Originally it was written as a gambling aid. We wanted ratings that would reflect the most recent results, but would still take into account past performance. When placing a bet most bookies pay on the result at 90 minutes (win, draw, or lose), so the only information we enter into the calculator is the result at 90 minutes. Extra time and penalty shoot outs are ignored. We feel this gives a much better and more accurate reflection of the teams performance.
The ratings are based on an average of 100 points. Teams nearer this mark are average, teams above are better and teams below are worse. Top teams will be around 115 to 120 points, the worst teams will be in the low 80's or below. The calculator works by calculating the expected result of the match at 90 minutes, then compares the actual result. Depending on how close the actual result is to the expected result affects the new rating. Stronger weight is given to competative matches over friendlies, and home advantage is also taken into concideration. From June 2011 a limit of 4 rating points was set to prevent massive swings from occasional high scores. From this time the maximum number of rating points a team could move from a single match was 4.
The ratings of two teams are compared. For example team a (home) has 115 points and team b (away) has 108. If team a is actually playing in their home country (as opposed to a neutral country), an extra two points are added for home advantage. This now gives the home team 117 points. The 108 is taken from 117 to give a 9 point difference. Each 4 points represents a goal so the home team would be expected to win in 90 minutes by a 2 to 3 goal margin. The calculator doesn't predict the actual score, just the result to be a win, draw or loss. Though in the above example you could place a bet on a 2-0 or 3-1 home win if that's your thing.
No, just use a pencil and paper if you prefer and the information from the web site. Ratings remain relatively accurate over a month.