Antibiotic Calculator

The antibiotic calculator is a benchtop tool for microbiologists, molecular biologists and cell-culture scientists who need to prepare selective antibiotics quickly and accurately. Whether you are pouring LB agar plates for bacterial selection, supplementing liquid media for a plasmid maintenance experiment, or reconstituting a fresh powder, the calculator removes the error-prone mental arithmetic from the equation. It supports three distinct workflows in one interface: building a stock solution from dry powder, performing a dilution using the classic C1V1 = C2V2 relationship, and computing the exact working concentration volume to add to a known media volume.

Every laboratory antibiotic has a recommended working concentration that balances effective selection against toxicity and cost. Ampicillin is typically used at 100 µg/mL, kanamycin at 50 µg/mL, and chloramphenicol at 25 µg/mL. Getting these numbers right matters: too little antibiotic allows non-resistant background colonies to survive and contaminate your selection, while too much can stress even resistant cells or waste expensive reagent. This antibiotic stock solution calculator encodes a curated table of common selection antibiotics together with their solvents, molecular weights and standard working concentrations so you can move from powder to plate with confidence.

The tool covers ten widely used antibiotics including ampicillin, kanamycin, chloramphenicol, tetracycline, streptomycin, gentamicin, spectinomycin, carbenicillin, hygromycin B and zeocin. For each, the calculator reports the resulting concentration in both mass units (mg/mL) and molar units (mM), making it straightforward to cross-check your preparation against published protocols and addgene plasmid datasheets.

The stock solution mode answers the most fundamental bench question: if I dissolve a measured mass of antibiotic powder into a measured volume of solvent, what concentration do I get? The calculator divides the powder mass by the solvent volume to produce a concentration in mg/mL, then converts that to molarity using the antibiotic's molecular weight.

Concentration is computed as mass divided by volume. To obtain molarity in millimolar, the calculator divides the mg/mL concentration by the molecular weight (g/mol, which equals mg/mmol) and multiplies by 1000 to express the result in mM rather than M. This dual readout is useful because some protocols quote stocks in mg/mL while structural and binding studies quote them in mM.

For example, dissolving 100 mg of kanamycin (MW 484.5 g/mol) in 2 mL of water yields 50 mg/mL, equal to (50 / 484.5) × 1000 ≈ 103.2 mM. Always dissolve water-soluble antibiotics in molecular-biology-grade water and ethanol-soluble antibiotics such as chloramphenicol and tetracycline in 70-100% ethanol, then filter-sterilize through a 0.22 µm filter rather than autoclaving, which would degrade the drug.

The dilution mode uses the universal dilution equation C1V1 = C2V2, the workhorse of every wet lab. Given a stock concentration, a desired final concentration, and a final volume, the antibiotic dilution calculator solves for the volume of stock you must pipette. The remainder of the final volume is made up with solvent or media.

The volume of stock needed equals the final concentration multiplied by the final volume, divided by the stock concentration. The solvent volume to add is simply the final volume minus the stock volume. The calculator also reports the dilution factor, computed as the stock concentration divided by the final concentration, so you can sanity-check the result as a familiar ratio such as 1:1000.

Because the equation is a ratio, the units on either side simply have to be consistent in the calculation itself. The tool intelligently switches the displayed stock-volume units, showing microliters (µL) when the required volume is less than 1 mL and milliliters (mL) otherwise, which keeps tiny additions readable at the bench. The calculator guards against impossible requests by returning no result if the final concentration exceeds the stock concentration, since you cannot dilute up.

The working concentration mode is purpose-built for supplementing growth media. You select an antibiotic, enter your stock concentration and your media volume, and the calculator returns the exact stock volume to add so the media reaches the antibiotic's standard working concentration. The working concentration is pulled automatically from the built-in antibiotic table, so you do not have to remember whether ampicillin runs at 100 µg/mL or kanamycin at 50 µg/mL.

Internally this mode applies the same dilution logic as C1V1 = C2V2: stock volume equals the working concentration times the final media volume, divided by the stock concentration. The volume of clean media is the final volume minus the stock volume, and the dilution factor equals stock concentration divided by working concentration. Note that stock concentration is entered in mg/mL while working concentration is tabulated in µg/mL; the numerical ratio (for example 100 mg/mL stock and 100 µg/mL working) gives the familiar 1:1000 dilution that most lab protocols call for.

This is the fastest route for routine plate pouring and overnight cultures. If you are pouring 500 mL of LB agar and need ampicillin selection, the calculator tells you to add exactly 0.5 mL of a 100 mg/mL stock. Add the antibiotic only after the agar has cooled to roughly 50 °C, because heat-labile antibiotics degrade in hot molten agar.

The table below lists the standard working and stock concentrations, recommended solvents, and molecular weights encoded in this antibiotic calculator. These values follow widely accepted molecular cloning conventions and addgene reference protocols. Use them as defaults, but always defer to the specific guidance on your plasmid or strain datasheet, since some constructs require higher selection pressure.

Notice that most antibiotics use a 1000-fold concentrated stock, meaning 1 µL of stock per 1 mL of media. Zeocin is an exception: its 100 mg/mL stock against a 25 µg/mL working concentration gives a 4000-fold dilution, so add 0.25 µL per mL, or scale up to keep pipetting accurate.

Accurate calculation is only half the job; proper handling preserves potency. Always filter-sterilize antibiotic stocks through a 0.22 µm syringe filter rather than autoclaving, because heat destroys most of these molecules. Aliquot stocks into single-use volumes and store at -20 °C to minimize freeze-thaw cycles, which slowly degrade the antibiotic. Light-sensitive drugs such as tetracycline should be stored in amber or foil-wrapped tubes.

When supplementing molten agar, let the flask cool to approximately 50 °C before adding antibiotic, swirl gently to mix without introducing bubbles, and pour plates promptly. Ampicillin in particular loses activity over time even in poured plates because residual beta-lactamase from satellite colonies degrades it; carbenicillin is a more stable alternative for the same beta-lactam selection. Label every stock and plate with the antibiotic name, concentration and date so the next person at the bench can trust your selection. Using this antibiotic calculator consistently across your lab keeps stock and working concentrations reproducible from experiment to experiment.