Mitotic Rate per 2 mm²

Converts a raw HPF mitotic count into mitoses/2 mm² using your microscope's actual field diameter.

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This calculator performs mathematical normalization of an observed mitotic count to 2 mm² based on the entered microscope field diameter and number of HPFs examined. It does not independently determine tumor grade, diagnosis, or prognosis. Apply tumor-specific counting methodology and grading criteria according to the current WHO classification, CAP protocol, and other applicable validated guidelines.

Reference

Field diameter

Field diameter must correspond to the actual microscope field being used for mitotic counting. Do not assume that all 40× HPFs have the same area, and do not calculate HPF area from objective magnification alone — the actual field diameter is required because HPF area varies between microscopes.

Counting rule

Count only clearly identifiable mitotic figures. Do not count hyperchromatic nuclei, karyorrhectic nuclei, apoptotic nuclei, degenerative nuclear changes, or ambiguous nuclear fragments.

The mitotic count should be performed in the appropriate mitotically active area according to the relevant tumor classification/protocol.

Why mitoses/HPF is not the answer

Dividing mitoses by the number of HPFs gives mitoses per field, which depends entirely on how large that particular microscope's field happens to be. Counting 25 mitoses in 42 HPFs is not simply "0.60 mitoses/HPF" as a reportable figure — that number means something different on every microscope.

This calculator instead determines the physical area those 42 fields represent (about 9.98 mm² for a 0.55 mm field) and normalizes to a fixed area, giving 5.0 mitoses/2 mm², which is comparable across microscopes.

Formulas used

HPF area A = π(d/2)²

Total area examined = A × N

HPFs for 10 mm² = 10 ÷ A

HPFs equivalent to 2 mm² = 2 ÷ A

Mitoses/2 mm² = (M × 2) ÷ (A × N)

All calculations are carried at full internal precision; the HPF area is never rounded before being used in the total-area or rate calculations. Rounding is applied only to displayed values.

Field diameter reference table

Generated from the formula, not from stored values.

Field diameter	HPF area (mm²)	HPFs for 10 mm²	HPFs ≈ 2 mm²
0.40 mm	0.1257	≈80	≈15.9
0.41 mm	0.1320	≈76	≈15.1
0.42 mm	0.1385	≈72	≈14.4
0.43 mm	0.1452	≈69	≈13.8
0.44 mm	0.1521	≈66	≈13.2
0.45 mm	0.1590	≈63	≈12.6
0.46 mm	0.1662	≈60	≈12.0
0.47 mm	0.1735	≈58	≈11.5
0.48 mm	0.1810	≈55	≈11.1
0.49 mm	0.1886	≈53	≈10.6
0.50 mm	0.1963	≈51	≈10.2
0.51 mm	0.2043	≈49	≈9.8
0.52 mm	0.2124	≈47	≈9.4
0.53 mm	0.2206	≈45	≈9.1
0.54 mm	0.2290	≈44	≈8.7
0.55 mm	0.2376	≈42	≈8.4
0.56 mm	0.2463	≈41	≈8.1
0.57 mm	0.2552	≈39	≈7.8
0.58 mm	0.2642	≈38	≈7.6
0.59 mm	0.2734	≈37	≈7.3
0.60 mm	0.2827	≈35	≈7.1
0.61 mm	0.2922	≈34	≈6.8
0.62 mm	0.3019	≈33	≈6.6
0.63 mm	0.3117	≈32	≈6.4
0.64 mm	0.3217	≈31	≈6.2
0.65 mm	0.3318	≈30	≈6.0
0.66 mm	0.3421	≈29	≈5.8
0.67 mm	0.3526	≈28	≈5.7
0.68 mm	0.3632	≈28	≈5.5
0.69 mm	0.3739	≈27	≈5.3