How to use
- Pick the antibiotic. Its standard stock and working concentrations fill in, along with the solvent it dissolves in.
- If your stock differs from the standard, edit the concentration — the name switches to “Enter my own” when you do.
- Enter the medium volumes. Several at once, separated by commas or line breaks, gives one row of answers each.
- Add the listed volume of stock to the medium and mix. For plates, wait until the agar has cooled to about 60°C.
Formula and practical notes
The dilution itself is C₁V₁ = C₂V₂, one line. What makes this its own page is the two things around it: the per-antibiotic concentrations people look up every time, and doing several bottles of medium at once.
The arithmetic
Fixing the units at mg/mL and μg/mL makes the conversion disappear, because 1 mg/mL is the same as 1 μg/μL.
- Volume to add (μL)
- working (μg/mL) × medium (mL) ÷ stock (mg/mL)
- Fold dilution
- stock (mg/mL) × 1000 ÷ working (μg/mL)
Practical notes
- These standards are for selecting E. coli. Strain, plasmid copy number and medium all shift them, so follow your own protocol where you have one.
- Ampicillin degrades quickly, in plates and in stock. Carbenicillin is often used in its place: it is more stable and gives fewer satellite colonies, though it costs more.
- Chloramphenicol dissolves in ethanol and coumermycin in DMSO; the rest go into water. A solvent-based stock can affect the culture if too much of it goes in, which is another reason to keep stocks concentrated at around 1000× and add very little.
- This calculator does arithmetic on concentrations and volumes. Which antibiotic and how much of it is an experimental design question, and the standards here are a starting point.
Frequently asked questions
Because 1 mL then goes into 1 L of medium. The added volume is small enough not to change the medium, and the arithmetic becomes mental: medium in mL divided by 1000. All nine standards listed here work out to exactly 1000×.
The formula is the same. Two things differ: this one carries the standard stock and working concentrations and the solvent for each antibiotic, so there is nothing to look up, and it takes several medium volumes at once.
Filter-sterilise, aliquot, and keep at −20°C. Without repeated freeze-thaw cycles they are good for at least 3–6 months. Do not heat-sterilise: autoclaving degrades antibiotics.
Once the autoclaved agar has cooled to around 60°C. At that temperature the agar is still liquid and most antibiotics will not break down. If you cannot hold the bottle with gloves on, let it cool further.
That volume is hard to pipette accurately. Dilute the stock tenfold and add ten times as much, or make a larger batch of medium and split it. The calculator flags those rows but still shows the number.