To estimate a cow's weight without a scale, measure its heart girth and body length in inches, multiply girth by girth by length, and divide by 300 to get the weight in pounds.
One tape, one formula, and a fair answer to the oldest question at the stock pen.
Cattle have been bought and sold by weight for as long as there have been cattle markets. On the old trail drives the herd was walked to a railhead and priced there. On a small farm today, a scale is still the one tool most people do not own.
You do not need one to get close. You need a soft tape measure, a calm animal, and one line of arithmetic.
Take two measurements in inches:
Then:
Weight in pounds = heart girth x heart girth x body length / 300
Girth gets squared because it stands in for the animal's cross section, which is an area. Length stretches that area into a volume. Dividing by 300 turns a volume in cubic inches into pounds of beef on the hoof. It is the same idea as estimating any lump by treating it as a rough cylinder.
A cow measures 72 inches around the heart and 66 inches from shoulder to pin bone.
That is a sensible number for a grown beef cow. If your answer comes out at 400 pounds for an animal that fills the chute, check where the tape went.
These are worked straight from the formula. Real animals vary with breed, gut fill and condition, so treat each row as a starting point.
| Heart girth (in) | Body length (in) | Estimated weight (lb) |
|---|---|---|
| 60 | 56 | 670 |
| 66 | 62 | 900 |
| 72 | 66 | 1,140 |
| 78 | 70 | 1,420 |
| 84 | 74 | 1,740 |
Notice how fast the numbers climb. Add 6 inches of girth and the estimate jumps by 250 pounds or more. A sloppy tape costs you more here than anywhere else.
Most bad estimates come from the tape, not the maths.
Farm stores also sell weight tapes printed with pounds instead of inches. They use girth alone and assume an average body shape, so they are quicker but less exact on unusually long or short animals.
Experienced buyers can look at a pen of steers and call the weights surprisingly well. They have seen thousands and checked their guesses against the scale for years. Most of us have not.
The untrained eye tends to read size from the outline: how tall the animal stands and how long it looks from the side. Weight lives in the depth of the chest and the spring of the ribs, which is easier to miss from a distance. A tall, narrow heifer looks heavy. A short, deep cow looks light. The tape corrects for both.
There is a famous story here too. In 1906 the scientist Francis Galton looked at the entries in a guess-the-weight contest for an ox at a country fair. Individual guesses were all over the place, but the middle guess of the crowd landed within about 1 percent of the true weight. A crowd can be right when no single person is.
Judging how much stuff is in a shape, rather than how big the shape looks, is the whole skill behind Halfsies. Each day you get three hand-drawn objects and one cut each. A scale then weighs the two halves, and your score is the lighter half divided by the heavier half, times 100.
The weight comes from the drawn area of the object, so a long thin handle counts for very little and a fat body counts for a lot. It is the cow problem again. The outline tricks you, the mass sits somewhere else, and the only way to get better is to check your eye against a scale every day. Three cuts, a best possible 300, and a round takes a few minutes in a browser or on iPhone and Android.
Multiply heart girth in inches by itself, multiply by body length in inches from point of shoulder to pin bone, then divide by 300. The result is an estimate in pounds.
It is close enough for rough feeding and dosing decisions on average animals, but it can be off for very long, short, thin or heavily pregnant cattle. A scale is still the only exact answer.
Around the body just behind the front legs and the shoulder blades, where the chest is deepest. Keep the tape vertical and snug.
Experienced buyers often get close because they have checked thousands of guesses against a scale. Most people misread it, because height and length catch the eye while chest depth carries the weight.
Similar girth and length formulas are used for pigs, sheep and goats, but each uses its own divisor. Do not use the cattle number of 300 for other species.