Antibiotic stock dilution calculations
Concentrated antibiotic stocks allow small, repeatable volumes to be added to growth media. The required volume follows the standard dilution relationship C1V1 = C2V2.
Correct arithmetic is only part of preparation. Solvent compatibility, sterility, storage, stability, and the requirements of the biological system must come from an approved protocol or product documentation.
How to use the antibiotic stock calculator
- Enter stock concentration: Use the concentration stated for the prepared and verified stock.
- Enter the target: Provide the required final concentration and total medium volume.
- Match units: Select compatible mass-per-volume and volume units.
- Calculate and verify: Confirm the result against the laboratory protocol before preparation.
Formula and variables
Stock volume equals the desired working concentration multiplied by final volume, divided by stock concentration.
V1 = (C2 × V2) / C1- C1 — Stock concentration
- Concentration of the prepared stock solution
- V1 — Stock volume
- Volume of stock to add
- C2 — Final concentration
- Target antibiotic concentration in the medium
- V2 — Final volume
- Total prepared medium volume
Ampicillin working solution example
Prepare 500 mL medium at 100 µg/mL from a 100 mg/mL stock.
- C1
- 100 mg/mL
- C2
- 100 µg/mL
- V2
- 500 mL
- 100 mg/mL = 100,000 µg/mL
- V1 = (100 × 500) / 100,000 = 0.5 mL
Result: Add 0.5 mL (500 µL) of stock.
The stock is a 1,000× concentrate relative to the target working concentration.
Understanding your results
Preparation check
A very small calculated volume may be difficult to pipette accurately and may justify an intermediate dilution.
- Confirm unit conversion before dispensing.
- Use calibrated equipment appropriate to the volume.
- Follow protocol-specific solvent and storage instructions.
Assumptions
- Stock and final concentrations use compatible mass-per-volume units.
- Volumes are additive for the intended laboratory precision.
Limitations
- The calculator does not assess antibiotic potency, degradation, or biological suitability.
- It does not provide medical dosing advice or replace institutional laboratory procedures.
Common mistakes
- Failing to convert mg/mL to µg/mL.
- Using media volume as stock volume.
- Adding heat-sensitive antibiotics before autoclaved media has cooled as required by the protocol.
Practical use cases
Selective media
Calculate stock additions for bacterial selection media.
Cell-culture preparation
Plan working dilutions when an established culture protocol specifies an antibiotic concentration.
Frequently asked questions
What does a 1,000× stock mean?
Its concentration is 1,000 times the working concentration, so one part stock is diluted to 1,000 parts final solution.
Does C1V1 = C2V2 handle unit conversion?
The equation requires compatible units; the calculator converts supported concentration and volume units before solving.
Can this be used for medication dosing?
No. It is intended for laboratory solution preparation, not clinical or veterinary dosing.
Sources and review
- Addgene Protocol: How to Create a Bacterial Glycerol Stock — Addgene. Accessed 2026-07-13.
- Laboratory Biosafety Manual, Fourth Edition — World Health Organization. Accessed 2026-07-13.
Reviewed 2026-07-13.