Enter the weights of 20 tablets, capsules or other single-dose units to check weight variation (uniformity of mass). The calculator finds the average and each unit’s % deviation, applies the Ph. Eur. 2.9.5 / BP / IP limit for your dosage form and average weight, and tells you whether the batch complies.
- Units (n)
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- Average weight
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- Highest (deviation)
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- Lowest (deviation)
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- Units beyond ± limit
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- Units beyond ± 2 × limit
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- Allowed range (± limit)
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- Allowed range (± 2 × limit)
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Show the deviation of each unit
What is the weight variation (uniformity of mass) test?
The weight variation test, called uniformity of mass in Ph. Eur. 2.9.5, BP and IP, checks that individual tablets, capsules or single-dose units have a consistent weight. Weigh 20 units taken at random, calculate the average, and compare each unit’s deviation from the average with the permitted percentage for that dosage form and average weight. For capsules and single-dose powders for parenteral use, weigh the contents of each unit.
Formula
% deviation = (individual weight − average weight) ÷ average weight × 100
The average weight is the mean of all 20 units.
Acceptance criteria (Ph. Eur. 2.9.5 / BP / IP)
| Dosage form | Average mass | Permitted deviation |
|---|---|---|
| Tablets (uncoated and film-coated) | 80 mg or less | ± 10% |
| More than 80 mg and less than 250 mg | ± 7.5% | |
| 250 mg or more | ± 5% | |
| Capsules, uncoated single-dose granules and single-dose powders | Less than 300 mg | ± 10% |
| 300 mg or more | ± 7.5% | |
| Powders for parenteral use (single-dose) | More than 40 mg | ± 10% |
| Suppositories and pessaries | All masses | ± 5% |
| Powders for eye drops and eye lotions (single-dose) | Less than 300 mg | ± 10% |
| 300 mg or more | ± 7.5% |
The test passes when not more than 2 of the 20 individual masses deviate from the average by more than the permitted percentage, and none deviates by more than twice that percentage. For powders for parenteral use with an average mass of 40 mg or less, use the uniformity of content test (2.9.6) instead.
Worked example: film-coated tablets
The 20 tablet weights in the calculator average 301.38 mg. Because the average is 250 mg or more, the permitted deviation is ± 5%. That gives an allowed range of 286.31–316.45 mg (± 10% would be 271.24–331.52 mg). The heaviest tablet, 315.8 mg, deviates by +4.78% and the lightest, 296.1 mg, by −1.75%. No unit is outside ± 5%, so the batch complies.
What about USP?
USP does not use this percentage table for release testing of drug products. USP <905> Uniformity of Dosage Units applies, either by content uniformity or by weight variation. Weight variation there means calculating an estimated content for each unit from its weight and the assay, then applying the acceptance value (AV) criteria. If you work to USP, use <905>. The percentage-deviation test above is still widely used for in-process control.
Common mistakes
- Using the label weight instead of the measured average weight.
- Choosing the wrong band near a boundary. The band depends on the actual average mass, for example 249 mg vs 251 mg for tablets.
- Weighing whole capsules instead of the contents, when the method requires contents.
- Testing fewer than 20 units for release, or not selecting them at random.
- Applying this test to sugar-coated tablets or to forms where the monograph requires content uniformity.
Frequently asked questions
How many tablets are used for weight variation?
20 units, taken at random.
What is the weight variation limit for a 500 mg tablet?
± 5%, because the average mass is 250 mg or more. Not more than 2 tablets may deviate by more than 5%, and none by more than 10%.
What is the limit for capsules?
± 10% if the average content mass is less than 300 mg, and ± 7.5% if it is 300 mg or more.
Is weight variation the same as content uniformity?
No. Weight variation checks mass. Content uniformity measures the drug content of individual units. The pharmacopoeia specifies which test applies, for example content uniformity for low-dose products.
Related tools and guides
References
- Ph. Eur. 2.9.5 Uniformity of mass of single-dose preparations (reproduced in the BP; the IP has a corresponding test).
- USP General Chapter <905> Uniformity of Dosage Units.