Up there, about 12 kilos. Down here, maybe 25 lbs

Here's one formula:

Northern Pike Weight

The formula to calculate the weight for a Northern Pike that is 40 inches long and has a girth of 20 inches is as follows. Northern Pike have a shape factor of 1000 and a girth ratio of (.52)

WEIGHT = LENGTH x GIRTH² / SHAPE FACTOR

WEIGHT= 40 x (20²) / 1000
WEIGHT= 40 x 400 / 1000
WEIGHT= 16000 / 1000
WEIGHT = 16.0 pounds
Please Note: A 40 inch pike might be thicker than normal and have a girth of 22 inches, which would make the fish 19.4 pounds.

Here's link to age/length/weight chart

http://www.practicalsportman.com/northern-pikefish-size-chart/
and a portion of the table.

NORTHERN PIKE
AVERAGE LENGTH AVERAGE
AGE Inches cm WEIGHT

20 40 101 21
21 41.5 105 23
22 43 109 25
23 44.5 113 28
24 46 117 31
25 46.7 118 32.5
26 47 119 34
27 47.5 120 35.5
28 48 122 37
29 48.5 123 38.5
30 49 124 40
30+ 49.5+ 124+ 40+