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.
Russian-speaking traders coined «одноногий арбитраж» — one-legged arbitrage, where a leg is one side of a paired trade. English communities say single-leg, one-sided, asymmetric or half-hedged arbitrage. The "arbitrage" label is loose: without a second real leg there is no perfect hedge. It stuck because the entry and exit logic comes from cross-exchange spread analysis, not from a chart pattern.
Classic vs. One-Leg: Side by Side
Classic, two legs
- Delta neutral. A 5% jump in the coin moves the long leg +5% and the short leg −5%, netting zero.
- You profit only from the spread narrowing. Boring by design, which is what makes it safe.
- Margin on both venues: about $200 each per $1 000 of notional at 5x, so $400 locked.
One-leg
- Unhedged. The same 5% jump moves your PnL by 5% of notional, in the direction of the live leg.
- Half the margin: $200 on one venue for the same $1 000 of notional.
- Across ten cards that is $2 000 locked instead of $4 000. The freed capital is paid for in directional risk on every card.
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.
How It Works Inside Arbitron
- 1
Both feeds stay live
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.
- 2
One real order at open
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.
- 3
One real order at close
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
- 1
Upper direction: A is cheaper
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.
- 2
Lower direction: A is pricier
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
PnL = (close − open) × quantity − fees on the one venue- close − open
- The real leg's own price move between your open fill and your close fill.
- fees
- Taker fees on one exchange, not two. Half the fee drag of a classic cycle.
The counter-leg never enters cash flow. Its price is recorded as a synthetic reference so you can see the spread you traded against, nothing more.
Classic, two legs
- 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
- 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 signal fires with ARB at $1.5200 on Exchange A and $1.5230 on B, a 0.20% spread. You configured Buy on A for the upper direction, so Arbitron opens 1 000 ARB long on A at $1.5200: $1 520 of notional. B's $1.5230 is reference only. The spread mean-reverts, A prints $1.5225, and your real close gives $2.50 gross, about $1 net after round-trip taker fees. Had A drifted to $1.5150 while the spread still converged, you would be down $5 real on a "good" spread. That is the directional exposure.
The Risks You Must Understand
Directional exposure
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 it
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 is the slow leak
Funding rate cost is the slow leak. Every 1, 2, 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.
A widening spread can keep widening
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.
Venue outages leave you naked
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.
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.
When One-Leg Is the Right Choice
You have a directional view
One-leg makes sense when you hold a directional view the spread signal does not capture. Say you think a mid-cap has bottomed, you see consistently wide spreads on it, and you want to be long anyway. One-leg gives you that long, with entries and exits timed by the spread. The spread provides the edge on when; your thesis provides the direction.
One venue is not tradable for you
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.
- 1
Paper-track the spread on your pair for a few days: confirm it genuinely mean-reverts instead of being structurally biased.
- 2
Pick the exchange with the deepest book, the lowest fees or the friendlier funding. That becomes your active leg.
- 3
Open a one-leg card at the smallest order size the venue allows, 1x or 2x leverage at most, with conservative thresholds.
- 4
Run it through different market regimes for a full week, not one afternoon.
- 5
Compare realized PnL against what a classic card would have made on the same signals, and scale size only if the directional drift went your way more often than not.
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.