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Antibiotic Stock and Working Concentration Calculator

Calculate the powder to weigh for an antibiotic stock solution, and the volume of stock to add to medium for a working concentration, with typical starting concentrations.

Formula

m=C×Vpotencym = \dfrac{C \times V}{\text{potency}}
Vstock=Cwork×VmediumCstockV_{\text{stock}} = \dfrac{C_{\text{work}} \times V_{\text{medium}}}{C_{\text{stock}}}
potency\text{potency}
the active fraction of the powder, given on the label
CworkC_{\text{work}}
the concentration in the medium, usually in µg/mL

How it works

Antibiotics are made as concentrated stocks, often 1000 times the working concentration, stored in aliquots and diluted into medium. A 50 mg/mL ampicillin stock diluted 1:500 gives 100 µg/mL. The mass of powder follows from the stock concentration and volume, corrected for potency if the powder is not pure.

The presets are typical starting concentrations for selecting bacteria that carry a resistance gene (from laboratory manuals) and for selecting mammalian cells. They are only starting values: the concentration that works depends on the strain, the plasmid copy number, the medium and the batch of antibiotic, and for mammalian cells must be found with a kill curve on untransfected cells.

Worked example

500 mL of medium with ampicillin at 100 µg/mL, from a 50 mg/mL stock.

  1. The stock is 50,000 µg/mL, so the dilution is 50,000 / 100 = 500.
  2. Volume = 500 mL / 500 = 1 mL.

Add 1 mL of stock to 500 mL of medium: 2 µL per mL. To make 10 mL of the stock itself takes 500 mg of ampicillin.

These are the values the calculator opens with, so you can check its output against this example.

Assumptions

  • The stock is sterile and still active. Many antibiotics lose activity when stored for months or freeze-thawed repeatedly.
  • The working concentrations shown are typical values, not requirements.

Common mistakes

  • Adding antibiotic to agar that is still hot, which destroys it.
  • Making a stock from a salt or hydrate without correcting for potency or molecular weight.
  • Using the concentration for an E. coli plasmid on mammalian cells, which need very different amounts.

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Related equipment

Service documentation, failure modes and parts for the instruments this calculation is used with.