The system is primarily based on the principal that current data is 'more relevant' than older data. This is achieved by a mathematical process known as 'exponential smoothing' and means that for instance the fact that a team lost 5-0 at home last week is much more relevant than the fact that they lost 5-0 at home two months ago. This also means that the system reacts quickly to any change of form by a particular team. The system captures all the data needed to compute the spreads offered by the spread betting firms (goals, corners, yellow and red cards, time of each teams first goal, time of second and last goal,  the aggregate time of all goals and the referee).

The system calculates how much better a team performs at home than away. For instance the system might calculate that Newcastle score 0.65 more goals at home than they do when playing away and will adjust the Newcastle away goals accordingly if they are playing at home. The reverse adjustment is made if a team is playing away (i.e. the home goals are adjusted). This calculated adjustment is made for all the data that is relevant to the various spreads (goals, corners, yellow and red cards, time of a team's first goal, wins, draws, losses and clean sheets).

The system also calculates whether a team is playing a team that is better or worse than average (and how much better or worse) and adjusts all the spreads accordingly. It is not only the goals and corners gained that are taken into account. The system also uses the goals and corners conceded by each team when calculating the spreads with the same principal being applied to the time of goals conceded. This adjustment is also made for wins, draws, losses and clean sheets.

The past form of the referee in charge of the match is taken into account when calculating the projected Yellow and Red cards for each team. The system calculates the adjustment that should be made (i.e. whether the referee is stricter than average or less strict than average and by how much) and adjusts the card figures accordingly.

The system also adjusts the Yellow and Red cards for the time of the year (more cards are issued in the first few months of the season compared to the last few months of the season) and a further adjustment is then made depending on whether the fixture is regarded as a 'local derby' (it is evident that local derbies produce more Yellow and Red cards than non local derbies).

When a team has been recently promoted or relegated the system calculates how the past form for the team in the 'wrong' division should be adjusted. For instance if a team was relegated from the Premiership to the Championship then the form for the previous season in the Premiership has to be adjusted as the team is now playing in the Championship (a team that has recently been relegated would normally be expected to perform better in the lower division). The reverse adjustment is made for teams that are promoted from the Championship to the Premiership.

These are only a few of the calculations and adjustments that the system performs when evaluating a particular fixture. The system has been carefully tuned over the years to produce ratings as close as possible to what actually occurs. The system is constantly being updated and as more data becomes available the ratings will become increasingly accurate.

In 2005 the original Soccer Spreads system was greatly enhanced to also use a variation of the ELO ratings system. The ELO system was originally devised by Dr. Arpad Elo and is used by FIDE to rate chess players. In 1997 Bob Runyan adapted the system to international football and posted his results on the internet. Soccer Spreads uses a variation of this system adapted for the English Premiership and Championship. The Soccer Spreads ratings are now an amalgamation of the original system and the variation of the ELO ratings system.

Once the expected ratings for a particular match have been calculated the Soccer121 system then examines the previous history of the calculated ratings (from the last eight years) to determine how often a particular event occurs and determines the fixed odds for the relevant markets.

For instance the system might predict that a match will have a ratings supremacy of 0.64 and produce 2.74 goals. The Soccer121 system will then examine the historical database to find several hundred previous matches with a previous predicted ratings supremacy of around 0.64 and around 2.74 goals. The data held for these matches is then used to produce the fixed odds ratings for the various goal and supremacy related markets. The same process is then repeated for the Bookings and Corners markets although for these markets fewer previous matches are used.

Where the predicted supremacy or total goals is too high or too low then the system will not be able to find enough previous matches in the historical database to produce an accurate rating and no fixed odds for the match can be generated. In this case the odds will appear as N/A (not available) in the spreadsheet. This also applies to the Corners and Bookings markets.