Liveweight from Girth Calculator NZ
This estimates what an animal weighs from two tape measurements: the heart girth, taken right around the body just behind the front legs, and the body length from the point of the shoulder back to the pin bone. It is the tool for when you need a weight and have no scales, which on a lifestyle block or a back paddock is most of the time. Everything here is centimetres and kilograms. That matters more than it sounds, because the formula this rests on was published in inches and pounds, and most of the tools answering this question online are still working in them, which leaves New Zealanders converting twice and losing accuracy at both ends. Rather than approximate a metric constant, this page derives it from the exact definitions of the inch and the pound, so the metric answer is the imperial one to the last decimal place. Two honest limits. The formula assumes a body shape, so it reads high on a deep or fat animal and low on a lean, leggy one, and it will differ between breeds and between a full gut and an empty one. And the carcass estimate underneath is only as good as the dressing out percentage you give it, which is why yours from a recent kill sheet beats any default.
An estimate from a tape, not a weighing. Use a weigh crate wherever the number carries money or a medicine dose.
How it works
The estimate is girth squared multiplied by length, divided by a constant. In its published form that is inches and pounds with a divisor of 300. Working in centimetres and kilograms needs a different divisor, and rather than round one off, this page computes it from the definitions: there are exactly 2.54 centimetres in an inch and exactly 0.45359237 kilograms in a pound, both fixed by definition rather than measured, so the metric divisor is 2.54 cubed times 300 divided by 0.45359237, or 10,838.1876. That makes the metric result identical to converting the imperial one, without the rounding that creeps in when you convert a measurement, calculate, then convert back.
Taking the measurements
Heart girth is the full circumference of the body immediately behind the front legs, with the tape snug but not pulled tight, taken when the animal is standing square and has stopped moving. Body length runs from the point of the shoulder to the pin bone, which is the bony point at the rear. Both measurements matter about equally, and the girth matters twice over since it is squared, so a girth taken loose or in the wrong place moves the answer more than anything else you can get wrong.
Worked example
A lamb measuring 85 centimetres around the heart girth and 70 centimetres long comes to 85 squared, or 7,225, multiplied by 70, which is 505,750, divided by 10,838.1876. That gives an estimated liveweight of 46.7 kilograms, or 102.9 pounds. At a dressing out percentage of 44, the carcass would come in around 20.5 kilograms. Measure ten of them and the average will be a good deal more useful than any single animal, because the shape assumption in the formula averages out across a mob in a way it does not on one.
Related calculators
- Kill Sheet: what the carcass was actually worth.
- Average Daily Gain: two of these measurements apart in time.
- Drench Dose Checker: turning a weight into a label dose.
- Stock Units: what the mob demands in feed.