Horse Weight Calculator
Estimate your horse's body weight from heart girth and body length — no scale needed. Switch between the five published formula constants (÷330, KER ÷300+50, ÷301, ÷280, ÷299), convert weight-tape readings, adjust for body condition score, and work out dewormer doses.
Honest numbers, honest limits
Every formula here is an estimate — only a livestock scale gives a true weight. Published metric constants differ by under 1% between publications, but the formula options themselves (e.g. ÷330 vs the KER ÷300+50 variant) can differ by roughly 15% on the same horse — still smaller than the method's own error versus a scale. The one published worked example — 75″ girth × 64″ length — gives 1,091 lb here, matching the equine textbook it comes from.
Estimate weight from girth & length
Weight tape converter
A weight tape prints pounds (or kg) instead of inches. Enter its reading to convert it — and remember tapes use girth alone, so they can read high or low depending on conformation.
Body condition score & ideal weight
🐎 Animal-welfare note
This adjuster is a guideline only (~50 lb per Henneke point for an average light-breed horse). Never put a horse on a crash diet or heavy refeeding program without your veterinarian or an equine nutritionist.
Dewormer dose from weight
⚠️ Estimate — confirm with your vet
This computes the mg of active ingredient from published mg/kg rates. It is not a prescription. Always verify against your product's label (concentrations vary) and your veterinarian's parasite program.
How to measure your horse
- Stand the horse square on firm, level ground, head forward and relaxed.
- Heart girth: wrap a soft, non-stretch tape around the chest just behind the elbow and up over the withers. Take the reading just after the horse exhales.
- Body length: run the tape in a straight line from the point of the shoulder to the point of the buttock (not around the curve).
- Have a helper hold the tape, use a plastic-coated (not cloth) tape, and repeat the same method each time so you can track real weight changes.
Formula constants — the honest table
Published metric constants are within 1% of each other; the formula variants (÷330 vs KER) differ more — see the FAQ.
| Applies to | Units | Formula |
|---|---|---|
| Adult horse (mature) | in → lb | (girth² × length) ÷ 330 |
| Adult horse, KER variant | in → lb | ((girth² × length) ÷ 300) + 50 |
| Yearling (1–2 yrs) | in → lb | (girth² × length) ÷ 301 |
| Weanling (< 1 yr) | in → lb | (girth² × length) ÷ 280 |
| Pony | in → lb | (girth² × length) ÷ 299 |
| Adult horse (metric) | cm → kg | (girth² × length) ÷ 11,900 (variants 11,877–11,900) |
Why so many constants? Different research groups calibrated on different horse populations. This tool computes in inches and converts to kilograms exactly (1 lb = 0.45359237 kg), which is equivalent to ÷ 11,922 — inside the range of the published metric variants.
Horse weight vs unit conversion
This tool estimates a horse's body weight from body measurements — it computes a number you don't know from measurements you take. Our weight-converter only converts units you already know (lb ↔ kg ↔ oz ↔ g). If you just need to convert 1,100 lb to kilograms, use the weight-converter; if you need to find out what your horse weighs in the first place, you're in the right place.
Frequently asked questions
What is the horse weight formula?
The standard estimate is (heart girth² × body length) ÷ 330 with girth and length in inches and the result in pounds. It is an estimate, not a scale reading — developed for mature horses by equine researchers and widely cited by extension programs (e.g. Kentucky Equine Research, University of Florida).
Which constant should I use: 330, 300, or 299?
330 is the standard for mature horses. Kentucky Equine Research also publishes a variant (girth² × length) ÷ 300 + 50. For young or smaller horses use 301 for yearlings, 280 for weanlings, and 299 for ponies. This calculator lets you switch between all five — the adult options (÷330 vs the KER ÷300+50 variant) can differ by roughly 15% on the same horse
What is the metric formula (cm to kg)?
Measure girth and length in centimetres and divide: (girth² × length) ÷ 11,900 = kg (Hall 1971 / Carroll & Huntington 1988). Published metric constants range from 11,877 to 11,900 — the differences are under 1%. This calculator works in inches internally and converts to kilograms exactly, so you get the same answer either way.
How do I measure heart girth and body length?
Stand the horse square on level ground. Heart girth: wrap a non-stretch tape around the chest just behind the elbow and over the withers, reading just after the horse exhales. Body length: run the tape in a straight line from the point of the shoulder to the point of the buttock. Measure with a helper and use the same tape and method each time so you can track changes.
How accurate is this estimate compared to a scale or a weight tape?
No formula is a true weight — only a livestock scale gives that. A 1998 study by Ellis & Hollands found the girth-and-length calculation is the most accurate of the estimation methods, better than guessing by eye or using a weight tape alone, because it includes body length. Weight tapes use girth only, so they can read noticeably high or low depending on the horse's conformation (Kentucky Equine Research).
What is a weight tape and how do I convert its reading?
A weight tape is a tape printed in pounds (or kg) instead of inches, calibrated so the girth reading converts to an estimated weight. Use the Weight tape tab above to convert any reading to lb/kg. Treat the number as an estimate: tapes are calibrated for typical adult horses and tend to be off for ponies, young horses, or very heavy breeds.
What is the Henneke body condition score (BCS)?
The Henneke system (1983) scores a horse's fat cover from 1 (emaciated) to 9 (extremely fat) by visual appraisal and palpation of six areas: neck, withers, back, tailhead, ribs, and behind the shoulder. A score of 5 to 6 is considered ideal for most horses (Ohio State / UF Extension). It complements, but does not replace, weighing.
How much should my horse weigh for its body condition score?
A common rule of thumb is about 50 lb (~23 kg) per Henneke point for an average light-breed horse — so a horse scoring 7 needs to lose roughly 100 lb to reach a 5, and a horse scoring 4 needs to gain about 50 lb. It is only a guideline; frame size and breed shift it. Confirm feeding changes with your vet or an equine nutritionist.
How do I calculate dewormer dose from my horse's weight?
Multiply the horse's weight in kg by the drug's rate: ivermectin 0.2 mg/kg, moxidectin 0.4 mg/kg, fenbendazole 5 mg/kg (single) or 10 mg/kg daily for 5 days (encysted), pyrantel pamoate 6.6 mg/kg (13.2 mg/kg for tapeworm), praziquantel 1.0 mg/kg in combination products. Set the syringe to the next weight marking UP — never round down, because underdosing is a major cause of parasite resistance. This is an estimate only: confirm the dose with your veterinarian and your product's label.
How much hay should a horse eat per day?
As a guideline, feed 1.5–2% of ideal body weight in forage (hay) per day — e.g. about 20 lb of hay for a 1,000 lb horse at ideal weight. Fresh water should always be available (typically 8–12 gallons a day). Adjust for workload, pregnancy, age, and hay quality with your vet or nutritionist.
How is this different from the generic weight-converter tool?
This tool estimates a horse's body weight from body measurements (heart girth and body length) — it computes a number you don't know from measurements you take. Our weight-converter only converts units you already know (lb ↔ kg ↔ oz ↔ g). If you just need to convert 1,100 lb to kilograms, use the weight-converter; if you need to find out what your horse weighs in the first place, you're in the right place.
Can I use this for ponies, foals, or donkeys?
Use the Pony (÷299), Weanling (÷280), or Yearling (÷301) formula options for those groups. The constants were validated for horses; donkeys, mules, and draft extremes have different conformation, so treat results for them as rough guides and confirm with a scale or your vet before dosing medication.
Sources
- Formula constants (330 / 301 / 280 / 299): cowgirlmagazine.com — How To Weigh Your Horse Easily
- KER variant ÷300 + 50 & weight-tape honesty: Kentucky Equine Research — Horse Weight Estimates
- Worked example (75″ × 64″ = 1,091 lb): Univ. of Florida, via Chapter 9: Determining Age, Height, and Weight of Horses
- Metric constants (Carroll & Huntington 11,877): MDPI Animals — Evaluation of the Accuracy of Horse Body Weight Estimation Methods
- Henneke BCS 1–9 & 5–6 ideal: Ohio State Extension — Body Condition Scoring (Henneke et al. 1983)
- ~50 lb per BCS point: cowgirlmagazine.com — Scoring Your Horse's Body Condition
- Ivermectin 200 mcg/kg: FDA NADA 140-439 (EQVALAN); moxidectin 400 mcg/kg: Mad Barn
- Underdosing & resistance: NSW Dept. of Primary Industries — Estimating a Horse's Weight
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