Potion decanting for profit in OSRS
How decanting potions at the Grand Exchange turns dose-price gaps into free profit, and what actually limits how much you can make per 4 hours.
What decanting is
Bob Barter, at the Grand Exchange, will convert any potion between its dose counts (1 to 4 doses per potion) for free. Different dose counts of the same potion often trade at different prices per dose on the Grand Exchange, and that gap, not any change to the potion itself, is what decanting profits from.
How the profit works
Buy the dose count that is cheaper per dose, decant it for free into the dose count that sells for more per dose, then sell. The profit per finished potion is the sell price minus Grand Exchange tax minus what the doses cost to assemble; Varrock's decanting tool calculates this for every potion family automatically rather than requiring you to work out dose ratios by hand.
Decanting up (fewer, bigger potions from several smaller ones) is the straightforward direction. Decanting down into more, smaller doses needs empty vials to hold the extra potions, which is its own cost and constraint, so Varrock's decanting tool only shows combining into more doses, not splitting into less.
What actually limits your profit per 4 hours
The same buy limits and trading volume that cap ordinary flipping also cap decanting: you can only buy as many source doses as the buy limit and the market's real volume allow within a 4-hour window, on both the buying side and the selling side, whichever is thinner. A route that looks highly profitable per potion but only supports a handful of trades in 4 hours will earn less overall than a lower-margin route with real volume behind it, which is why Varrock ranks decanting routes by profit per 4h rather than profit per potion alone.