Measure the 'short length' of the fish in centimetres, from the tip of the lower jaw to the V of the tail.
Measure fish girth in centimetres all the way around the fattest part of the body. It may be easier to measure one side and multiply by two.
This fish weight calculator is based on the fish weight formula: (short length x girth squared) / 800.
This formula tends to work reasonably well for estimating the weight of gamefish like tuna, marlin, swordfish and kingfish, with an accuracy range of around +/- 10%.
Of course, this is just an estimate and can't ever replace a certified scale for an accurate fish weight - as the saying goes: if you don't weight it, you can't say it!
Remember this is a metric fish weight calculator, so you should enter the fish measurements in centimetres (cm) and not inches. Also if you're taking a half-girth measurement from one side of a fish, remember to double it before entering it into the fish weight calculator!