DIY Horse Vet Course
Free Tool · Designed by Dr. Louise Cosgrove BVSc

Free Horse Weight Estimator
Girth & Length Formula

Calculate your horse's weight accurately using the validated girth and body length formula — no scales needed. Essential for correct worming doses, feed planning, and medication calculations.

Takes 2 minutes
Validated formula
Results emailed instantly
±5–10%Accuracy vs scales
2 minsTo complete
All breedsSupported
FreeNo sign-up required
Educational Purposes Only: This tool provides weight estimates for general guidance. For precise drug dosages, always use a calibrated livestock scale or consult your equine veterinarian.
1Your Horse
2Measurements
3Your Details
✓Your Result
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Tell us about your horse

We'll apply the correct formula based on breed type.

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Enter your measurements

Use a soft measuring tape. Measure in centimetres for best accuracy.

📐 How to Measure

①

Heart Girth

Wrap the tape around the barrel just behind the elbow and withers. Measure after the horse exhales. Keep tape snug but not tight.

②

Body Length

Measure in a straight line from the point of the shoulder to the point of the buttock. Have an assistant hold the tape at the shoulder.

Typical ranges: Pony 120–150 cm · Average horse 165–195 cm · Large horse 195–220 cm · Draft 220–260 cm
Typical ranges: Pony 110–135 cm · Average horse 145–165 cm · Large horse 160–185 cm
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Get your results by email

We'll email your horse's weight estimate along with personalised feeding and worming dose guidance.

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Your Results

Results have been emailed to you. Here is a summary:

Want a personalised feeding plan or worming schedule based on this weight?

⚠️ Educational Purposes Only: This weight estimate is provided for general guidance only and should not replace professional veterinary advice. For precise drug dosage calculations, always use a calibrated livestock scale or consult your local equine veterinarian.

Why Knowing Your Horse's Weight Matters

Accurate body weight is one of the most fundamental pieces of information in equine health management, yet it is one of the most frequently overlooked. Research consistently shows that horse owners underestimate their horse's weight by an average of 10–15%, with some studies reporting underestimates of up to 30%. This systematic underestimation has real consequences — particularly for parasite control.

The Worming Dose Problem

Anthelmintic (worming) drugs are dosed by body weight. When a horse owner estimates their 550 kg horse weighs 450 kg and doses accordingly, they administer only 82% of the required dose. This sub-therapeutic dosing does not kill all parasites — it kills the susceptible ones and leaves the resistant ones to reproduce. Over time, this is the primary mechanism by which anthelmintic resistance develops. Australia already has documented resistance to ivermectin and oxfendazole in some horse populations. Accurate weight estimation is therefore a critical component of responsible parasite management.

Key Fact: Underdosing wormers is the single biggest driver of anthelmintic resistance in Australian horse populations. Always dose to your horse's actual weight — never guess.

The Girth-Length Formula Explained

The most widely used and validated formula for estimating horse weight from body measurements is the Caroll and Huntington (1988) formula:

Weight (kg) = (Girth cm² × Body Length cm) ÷ 11,877

For ponies, a modified divisor of 10,926 is used to account for their proportionally rounder barrel.

This formula has been validated across multiple studies and is accurate to within 5–10% for most light horse breeds when measurements are taken correctly. It is the formula used in this calculator and by most equine veterinarians when a weigh bridge is not available.

Breed-Specific Considerations

The standard formula works well for thoroughbreds, standardbreds, warmbloods, and stock horses. However, adjustments are needed for:

Breed TypeFormula AdjustmentNotes
Ponies (under 14.2hh)Modified divisor (10,926)Ponies have a proportionally rounder barrel; standard formula underestimates
Draft breedsStandard formulaMay slightly overestimate; weigh bridge recommended for drug dosing
Foals (under 6 months)Foal-specific formulaBody proportions differ significantly from adult horses
Pregnant mares (late gestation)Standard formulaGirth measurement includes foetal weight; actual horse weight is lower
Thoroughbreds in race trainingStandard formulaHighly accurate for lean, athletic horses

Other Uses for Accurate Horse Weight

Beyond worming, accurate body weight is essential for:

Daily feed requirements are calculated as a percentage of body weight (1.5–2.5% of BW per day). A 100 kg error in weight estimate translates to 1.5–2.5 kg of feed per day — significant over time.

NSAID dosing (phenylbutazone/bute, flunixin, Equioxx) is weight-based. Overdosing NSAIDs in horses causes gastric ulceration and right dorsal colitis — serious and potentially fatal conditions.

Sedation and anaesthesia doses are strictly weight-based. Underdosing can result in inadequate sedation; overdosing can cause cardiovascular depression.

Monitoring Weight Over Time

Regular weight monitoring — ideally monthly — is one of the most valuable health monitoring tools available to horse owners. Gradual weight loss is often the first sign of dental disease, internal parasites, chronic pain, PPID (Cushing's disease), or inadequate nutrition. By tracking weight over time using this calculator, you can detect problems early and intervene before they become serious.

When to Call Your Vet: If your horse loses more than 5% of body weight over 4 weeks without a clear dietary explanation, contact your equine veterinarian. Unexplained weight loss warrants investigation for dental disease, parasitism, PPID, or other systemic illness.

Frequently Asked Questions

Answers to the most common questions about estimating horse weight and why it matters.

How do you calculate a horse's weight without a scale?
The most accurate method is the girth-length formula: Weight (kg) = (Girth cm² × Body Length cm) ÷ 11,877. Measure the heart girth (circumference around the barrel just behind the elbow) and the body length (point of shoulder to point of buttock). This formula is accurate to within 5–10% for most horses. Weight tapes are a simpler alternative but are less accurate, particularly for ponies, draft breeds, and heavily muscled horses.
How accurate is the horse weight formula?
The girth-length formula (Caroll & Huntington, 1988) is accurate to within approximately 5–10% for most light horse breeds. It is less accurate for ponies (which tend to be underestimated) and draft breeds. For the most accurate weight measurement, a livestock scale or weigh bridge is recommended, particularly when calculating precise drug dosages.
Why is knowing my horse's weight important?
Accurate horse weight is essential for: (1) Calculating correct worming drug dosages — underdosing is a leading cause of anthelmintic resistance; (2) Determining daily feed requirements; (3) Monitoring body condition and detecting weight loss early; (4) Calculating correct doses for medications such as NSAIDs; (5) Assessing fitness and performance. Many horse owners significantly underestimate their horse's weight.
How do I measure my horse's heart girth?
Stand your horse on level ground and ensure they are standing square. Pass a soft measuring tape around the barrel, just behind the elbow and withers, at the deepest point of the girth groove. The tape should be snug but not tight. Take the measurement after the horse exhales. Measure in centimetres. Repeat twice and use the average.
How do I measure my horse's body length?
Body length is measured from the point of the shoulder (the bony prominence at the front of the shoulder) to the point of the buttock (the bony prominence at the back of the hindquarters). Measure in a straight line, not following the curve of the body. Have an assistant hold the measuring tape at the shoulder while you extend it to the buttock. Measure in centimetres.
What is the correct worming dose for my horse?
Worming doses are calculated based on body weight. Most equine wormers are dosed at a specific mg per kg of body weight. Always use the horse's actual or estimated weight — never guess. Underdosing is the single biggest driver of anthelmintic resistance in Australian horse populations. Use our Parasite Control Risk Checker for a personalised deworming program.
Dr Louise Cosgrove BVSc, Principal Equine Veterinarian at EEVS

Dr Louise Cosgrove

BVSc · Principal Equine Veterinarian, EEVS · Regency Downs, QLD

Dr Louise Cosgrove has been riding horses since she was 4 years old and has worked exclusively with horses since graduating from the University of Queensland. As the Principal Veterinarian at Exclusively Equine Veterinary Services (EEVS) in Regency Downs, QLD, she has a deep passion for equine health education and empowering horse owners with the knowledge to make better decisions for their horses.

"Accurate weight is the foundation of everything — correct worming doses, correct feed amounts, correct medication doses. It takes two minutes with a tape measure and it could save your horse's life." — Dr Louise Cosgrove

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