Assay Calculator (HPLC/UV): % Label Claim Formula & Example

Calculate HPLC or UV assay results as % of label claim for tablets and capsules, or as % w/w for APIs. Enter your standard and sample peak areas, weights, dilutions and standard potency. The calculator checks the standard %RSD, shows the mg per unit and the result for each injection, and lists every step so you can check it against your worksheet.

Calculate for Dosage form (% of label claim) API / raw material (% w/w)

Standard Standard peak areas (one per line) Standard weight, W_s (mg) Total dilution volume, D_s (mL) Standard potency, P (% as is)

Sample Sample peak areas (one per line) Sample weight, W_u (mg) Total dilution volume, D_u (mL) Average weight per unit (mg) Label claim per unit (mg)

Optional: salt/base conversion and limits

Conversion factor (e.g. MW base ÷ MW salt; 1 if not needed) Standard %RSD limit (system suitability) Assay limit, lower (%) Assay limit, upper (%)

Total dilution volume = first volume × (final volume ÷ aliquot) for each further dilution. Example: 100 mL, then 5 mL → 50 mL gives 100 × 50 ÷ 5 = 1,000 mL.

Assay–

Mean standard area (n): –
Standard %RSD: –
Mean sample area (n): –
Content per unit: –

Show the calculation step by step

What is an assay calculation?

An assay measures how much of the active ingredient is present. For a finished product it is reported as a percentage of the label claim, for example 99.3% of 500 mg. For an API or raw material it is reported as % w/w. In HPLC and UV methods with an external standard, the sample response is compared with the response of a reference standard of known concentration. The calculation then corrects for weights, dilutions and standard potency.

Assay formula (external standard)

% of label claim = (A_u ÷ A_s) × (W_s ÷ D_s) × (D_u ÷ W_u) × (P ÷ 100) × (Average weight ÷ Label claim) × 100

% w/w (API) = (A_u ÷ A_s) × (W_s ÷ D_s) × (D_u ÷ W_u) × P

A_u, A_s = mean peak areas of sample and standard · W_s, W_u = weights of standard and sample (mg) · D_s, D_u = total dilution volumes (mL) · P = standard potency (% as is) · average weight and label claim in mg per unit

If the standard is a salt and the label claim is expressed as the base (or the other way round), multiply by the conversion factor (MW of base ÷ MW of salt). Use the potency on the same basis as the label claim, for example “as is” or corrected for water.

Worked example: paracetamol 500 mg tablets

Item	Value
Standard	50.2 mg paracetamol RS (potency 99.8%) in 100 mL; 5 mL diluted to 50 mL → D_s = 1,000 mL
Sample	60.4 mg tablet powder in 100 mL; 5 mL diluted to 50 mL → D_u = 1,000 mL
Average tablet weight / label claim	600 mg / 500 mg
Standard areas (5 injections)	1,250,430; 1,248,876; 1,252,110; 1,249,954; 1,251,302 → mean 1,250,534.4, %RSD 0.10%
Sample areas (2 injections)	1,246,880; 1,248,210 → mean 1,247,545.0

Standard concentration: 50.2 ÷ 1,000 × 0.998 = 0.050100 mg/mL.
Area ratio: 1,247,545.0 ÷ 1,250,534.4 = 0.99761.
Sample concentration found: 0.99761 × 0.050100 = 0.049980 mg/mL.
Content per tablet: 0.049980 × 1,000 ÷ 60.4 × 600 = 496.49 mg.
% of label claim: 496.49 ÷ 500 × 100 = 99.3%.

These are the default values in the calculator above, so you can follow every step there.

System suitability comes first

Before calculating results, confirm that the system suitability criteria in the method are met. These typically include the %RSD of replicate standard injections (commonly NMT 2.0% for assays), tailing factor, theoretical plates and resolution. Many methods also require a check standard, prepared separately, to agree with the working standard within a set limit (for example 98.0–102.0%). If system suitability fails, sample results must not be reported. Use the RSD calculator to check your replicates.

Typical assay limits

Acceptance criteria come from the pharmacopoeial monograph or the registered specification, not from a general rule. Common examples include 90.0–110.0% or 95.0–105.0% of label claim for finished products, and 98.0–102.0% for many APIs (on the dried or anhydrous basis). Always use the limits in your specification.

Common mistakes

Ignoring standard potency, or using it on the wrong basis (as is vs dried or anhydrous).
Forgetting a dilution step. Write the full dilution scheme and calculate D_s and D_u before you start.
Salt/base mix-ups, for example a hydrochloride standard for a label claim expressed as the base.
Using one standard injection without checking replicate precision.
Averaging away a problem. Look at each injection and each preparation. A large difference between duplicates needs investigation, not averaging.
Rounding intermediate results. Round only the final reported value, as your SOP requires.

A result outside the specification is an OOS result and must be investigated. See the OOS procedure guide.

Frequently asked questions

What is the difference between % assay and % label claim?

For a finished product, assay is usually reported as % of label claim, meaning the amount found per unit divided by the amount stated on the label. For an API, assay is reported as % w/w of the substance, on the basis stated in the monograph.

Why do I need the average weight?

You weigh powdered tablets, not single tablets. The average weight converts the amount found in the weighed powder to the amount in one tablet.

What is the total dilution volume?

It is the overall volume into which the weighed material is effectively diluted. For 100 mL followed by 5 mL to 50 mL, it is 100 × 50 ÷ 5 = 1,000 mL.

Can I use this calculator for UV assays?

Yes. Enter absorbance values in place of peak areas, as long as the method uses a reference standard and the response is linear.

How many sample preparations should I make?

Follow the method. Duplicate sample preparations, each injected at least once, are common. Report the mean and check that the individual results agree.

Related tools and guides

References

USP General Chapter <621> Chromatography.
ICH Q2(R2) Validation of Analytical Procedures.
The relevant USP, BP or Ph. Eur. monograph for the product being tested.