How can I estimate my horse's weight without a scale?

Measure the heart girth, around the barrel just behind the elbow and over the withers, and the body length, from the point of the shoulder to the point of the buttock. In inches, girth times girth times length divided by 330 gives the weight in pounds; in centimeters, divide by 11,877 for kilograms. In a test on 600 horses weighed on a weighbridge, a tape-measure formula averaged 98.6 percent of the true weight, against 88 percent for estimates by eye.

How accurate is a horse weight tape?

It depends on the tape. In a 1998 study of 600 horses, one weight tape averaged 98 percent of the true weight and another 112 percent. Both were closer than estimating by eye, which averaged 88 percent with the widest spread of all.

Why does a horse's weight matter for deworming?

Dewormers are dosed by bodyweight. University of Florida extension advises using an accurate body weight when giving them, because underdosing leads to resistant parasites. Overestimating carries its own risk with drugs that have a narrow safety margin, which is why the 1988 study behind the weight formula warned against formulas that read high.

Tool

Horse weight calculator: your horse's weight from a tape measure

By Sebastien Kister

Last checked: 12 October 2026

Most horses are never weighed, and the eye makes a poor scale: in a study of 600 horses on a weighbridge, guesses came in 12% light on average, and some were out by far more. A tape measure and a published formula do much better. Put in two measurements — four if you have them — and this gives the weight in pounds and kilograms, and how far to trust it.

This is an estimate from published formulas, not a scale and not veterinary advice. For a drug with a narrow margin, or a horse whose weight really matters, ask your vet — most vet clinics have a scale. Nothing you type here leaves your browser.

How to measure

A plastic tape, long enough to go around the horse. The University of Arkansas prefers plastic to cloth, and warns off metal tapes, which can cut the horse. Have someone hold the horse rather than tying it up, and stand it square on level ground — a horse that fidgets changes both measurements.
Heart girth: all the way around the barrel, just behind the elbow and over the highest point of the withers. The 1988 study the formula comes from read it after the horse had breathed out.
Body length: from the point of the shoulder straight back along the side to the point of the buttock — around the corner of the hip, not to it. That takes two people on the tape and a third holding the horse.
Height and neck, for the second estimate: height at the highest point of the withers, and the neck all the way around, halfway between the poll and the withers.
Same time, same way. To follow a horse's weight over time, measure at the same time of day — best in the morning, before feeding.

A weight tape is a heart-girth tape printed in pounds instead of inches: it reads the girth alone and converts it. The formula adds the length, which is why it can tell a long horse from a short, deep one.

The two formulas

Girth and length (Carroll and Huntington, 1988). Weight in kilograms = girth × girth × length ÷ 11,877, with both in centimeters. The study weighed 372 horses and measured 281 of them, ponies to Thoroughbreds, from 160 to 680 kg, and girth × girth × length tracked their weight closely (r² = 0.90). With inches and pounds, extension services divide by 330 — an older figure from Hall (1971), which the 1988 result matched. The calculator works in centimeters and converts; the two agree within half a percent.

The same study found that thin horses weigh less for their measurements: below 3 on its 0 to 5 condition scale the constant was 12,265, against 11,706 for the rest. On a thin horse, then, the formula reads about 3% heavy — and the Arkansas extension sheet adds that it may not be accurate for pregnant mares or horses of unusual build.

Four measurements (Martinson and colleagues, University of Minnesota, 2014). Weight in kilograms = girth^1.486 × length^0.554 × height^0.599 × neck^0.173 ÷ a constant for the type: 3,441 for stock horses, 3,596 for Arabians, 3,606 for ponies, all in centimeters. It was fitted on 629 horses and ponies at least three years old and 112 cm tall, weighed on a livestock scale at two Minnesota horse shows (R² = 0.92).

How far to trust it

About 10% either way. The largest test, by Ellis and Hollands on 600 horses weighed on a weighbridge, found a formula from tape measurements averaged 98.6% of the true weight, with a standard deviation of 10.6%: for a typical horse within about a tenth, for some further out. It did better under 15 hands (99.6%, 5.2%) than at 15 hands and over (95.5%, 13.1%), where it tended to read light.
Weight tapes differ by brand. Of the two tapes in the same study, one averaged 98.1% of the true weight and the other 112%.
The eye is the worst of all: 88.3% on average, with a standard deviation of 20.1%.
No formula wins everywhere. A 2020 comparison on horses in Poland found Carroll and Huntington's formula closest for Thoroughbreds, and Martinson's Arabian-type equation closest for ponies and two warmblood breeds. When the two estimates here disagree by much, a scale settles it.
Not for foals, miniature horses, donkeys or mules. None was in the studies above. Donkeys and mules have formulas of their own, from studies of working animals in Morocco; for a foal or a miniature, the vet's scale is the answer.

Much of a formula's error for a given horse comes from its build, which does not change from one month to the next. Measured the same way each time, a girth that has grown by two inches has grown — the trend can be trusted further than the single number.

Why the number matters

Dewormers are dosed by bodyweight. The University of Florida's extension guide to deworming is plain about it: "Use an accurate body weight when administering anthelmintics", because underdosing breeds resistant worms. Reading high has its own cost: Carroll and Huntington warned that a formula which overestimates could have serious consequences for drugs with a low safety margin.

Feed is the other reason. Every ration in the feeding guide is a share of bodyweight: 2% of 1,100 lb is 22 lb of hay a day, 2% of 900 lb is 18.

Keep the measurements with the horse

In EquiDiary, make a type of your own — Weight, say — and log the girth, the length and the estimate as an entry with its date. Give it a monthly rhythm in Care schedule, and the next measuring day comes up alongside the farrier and the boosters, with the earlier readings in the horse's history to compare against.

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Sources

Carroll CL, Huntington PJ. Body condition scoring and weight estimation of horses. Equine Vet J 1988;20(1):41–45 (the girth-and-length formula, 11,877, Hall's 330, condition, dosing)
Martinson KL, Coleman RC, Rendahl AK, Fang Z, McCue ME. Estimation of body weight and development of a body weight score for adult equids using morphometric measurements. J Anim Sci 2014;92(5):2230–2238
Ellis JM, Hollands T. Accuracy of different methods of estimating the weight of horses. Vet Rec 1998;143(12):335–336 (formula, two weight tapes and the eye, against a weighbridge)
Górniak W, Wieliczko M, Soroko M, Korczyński M. Evaluation of the accuracy of horse body weight estimation methods. Animals 2020;10(10):1750 (which formula is closest, by breed; the measurement landmarks)
University of Arkansas Extension — Estimating a horse's body weight (÷ 330 in inches and pounds; how to measure)
University of Florida IFAS Extension VM251 — Developing a deworming program for Florida horses (dosing by weight, underdosing)
Pearson RA, Ouassat M. Estimation of the liveweight and body condition of working donkeys in Morocco. Vet Rec 1996;138(10):229–233, and Kay G, Pearson RA, Ouassat M, on working mules, Vet Rec 2004;154(3):85–88

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Next: horse feeding plan — what the weight is for: hay, starch and condition, in pounds.