Microbiology
Zone of Inhibition Calculator
Reads a Kirby-Bauer disk diffusion zone diameter as susceptible, intermediate or resistant.
Zone diameter
Enter the zone diameter in millimetres.
Zone measurement
Edge to edge of the clear zone, through the centre of the disk.
Try an example measurement
Petri dish visualisation
Zone drawn to scale against a standard plate.
CLSI zone diameter breakpoints (mm)
Examples of drug-specific breakpoints for comparison.
| Antibiotic | S (≥) | I | R (≤) |
|---|---|---|---|
| Ampicillin | 17 | 14‑16 | 13 |
| Cefotaxime | 26 | 23‑25 | 22 |
| Ciprofloxacin | 21 | 16‑20 | 15 |
| Gentamicin | 15 | 13‑14 | 12 |
| Tetracycline | 19 | 15‑18 | 14 |
Source: CLSI M100, 2023
There is no formula to calculate — the zone diameter itself is compared with breakpoints. This calculator uses one general set:
In practice, breakpoints differ by drug and organism because each antibiotic diffuses through agar at a different rate and has a different achievable blood level. The CLSI table shows how much they vary — gentamicin is susceptible from 15 mm, cefotaxime only from 26 mm.
Common questions
Hold the plate against a dark background and measure the diameter of the clear area, including the disk, to the nearest millimetre with a ruler or caliper. Measure from the back of the plate if the lid obscures the edge.
This calculator applies one general cut-off to every drug so students can learn the idea of S/I/R. Clinical reports must use the breakpoint for that specific drug–organism pair from the current CLSI M100 or EUCAST tables.
An inoculum that is too heavy or too light, agar that is not 4 mm deep, plates incubated too long, or disks that have lost potency. The lawn should be confluent and match a 0.5 McFarland standard.
Scattered colonies within the zone can indicate a mixed culture or resistant subpopulation. Re-check purity and, if they persist, treat the organism as resistant.
Prepared by the PharmaWallah student team. Verify results with your textbook or instructor. Report an error · Editorial policy