Tetris factor is defined as
tf = ( lpm × field_width ) / ( 4 × tpm )
Where tf is tetris factor, lpm is number of cleared lines per minute, field_width is the width of the tetris playfield, and tpm is the number of tetriominoes per minute. In a standard tetris game where the field_width variable is 10, the formula simplifies to:
tf = 2.5 × lpm / tpm
The tetris factor is the percentage of tetrominoes which were built to fill the lines (cleared lines). Tetris factor is 1 in case where all the lines in the playfield are cleared (no garbage left).
Normally, a tetris game ends with a playfield full of incomplete lines. Hence
1 − Tetris factor
is the percentage of the left-over tetrominoes in the playfield relative to all other tetrominoes in the game which were successfully stacked to cleared lines. The longer the tetris game lasted, the more tetris factor converges towards 1 because the left-over tetrominoes become a relatively small percentage of all the tetrominoes played.
Tetris factor is therefore also usable as am additional measure of tetris gameplay skill.