Animal Dose Calculator — mg/kg to Injection Volume & Batch Preparation

Formula

In laboratory animal dosing, the dose is expressed in mg/kg, but body weight is measured in grams and the stock solution is prepared in mg/mL — this unit mismatch 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 you can verify

• 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 preparation: 6 animals × 0.11 mL + 0.1 mL dead volume = 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; use substantially lower volumes for organic solvents or irritant formulations.

Upper limits vary across sources — for the same route, some references give a “typical volume” and others give the “maximum acceptable volume,” and the two can differ by a factor of two (intraperitoneal injection commonly appears as both 10 and 20 mL/kg). This tool consistently uses the more conservative set, because over-injecting here means animal suffering. Your institution’s IACUC guidelines are the authoritative standard — they take precedence over this table.

The values in this table and the thresholds used internally by the tool are the same numbers, enforced by assertions so they cannot drift apart — a tool whose documentation contradicts its code is more dangerous than no tool at all.

When the calculated volume exceeds the limit, the only correct response is to increase the stock concentration, not to inject a larger volume.

Free Base vs. Salt Form

Drug doses are conventionally expressed as free base, but commercially supplied reagents are often hydrochloride, phosphate, or other salt forms. When using a salt:

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 salt MW = 236.5, and the protocol calls for 10 mg/kg of free base, the actual amount to weigh is 10 × 236.5 ÷ 200 = 11.8 mg/kg of the hydrochloride salt. When in doubt, check PubChem or the SDS — do not assume from experience.

Batch Preparation

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

Total stock volume (mL) = Volume per animal × Number of animals + Dead volume (mL)

This tool uses a single dead-volume value per batch (entered in the “dead volume” field). If you draw a separate syringe for each animal, multiply the dead volume by the number of syringes, or add a 10–15% overage to the total prepared volume.

Scope and Limitations

• The formula is valid for any species and dose range; constraints come from the maximum injectable volume the animal can tolerate, not from the formula itself.
• Maximum volume references cover only mice and rats; for other species consult your institution’s IACUC animal welfare standards.
• This tool does not calculate pharmacokinetic parameters (half-life, bioavailability, dosing frequency) — those are determined by the study protocol. This tool answers only “how many millilitres to inject.”
• For high-concentration compounds near the solubility limit, verify dissolution separately; this tool cannot detect precipitation or crystallisation risk.