Animal Dosing Calculator — mg/kg to Injection Volume & Batch Prep

Formula

In laboratory animal dosing, dose is expressed in mg/kg, but body weight is measured in grams and stock solutions are prepared in mg/mL — the unit mismatch across these three quantities is the most common source of calculation errors.

The conversion is a single step:

Injection volume (mL) = Dose (mg/kg) × Body weight (g) ÷ 1000 ÷ Stock concentration (mg/mL)

The ÷ 1000 converts grams to kilograms. Dimensional check: (mg/kg) × (g ÷ 1000) ÷ (mg/mL) = mL. ✓

Worked examples

• 10 mg/kg, 22 g mouse, 2 mg/mL stock: 10 × 22 ÷ 1000 ÷ 2 = 0.11 mL
• 50 mg/kg, 250 g rat, 10 mg/mL stock: 50 × 250 ÷ 1000 ÷ 10 = 1.25 mL
• Batch prep: 6 animals × 0.11 mL + dead volume 0.1 mL = 0.76 mL; drug required: 0.76 × 2 mg/mL = 1.52 mg

Maximum Injection Volume by Route

Reference: Turner PV et al., Administration of substances to laboratory animals: routes of administration and factors to consider, J Am Assoc Lab Anim Sci, 2011, 50(5):600-613 (PMID: 22330705).

The table below shows the conservative upper limits used by this tool, applicable to isotonic solutions; organic solvents or irritating compounds should use substantially lower volumes.

Published upper limits vary across sources — for the same route, some references give “typical volumes” and others give “maximum acceptable volumes,” which can differ by twofold (IP is commonly cited as either 10 or 20 mL/kg). This tool consistently uses the more conservative set, because erring high here costs animal welfare. Your institution’s IACUC standard is the binding document; defer to it.

The values in this table are the same numbers the tool uses internally, with assertions that prevent them from drifting apart — a tool whose documentation disagrees with its code is more dangerous than no tool at all.

When the calculated volume exceeds the limit, the only correct fix is to increase the stock concentration, not to force a larger injection volume.

Free Base vs. Salt Form

Drug doses are conventionally stated as free base, but commercial reagents are often supplied as hydrochlorides, phosphates, or other salts. When using a salt form:

Actual mass to weigh = Required free-base mass × Salt MW ÷ Free-base MW

For example, if a compound has free-base MW = 200 and its hydrochloride has MW = 236.5, and the protocol calls for 10 mg/kg free base, you would weigh out 10 × 236.5 ÷ 200 = 11.8 mg/kg of the hydrochloride salt. When in doubt, check PubChem or the SDS; do not assume based on experience.

Batch Preparation

When preparing drug for N animals, include extra volume to cover handling losses (syringe dead volume, transfer losses, etc.):

Total volume (mL) = Single-animal injection volume × Number of animals + Handling loss (mL)

This tool uses a single handling-loss value per batch (the “dead volume” field). If each animal gets an individually drawn syringe, multiply the dead volume by the number of syringes, or simply add a 10–15% overage to the total.

Scope and Limitations

• The formula applies to any species and any dose; the limiting factor is the maximum injection volume the animal can tolerate, not the formula itself.
• Maximum volume references cover mice, rats, and rabbits; values are taken from Diehl et al. 2001, the guide widely cited by institutional IACUCs (J Appl Toxicol 21:15–23, Table 1, “good practice” column). When a species not in the table is selected (guinea pig, hamster, etc.), the tool explicitly states “no volume limit check was performed for this species” — it does not silently skip the check. Consult your institution’s IACUC guidelines for those species.
• Note that the IM row uses inconsistent units: values for mice and rats are mL per injection site (independent of body weight, corresponding to footnote b of the original table), whereas the rabbit value is mL/kg. This follows the original table and is not a typo.
• The tool does not calculate pharmacokinetic parameters (half-life, bioavailability, dosing frequency) — those are determined by the study protocol. This tool answers only “how many mL to inject.”
• When a high-concentration stock approaches the solubility limit, actual dissolution must be verified separately; the tool cannot detect precipitation or crystallization risk.