Mitotic Index: How to Calculate the Fraction of Dividing Cells
The mitotic index is the percentage of cells in a sample that are in mitosis: mitotic cells divided by total cells counted, times 100. Count 24 dividing cells among 300 and the index is 8%. It is a simple, reproducible measure of how fast a tissue or culture is proliferating, used from onion-root-tip lab exercises to research on growth and drug response. This guide covers the formula, what to count, and how it relates to the mitotic counts pathologists use.
What the Mitotic Index Measures
In a stained tissue section most cells are in interphase. A few are caught mid-division, with condensed or separating chromosomes. The mitotic index is the share of cells in any visible mitotic phase (prophase, metaphase, anaphase or telophase) at the moment the sample was fixed, expressed as a percentage. More dividing cells means a higher index and a faster-growing population.
It is used to map growth zones in developing tissue, to confirm that a cell culture is proliferating rather than stalled, and to compare how treatments change division rates. It needs no expensive markers, only a stain, a microscope and careful counting, which is why it remains a staple of teaching labs.
How to Calculate the Mitotic Index
Mitotic index (%) = (Mitotic cells ÷ Total cells counted) × 100
A mitotic cell is any cell showing a recognizable stage of mitosis. The total includes every cell in the fields you examine, dividing and non-dividing alike, so interphase cells go in the denominator but never the numerator.
Worked example. Counting a stained onion-root-tip preparation:
- Mitotic cells (clearly in some phase of division): 24
- Total cells counted across your fields of view: 300
1. 24 ÷ 300 = 0.08
2. 0.08 × 100 = 8%
Roughly 8 in every 100 cells were dividing when the root was fixed, which fits the actively growing meristem just behind the tip. The Mitotic Index Calculator takes the mitotic count and the total count and returns this percentage.
If you are counting cells in suspension rather than in tissue, you first need the concentration; the Cell Density Calculator converts a hemocytometer count, the number of squares counted and the dilution factor into cells per milliliter.
Mitotic Index Versus the Clinical Mitotic Count
Pathologists grading tumors do not report a percentage. In the Nottingham system for breast cancer, for example, mitotic figures are counted in 10 high-power (400×) fields, and scoring thresholds depend on the microscope's field area; current standards prefer mitoses per square millimeter so counts are comparable between microscopes. That mitotic count is a density per area, not a fraction of cells, so a research or classroom mitotic index cannot be dropped into a tumor-grading table.
Putting the Index to Work
Comparing treatments. Count treated and control cultures under the same protocol. A falling index after exposure to a compound suggests it is suppressing division.
Mapping growth. In a root or an embryo, the index is highest in meristematic or stem-cell regions and falls toward mature tissue.
Tracking a culture over time. Sampling at intervals shows the index rise during exponential growth and fall as cells reach confluence or exhaust nutrients.
Standardizing counts. Count a consistent, large number of cells per sample, often several hundred to a thousand, with the same field-selection strategy each time.
Common Mistakes
Counting too few cells. A handful of mitotic figures in a small sample gives an unstable percentage. With 300 cells, one miscounted figure moves the index by 0.33 points; with 50 cells it moves it by 2 points.
Misidentifying figures. Pyknotic (dying) nuclei and densely stained debris can pass for mitoses. Confirm the chromosomal pattern of a genuine phase before counting a cell as dividing.
Choosing eye-catching fields. Picking the densest patch of dividing cells inflates the index. Select fields systematically or at random.
Dropping cells from the total. Faint or overlapping cells still belong in the denominator. Omitting them overstates the index.
Comparing across protocols. Stain, fixation time and counting rules all change what is visible. Hold the method constant when comparing samples.
Related guide: Hardy-Weinberg Equilibrium: How to Calculate Genotype Frequencies in a Population covers another count-based calculation from the biology lab.