Getting an injection volume wrong on a 600 kg cow or a 4 kg lamb is easy to do in your head at the chute. This calculator turns a label dose rate in mg/kg and a bottle concentration in mg/mL into the exact volume to draw for that animal’s weight.
How it works
Two steps, both pure arithmetic:
total dose (mg) = body weight (kg) × dose rate (mg/kg)
volume (mL) = total dose (mg) ÷ concentration (mg/mL)If you enter weight in pounds, it is divided by 2.20462 to convert to kilograms
first, because label dose rates are almost always per kilogram. Dividing dose by
concentration cancels the milligrams and leaves millilitres.
Worked examples
Cattle antibiotic: A 600 kg steer needs a product labelled at 10 mg/kg and
bottled at 200 mg/mL. Total dose = 600 × 10 = 6,000 mg. Volume = 6,000 ÷ 200 = 30 mL.
Check the label for maximum volume per injection site — many products limit a single
site to 10 mL, so this dose would need to be split across three sites.
Small ruminant treatment: A 40 kg sheep requires a product at 2.5 mg/kg and
50 mg/mL concentration. Total dose = 40 × 2.5 = 100 mg. Volume = 100 ÷ 50 = 2 mL.
A straightforward single injection at this volume.
Lamb / young pig: A 4 kg piglet needs a product at 5 mg/kg and 100 mg/mL.
Total dose = 4 × 5 = 20 mg. Volume = 20 ÷ 100 = 0.2 mL. At this small volume,
accuracy requires an appropriate small-bore syringe. Eyeballing 0.2 mL on a 20 mL
syringe will be inaccurate — use a 1 mL or insulin syringe for small animals.
Common dosing mistakes to avoid
Using outdated weight estimates. Drug dose scales directly with body weight. A steer estimated at 550 kg but actually weighing 620 kg will be underdosed by more than 10%, which can mean treatment failure and potential antibiotic resistance concerns. Use a weigh scale or calibrated weight band before dosing where practical.
Confusing product concentration. The same active drug is sold at different concentrations under different brand names — for example, one formulation may be 100 mg/mL and another 200 mg/mL. Always read the concentration from the specific bottle in hand, not from memory or a previous product.
Ignoring maximum site volumes. Injectable products have a maximum volume per injection site on the label (often 10 mL for cattle, less for small ruminants). Exceeding this can cause tissue damage, abscess formation, and condemnation of meat at slaughter. Split the calculated volume across multiple sites when needed.
Missing the route restriction. Some products are approved only for subcutaneous (SC) injection, others for intramuscular (IM) or intravenous (IV). The calculated volume is the same regardless of route, but injecting into the wrong tissue type can cause adverse reactions or treatment failure.
Withdrawal times: the critical reminder
This calculator does only the dose arithmetic. Meat and milk withdrawal periods are entirely your responsibility. They vary by drug, dose rate, and route of administration and are printed on the label. Extra-label use (any use outside label instructions, including a different dose rate or species not listed) legally requires veterinary oversight and typically extends withdrawal times.
Record every treatment with the product name, lot number, dose given, date, animal identification, and withdrawal end date. Most livestock operations use a treatment record book or farm management software for this purpose.