What this calculator does
Two tape measurements give a usable weight estimate without a weighbridge. Heart girth is measured around the barrel just behind the withers and elbows, body length from the point of shoulder to the point of buttock. The formula squares the girth, multiplies by the length and divides by 330.
Girth is squared, so it dominates the result and measuring it accurately matters far more than measuring the length. A one inch error in girth on a 72 inch measurement shifts the estimate by about 29 pounds, while a one inch error in length shifts it by about 16. Take the girth measurement twice.
The formula
Weight in pounds is heart girth in inches squared, multiplied by body length in inches, divided by 330. The result converts to kilograms. The formula is the standard weight-tape relationship and is accurate to roughly 10% for adult horses of ordinary build. It is less reliable for ponies, draught breeds, heavily pregnant mares and young stock.
| Term | Meaning |
|---|---|
| Heart girth | Circumference around the barrel just behind the withers and elbows, taken at the end of an exhale. |
| Body length | Point of shoulder to point of buttock, measured in a straight line rather than around the curve. |
| 330 | The divisor in the standard formula, which carries the unit conversion and the body-density assumption together. |
The inputs explained
| Field | What to enter |
|---|---|
| Heart girth (circumference behind the withers/elbow) (in) | Heart girth in inches. Measure at the end of a normal exhale with the tape snug but not tight, and take it twice since this figure is squared. |
| Body length (point of shoulder to point of rump) (in) | Body length in inches, point of shoulder to point of buttock. |
When to use it
Dosing wormer or medication
Most equine medications are dosed by body weight, and guessing overdoses light horses and underdoses heavy ones. This is the most common reason the measurement is taken.
Monitoring condition over time
The trend matters more than the absolute figure. Taking the same two measurements monthly tracks gain or loss reliably even if the formula is slightly off in absolute terms.
Working out feed rations
Feed is specified as a percentage of body weight, so the ration depends directly on this figure.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does heart girth change the weight estimate?
A range of heart girths with body length held constant.
| Heart girth | Estimated body weight | In kilograms | Formula used |
|---|---|---|---|
| 66 in | 871 lb | 395 kg | Weight (lb) = girth² × length ÷ 330, accurate to roughly ±10% for adult, mature-build horses |
| 72 in | 1,037 lb | 470 kg | Weight (lb) = girth² × length ÷ 330, accurate to roughly ±10% for adult, mature-build horses |
| 78 in | 1,217 lb | 552 kg | Weight (lb) = girth² × length ÷ 330, accurate to roughly ±10% for adult, mature-build horses |
Questions
How accurate is the weight tape formula?
Within about 10% for adult horses of ordinary conformation, which is good enough for feed rations and most routine dosing. It is less reliable for ponies, draught breeds, very young horses and heavily pregnant mares, all of which have proportions the formula was not fitted to.
Where exactly do I measure heart girth?
Around the barrel just behind the withers and the elbows, with the tape passing over the lowest part of the withers and snug against the coat. Take it at the end of a normal exhale, since a horse that has just breathed in reads several inches larger.
Should I use this for medication dosing?
It is better than estimating by eye, which is what the alternative usually is, but discuss anything with a narrow safety margin with a vet. Some medications need a more accurate weight than a tape formula provides, and a weighbridge is the answer where one is available.
Does the formula work for ponies?
Less well. Ponies are proportionally rounder than horses, and the standard divisor of 330 tends to underestimate them. Pony-specific variants using a different divisor exist and give better results in that range.
For sizing a round pen, see the round pen panel calculator. For estimating the weight of a fish instead, see the fish weight calculator.