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HPLC System Suitability Calculator: Plates, Tailing, Resolution & %RSD
Check HPLC system suitability in seconds: theoretical plates (half-height or baseline width), tailing factor, retention factor k′, resolution and selectivity between two peaks, and %RSD of replicate injections – with pass/fail against your limits and the Ph. Eur./USP maximum %RSD. Free, with a printable record.
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How it works
Plates (baseline tangent): N = 16 × (tR ÷ W)²
Tailing factor: T = W₀.₀₅ ÷ 2f
Retention factor: k′ = (tR − t₀) ÷ t₀
Resolution (half height): Rs = 1.18 × (t₂ − t₁) ÷ (W½,₁ + W½,₂)
Selectivity: α = k₂ ÷ k₁
Maximum %RSD = K × B × √n ÷ t(90 %, n − 1), K = 0.349
W½ is the peak width at half height, W₀.₀₅ the width at 5 % of the peak height and f the distance from the peak front to the perpendicular dropped from the apex, at 5 % height. Use the same time unit for all inputs. Most chromatography software reports these values directly.
Worked example
Assay peak at 6.42 min
W½ = 0.112 min gives N = 5.54 × (6.42 ÷ 0.112)² = 18,203 plates. With W₀.₀₅ = 0.215 min and f = 0.098 min, tailing = 0.215 ÷ (2 × 0.098) = 1.10. A neighbouring peak at 5.80 min with W½ = 0.105 min gives Rs = 1.18 × 0.62 ÷ 0.217 = 3.37.
Typical acceptance criteria
| Parameter | Common limit |
|---|---|
| %RSD of replicate injections (assay) | Not more than 2.0 % (5 or 6 injections), or the Ph. Eur./USP maximum %RSD |
| Tailing (symmetry) factor | 0.8 – 1.8 by default in Ph. Eur./USP for assays; many monographs say not more than 2.0 |
| Resolution | Not less than 1.5 – 2.0 between the critical pair |
| Theoretical plates | Method-specific, often not less than 2000 |
Always use the limits in your monograph or validated method; these are common values only.
Frequently asked questions
Should I use the half-height or the baseline formula for plates?
The pharmacopoeias (USP <621>, Ph. Eur. 2.2.46, IP) use the width at half height, N = 5.54 (tR/W½)². The tangent method, N = 16 (tR/W)², gives similar results for symmetrical peaks but differs for tailing peaks. Use the one your method specifies.
What is the difference between tailing factor and asymmetry factor?
The USP tailing factor and the Ph. Eur. symmetry factor are both W₀.₀₅ ÷ 2f, measured at 5 % of peak height. The older asymmetry factor (b/a at 10 % height) gives different numbers and is not the pharmacopoeial value.
What is the maximum %RSD in the harmonised chapter?
For assays of active substances where the content limit is close to 100 %, Ph. Eur. 2.2.46 and USP <621> set the maximum %RSD from the upper limit B and the number of injections n: for B = 2.0 % it is 0.41 % (3 injections), 0.59 % (4), 0.73 % (5) and 0.85 % (6). Enter B in the replicate tab to apply it.
Can I enter areas copied from Excel?
Yes. Values separated by spaces, new lines or commas work, and numbers with thousands separators (10,00,000 or 1,000,000) work when values are separated by spaces or new lines. If you separate values with commas, put a space after each comma. The result lists the values it read so you can check them.