HPLC Resolution, Plate Count & Tailing Factor Calculator (USP <621>)

Check a critical pair against USP <621> system suitability.

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Peak 1 tR min

Peak 2 tR min

Peak 1 width min

Peak 2 width min

Width measured at

Peak 2 width at 5% height min

Peak 2 front half at 5% min

Example values shown. Tailing uses the width at 5% height and its leading half (f).

Resolution Rs–

Plates N (peak 2)–

Tailing T–

Rs = 1.18(t₂ − t₁)/(W0.5,1 + W0.5,2) or 2(t₂ − t₁)/(Wb,1 + Wb,2) · N = 5.54(tR/W0.5)² or 16(tR/Wb)² · T = W0.05/2f

How it works

Resolution tells you whether two neighbouring peaks are separated enough to integrate reliably. Rs = 1.5 is baseline resolution for two equal, symmetrical peaks; most methods ask for Rs ≥ 2.0 for the critical pair to leave margin for column ageing and small changes in conditions.

Plate count (N) measures column efficiency, and the USP tailing factor (T) measures peak symmetry at 5% of the peak height. T = 1.0 is perfectly symmetrical; values above 2.0 usually point to secondary interactions with silanols, column overload, a void, or extra-column volume.

Use the same width type for both peaks. Half-height widths are easier to measure on noisy or partly co-eluting peaks, which is why most data systems report them.