Formulas
General
The ESI formula is built up of different parts for NO_{X}, SO_{X} and CO_{2}; additionally a bonus is awarded for the presence of an Onshore Power Supply installation (OPS). The ESI Score ranges from 0 for a ship that meets the environmental performance regulations in force to 100 for a ship that emits no SO_{X} and no NO_{X} and reports or monitors data to establish its energy efficiency; in other words a ship with a score of 0 points is actually in conformity with the applicable requirements and thus OK and the ship with 100 points has zero emissions to air.
The best performing ships have scores that are close to 100; these ships use fuels with sulphur contents close to or equal to 0 % and are fitted with NO_{X} reduction systems. Adding points for ESI CO_{2} and/or fitting such vessels with OPS would result in an ESI Score over 100 and to prevent this, the ESI Score is capped at 100.
By comparing the actual performance of a ship with set baselines, the ESI score can be calculated. The baselines are based on the IMO regulations in force, additional for MDO/Gasoil the ESI Working Group has set another baseline.
The weighing factor of the NO_{X} sub points in the ESI Score is twice that of the SO_{X} sub points. This reflects the fact that the average environmental damage from NO_{X} in ship emissions to air is approximately twice the damage from SO_{X}.
ESI SCORE = ESI NO_{X} + ESI SO_{X} + ESI CO_{2} + OPS (max. 100) 

ESI NO_{X}  =  x  2 x NO_{X} sub points ranging between 0 and 100  divided by 3;  
ESI SO_{X}  =  y  SO_{X} sub points ranging between 0 and 100  divided by 3;  
ESI CO_{2}  =  5  for reporting (every half year) of fuel and distance;
efficiency increase in % is added as points; total capped at 15; 

OPS  =  10  if Onshore Power Supply (OPS) installation is fitted. 
NO_{X} sub points
The NO_{X} sub points are calculated with the NO_{X} emissions levels based on the rated power per engine.
The data appearing in the EIAPP certificate(s) of the engine(s) on board a ship are used for that purpose. It should be noted that where IMO approved abatement technologies of primary or secondary nature are applied, their effects have been included in the respective EIAPP certificate(s) issued.
The baseline used in the calculation of the NO_{X} sub points is Tier I and this approach will be maintained for the next few years.
Ships that do not have an EIAPP certificate cannot obtain points for ESI NO_{X}, unless such ships have been issued with an approved statement to the effect that engines meet Tier I requirements. Alternatively, the value zero can be entered.
All Main and Auxiliary Engines must be included.
ESI NO_{X} is defined as:
ESI NO_{X} =  100  X  (NO_{X} limit value  NO_{X} rating) x Rated Power  Σ of all Engines 

Rated Power Σ of all Engines  NO_{X} limit value 
SO_{X} sub points
The SO_{X} sub points reflect the reduction in sulphur content of the fuels below the limit values set by IMO and that determined by the ESI working group. IMO limit values determine the baselines for fuels that would normally be used at the high seas and in (S)ECA’s and these will be tightened in accordance with IMO regulations. In addition there is another baseline for MDO/Gasoil set by the ESI Working Group at 0.50 % sulphur. For the calculation of the SO_{X} sub points the following fuel categories are defined:
 HIGH: Heavy Fuel Oil with a sulphur content greater than 0.50 % S but not exceeding 3.50 % S
 MID: Marine Diesel Oil / Gasoil with a sulphur content equal to or less than 0.50 % but greater than 0.10 % S
 LOW: MDO/Gasoil with a sulphur content equal to or less than 0.10 % S
The value of 0.50 % is chosen to allow more ships to benefit from the subpoints that are given for the use of MID category fuel oil.
The value of 0.50 % has been introduced by the ESI Working Group to allow more ships to benefit from the subpoints that are given for the use of MID category fuel oil.
The cleaner emission characteristics of fuel oils of the MID and LOW categories result in their preferred use in ports and their approaches with mandatory requirements in place in certain ports and areas. This larger impact on improving conditions in ports and their approaches is the reason that these fuels carry more weight in the formula for determining the SO_{X} sub points.
To establish the SO_{X} sub points for a next period of validity of the system is set up as follows: once a ship has been entered into the database, the first SO_{X} sub points will be established at the first day of the next quarter of the year and will have a validity of half a year. Consequently the system establishes the scores on every 1st January, 1st April, 1st July and 1st October for newly entered ships, while for each ship that is already included in the database, the calculation is only performed twice a year.
Alternatively where scrubbers are used the equivalent sulphur percentage established in accordance with IMO procedures may be used.
For all bunker operations, Bunker Delivery Notes (BDN) shall be issued. At the date of submission of data for calculation of the SO_{X} sub points, those BDNs which have been issued during the two preceding quarters shall have been recorded. The data of each BDN such as type of fuel oil, mass and percentage (m/m) of sulphur must be accurately entered into the database.
SO_{X} sub points are defined as:
SO_{X} sub points = x * 30 + y * 35 + z * 35 where:
 x = the relative reduction of the average sulphur content of HIGH
 y = the relative reduction of the average sulphur content of MID
 z = the relative reduction of the average sulphur content of LOW
More calculation scenarios for SO_{X} sub points have been developed. General information may be found in the FAQ section and at request the ESI administrator will provide additional information to interested parties.
ESI CO_{2}
The principles used in establishing the Ship EEOI have been selected by the ESI Working Group to increase fuel efficiency in ships. As a first step ships should report on two of the EEOI data sets viz. fuel consumption and distance sailed; the data provided should be as described in the definitions section of the Guidelines for Voluntary Use of the Ship Energy Efficiency Operational Indicator (MEPC.1/Circ.684).
ESI CO_{2} is a measure for the efficiency of a vessel and is calculated based on comparison between a base line period of 3 (three) years in which the totals of fuel consumption and distance sailed in that period are reported; the efficiency over a full year following that baseline period is also calculated.
Reporting during the 3 year base line period and any increase of efficiency in the following year(s) (2 BDN reporting periods) compared to the base line period, adds to the ESI score as shown above.
The calculation of efficiency is executed at the ESI Score calculation date for the same period over which the BDNs received, are entered; the half year period over which fuel used and distance sailed are requested, coincides with previous BDN reporting period.
OPS
* Click here for explanation of OPS in FAQ section