this is just a general idea, I can’t give a specific example because information about the fields your using is incomplete.
$total_footage = 0;
if (have_rows('repeater')) {
while (have_rows('repeater')) {
the_row();
if (get_sub_field('available')) {
$total_footage += get_sub_field('square_footage');
}
}
}
echo $total_footage;