About this calculator
Ship fuel, or bunker, is often the biggest cost of a voyage, up to half of the voyage costs for a bulk carrier. This calculator estimates how many tonnes of fuel are burned and what they cost, and shows how much you save when the ship slows down.
The method follows what operators use. Sea days = distance ÷ speed ÷ 24, plus a weather margin. Main engine consumption is entered in tonnes per day at a reference speed, for example 20 tonnes per day at 12 knots. When you choose another speed, consumption is adjusted with the cube law (propeller law): fuel per day is proportional to the cube of speed, so fuel per mile is proportional to its square. Dropping from 12 to 10 knots cuts fuel per mile by about 31%, but the voyage takes longer. Auxiliary engine consumption at sea and consumption in port are counted separately, usually as MGO (marine gas oil), while the main engine burns VLSFO, or HSFO when the ship has a scrubber.
The sensitivity table shows sea days, tonnes and cost for several speeds around the one you picked. The cube law is reasonably accurate within about 20% of the reference speed; beyond that use the ship's own consumption curve from noon reports or sea trials. Bunker prices change daily per port, so enter the latest price from your supplier. The result is a planning estimate, not an official consumption claim.
Worked example
A 1,000 nm passage at 12 knots, main engine 20 t/day at 12 knots, auxiliaries 2 t/day, 2 port days × 3 t/day, VLSFO USD 600/t, MGO USD 800/t, no margin:
- Sea days: 1,000 ÷ 12 = 83.33 hours = 3.47 days
- Main engine: 3.47 × 20 = 69.44 t × 600 = USD 41,666.67
- MGO: 6.94 t (sea) + 6 t (port) = 12.94 t × 800 = USD 10,355.56
- Total USD 52,022.22, or USD 52.02 per nautical mile
At 10 knots the main engine drops to 20 × (10/12)³ = 11.57 t/day; the 4.17-day passage burns 48.23 t, about 31% less than 69.44 t.
How to use it
Enter distance and speed
Enter the voyage distance in nautical miles, the planned speed and a weather margin in percent.
Enter the ship consumption
Enter main engine consumption at the reference speed, auxiliary consumption at sea, port days and port consumption.
Enter bunker prices
Enter the VLSFO (main engine) and MGO (auxiliaries and port) price per tonne, then compare speeds in the sensitivity table.