FX Option Quoting Conventions

Ask an equity options desk for a price and they'll quote a strike and a premium: "the $100 call trades at $3.50." Ask an FX options desk for the same thing and they'll answer with two numbers that have no strike or dollar price in them at all: "25-delta, 8.5 vol." Learning to read that sentence is the first hurdle in FX options, and it exists for a good practical reason — a strike quoted in absolute terms goes stale the moment spot moves, while a delta and a volatility level stay meaningful all day.

Why delta instead of strike

Delta measures roughly how sensitive the option's value is to a small move in spot, and conveniently that number also behaves like the option's approximate probability of finishing in the money. Quoting by delta means a market maker's "25-delta call" and "25-delta put" always describe options a similar distance out of the money on either side, no matter where spot happens to sit that day:

$$K_{25\Delta \, call} = F \times e^{(\sigma \sqrt{T} \, N^{-1}(0.25) + 0.5 \sigma^2 T)}$$

In words: given the forward rate, volatility, and time to expiry, this formula spits out the exact strike that currently has a 25% delta. Traders never actually compute it by hand in real time — systems do it — but the point is that the strike is the derived, moving number, while the delta is the fixed label everyone quotes and trades on.

Worked example

A trader wants to price a 3-month 25-delta EUR/USD call. The desk isn't asked for a strike; it's asked to quote implied vol for that delta point, currently 8.6%. Feeding the forward (1.0820), the 8.6% vol, and 3 months into the delta formula above locates the strike — say it comes out to 1.1015. Tomorrow, if spot rallies and vol ticks up to 8.9%, the same 25-delta call now corresponds to a different strike, maybe 1.1090 — the quote (25-delta, 8.9 vol) didn't need to change at all for the desk to keep making a consistent two-way market, even though the actual strike moved.

What this means in practice

The whole FX implied-volatility surface is built and traded in this delta space — 10-delta, 25-delta, and at-the-money points for calls and puts, at each tenor — which is also how risk reversals and butterflies (the standard measures of skew and smile) are defined. A quant reading a term sheet with "25D RR" or "10D BF" is reading the vol surface's shape, not a strike grid.