underlying with roughly zero PnL impact, because both legs move together. A one-leg card on the same pair will gain or lose 10% of notional on the same move. Over the lifetime of a long-held position, price volatility dwarfs any spread profit you are likely to capture.
Leverage amplifies the problem. At 2× leverage, a 5% adverse move is a 10% loss on margin. At 5×, it is 25%. At 10×, a single flash crash can trigger liquidation before any safety stop fires. Classic arbitrage tolerates leverage because net delta is near zero; one-leg does not. Keep leverage low — 2× or less is strongly recommended — and size positions by the amount you can afford to lose on a directional move, not by the spread capture math.
Funding rate cost is the slow leak. Every 1, 4, or 8 hours, depending on the exchange and symbol, the active leg either receives or pays funding. In classic arbitrage, the two legs partially offset each other; in one-leg, you pay or collect the full amount. A negative funding rate of 0.03% per 8h against your direction compounds to about 2.7% per month — enough to erase a moderate spread profit if you hold too long on the wrong side. See Funding Rate Explained for how to read the rates.
Widening spreads can keep widening. The premise of the signal is that extreme spreads mean-revert. Most of the time they do. Sometimes they do not — during exchange outages, delistings, cascading liquidations, or major news events, one venue can drift dramatically away from the other. In classic arbitrage, a spread that keeps widening produces unrealized loss bounded by the spread itself. In one-leg, you are exposed to the full underlying move on top of any spread drift, with no natural upper bound until you close.
Arbitron includes a safety stop that auto-closes one-leg positions at −$500 absolute loss or −10% of notional, whichever comes first. This is a floor, not a plan. It exists to prevent account-wipeout scenarios from illiquid venues or severe gaps. You should still set your own position sizing and discipline to exit far earlier than that floor. Read Risk Management before running one-leg cards with meaningful size.
Exchange-side risk amplifies directional exposure in subtle ways. If your active leg's exchange experiences API rate limiting, websocket disconnection, or scheduled maintenance during a volatile move, you can be locked out of closing exactly when you need to. Classic arbitrage on a working counter-venue at least keeps you hedged while you wait; one-leg leaves you exposed. Plan around announced maintenance windows on the venue you trade, watch the exchange's status page, and consider closing positions before known high-impact events — CPI releases, FOMC decisions, major funding settlements — when slippage and outages spike together.
When One-Leg Is the Right Choice
One-leg makes sense when you have a directional view the spread signal does not capture on its own. For example: you believe BTC has bottomed, you see consistently wide spreads on a pair where BTC is cheaper on one venue, and you want to be long BTC anyway. One-leg gives you the long position you want, with entries and exits timed by the spread signal. The spread provides the edge on when; your thesis provides the direction.
One-leg also makes sense when one of the two exchanges is not realistically tradable for you — because of withdrawal restrictions, KYC limitations, regulatory concerns, or extremely thin liquidity on the counter side. Classic arbitrage needs capital on both venues; one-leg needs capital only on the one you actually trust. When you pick the same leg for both Upper and Lower directions (for example Buy-on-A on Upper and Sell-on-A on Lower), you only need an API key on that one exchange — the reference venue is read from public orderbook feeds and requires no credentials at all. This is the original Russian-language meaning of одноногий арбитраж: traders treat one venue as a reference and execute only where they can reliably settle.
When in doubt, stay classic. Classic arbitrage is the default for a reason: it is market-neutral, predictable, and downside is bounded. Choose one-leg only when you have a concrete directional thesis, you accept the added volatility, you size positions accordingly, and you have explicitly decided — not drifted — into taking directional risk. See Trading Modes for how Long, Short, and Both interact with the active leg you pick.
A practical onboarding workflow looks like this. Start by paper-tracking spread signals on a pair for a few days to confirm the spread genuinely mean-reverts and is not just structurally biased. Pick the exchange where you have the deepest liquidity, lowest fees, or favorable funding — that becomes your active leg. Open a one-leg card at the smallest order size your exchange allows, 1× or 2× leverage at most, with conservative Open and Close thresholds. Run it through different market regimes for a full week. Compare realized PnL to what a classic card would have produced on the same signals — Arbitron's signal history makes this comparison straightforward. Only scale size once the directional drift across the test period worked in your favor more often than against you.
Pro Tier and the Safety Gate
One-leg arbitrage is a Pro-tier feature. Basic accounts cannot create or modify one-leg cards. This gating is deliberate: directional trading requires more experience and more active risk management than classic arbitrage, and we want users to opt in consciously rather than stumble into higher risk accidentally.
The first time you toggle "One-Leg Arb" in the card creation dialog, a risk acknowledgment modal appears with a plain-language summary of the directional exposure, liquidation risk, and funding cost. You must check the acknowledgment box and confirm before the feature unlocks for your session. The acknowledgment is saved locally so it does not repeat on every card — but the risks do not disappear just because the modal does.
If you are on the Basic tier and want to try one-leg, upgrade from the Subscription page. If you are already Pro and still see the option as disabled, reload the page — the tier check runs at card creation time, not on every render. Once unlocked, one-leg appears as a toggle at the top of the New Card dialog, next to Classic Arb.