[robert waddell]

there is a formula  length by girth or something  i just pop them in the weigh crate

[HappyHippy]

The formula is heart girth x heart girth (round their body, just behind the front leg) multiplied by length (from base of ear to base of tail) all measured in meters multiplied by 69.3 - gives you the liveweight, within 5 Kg apparently  Dunno if it really is that exact (not having a weigh crate to double check) but it does give you a 'ballpark' 
So, if they measure 0.9m (or 90cm or 36 inches in old money) round and 0.95m long, the calculation would look like this:
0.9 x 0.9 (Hearth girth squared) = 0.81
0.81 x 0.95 = 0.7695
0.7695 x 69.3 = 53.3Kg

HTH
Karen