The overall ESI formula is built up of different parts for NOX, SOX and CO2; additionally a bonus is awarded for the presence of an 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 SOX and no NOX and reports or monitors its energy efficiency; in other words a ship with a score of 0 points is actually in full conformity with the applicable requirements and thus OK and the ship with 100 points has zero air emissions. Actually apart from certain LNG carriers, the best performing ships, have scores that hover around 60 points; LNG carriers using boil-off gases as fuel top the list with around 80 points.

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, except for MDO/Gasoil where an additional baseline set by the ESI Working Group is used.

The weighing factor of ESI NOX in the overall index is twice that of ESI SOX. This reflects the fact that the average environmental damage from NOX in ship air emissions is approximately twice the damage from SOX. The emission characteristics of MDO/Gasoil 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 ESI SOX .

The formula for the ESI Score is: 2 x ESI NOX + ESI SOX + ESI CO2 + OPS


  • ESI NOX represents the sub-points for NOX and ranges from 0 to 100 sub-points
  • ESI SOX represents the sub-points for SOX and ranges from 0 to 100 sub-points
  • ESI CO2 is the bonus for reporting on two data sets of EEOI and is fixed at 10 sub-points
  • OPS is the bonus for the presence of an OPS* on board irrespective of its use and is fixed at 35 sub-points

In the formula the sub points for ESI NOX, ESI SOX, ESI CO2 OPS are included independent of one another: if no points are included for one or more of these constituents, the ESI Score is calculated with the available data.

A maximum of 345 sub-points may theoretically be reached; since this would result in an ESI score exceeding 100, the ESI score is capped at 100 points.


ESI NOX is calculated with the NOX 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 for defining the ESI NOX score 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 NOX, 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 NOX is defined as:

ESI NOX = 100 X (NOX limit value - NOX rating) x Rated Power Σ of all Engines
Rated Power Σ of all Engines NOX limit value


The ESI SOX reflects 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 ESI SOX 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 sub-points 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 ESI SOX.

To establish the ESI SOX 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 ESI SOX 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 ESI, those BDN which have been issued during the two preceding quarters shall be 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.

ESI SOX is defined as:

ESI SOX = x * 30 + y * 35 + z * 35


  • 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 ESI SOX 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.


In IMO decisions have been reached on Green House Gas emissions that will affect the way in which the CO2 part of the ESI formula will be determined. In this respect it is recalled that in the ESI approach, a ship only scores ESI sub points, when the performance exceeds minimum standards set by IMO.

As of 1 January 2013 the Ship Energy Efficiency Management Plan (SEEMP) has become mandatory for all ships; therefore, the 10 sub points that were granted to ships where a SEEMP was available, have been discontinued.

Furthermore on 1 January 2015 the IMO requirements for an Energy Efficiency Design Index will enter into force for New Ships. The Energy Efficiency Operational Indicator (EEOI) remains a voluntary non-mandatory initiative

The principles used in establishing the Ship EEOI have been selected by the ESI Working Group to define the way forward to promote fuel efficiency in ships. As a first step on 1 January 2013 an incentive of 10 ESI sub points is introduced for voluntary reporting 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).

Supply of data on the type of cargo and/or the value of the EEOI is not required but may be considered.

This approach for EEOI related incentives will be further developed in the next few years.

In order to simplify the entry of data, the period of time over which the two EEOI data sets would need to be provided to ESI should coincide with the half year period over which data on BDNs are provided for a particular ship i.e. where in order to calculate the SOX part of the ESI score on 1 April, an Owner would need to provide the BDNs for a ship over the 3rd and 4th quarter of the previous year, the two EEOI data sets should be provided over that same period.

To allow sufficient time for provision of data related to EEOI, the first calculation of ESI scores in which use is made of sub points related to EEOI will take place on 1 July 2013 for all vessels using the formula referred to above. From that date the ESI Score CO2 is calculated with the EEOI data provided.


* Click here for explanation of OPS in FAQ section