MARPOL Annex VI does not mandate a specific reserve quantity, but best-practice guidance from flag states and OCIMF recommends a minimum 48-hour steaming reserve on arrival at the next port. Many operators maintain 10–15% of voyage consumption as reserve. The IMO III Code (flag state instrument) may impose additional requirements for vessels on unmanned machinery spaces.

A 48-hour reserve is widely recognised as the minimum safety margin for commercial vessels. It covers weather delays, port congestion, engine efficiency drops, and deviation to an alternative port in an emergency. Many P&I clubs and charterers set this as a standing instruction in voyage orders.

Weather routing services (e.g., Applied Weather Technology, Bon Voyage System) can reduce fuel consumption 3–8% by avoiding heavy weather that increases resistance. ETA calculations should treat the fuel rate as a calm-water baseline and add a weather factor of 5–10% for exposed ocean passages, especially North Atlantic winter and Cape of Good Hope routes.

Industry practice is 10–15% of estimated voyage consumption as a reserve, but voyage orders and SMS procedures vary. Tankers under SIRE inspection are expected to hold at least 48 hours at sea speed. The calculated reserve in this calculator is a separate fixed buffer based on reserve hours — it is additional to the voyage fuel.