Average Weight & Weight Variation of Tablets
Weighs 20 tablets individually, calculates the average weight, and checks how far each tablet deviates from the average against the limit for that tablet size.
- Gravimetry (analytical balance)
- Analytical balance
Reference: Typical method based on IP Uniformity of weight and Ph. Eur. 2.9.5. Always follow your approved specification and STP.
Why this method works
If the tablet press fills each die evenly, each tablet has nearly the same mass and, for a uniform blend, the same dose. Weighing 20 tablets individually shows the spread. Each tablet's percentage deviation from the average is compared with a limit that depends on the average weight: wider limits for small tablets, tighter limits for large ones.
Instruments used
Reagents & standards
| Reagent | Grade / specification |
|---|---|
| Reagents | None required |
| Tools | Clean forceps, weighing boat, tablet tray numbered 1–20 |
Preparing the sample
- 01
Select
Take 20 tablets at random from the sample.
- 02
Check balance
Confirm the daily check passed and the balance is level.
- 03
Weigh
Weigh each tablet individually and record the weight.
- 04
Average
Calculate the average of the 20 weights.
- 05
Compare
Calculate each % deviation and compare with the limit.
Procedure
- 1Take 20 tablets at random from the sample, dedusting them if necessary.
- 2Tare the balance with a weighing boat in place.
- 3Weigh each tablet individually using forceps, recording each weight to 0.1 mg (print or transfer to the LIMS directly).
- 4Calculate the total and the average weight of the 20 tablets.
- 5Calculate the percentage deviation of each tablet from the average.
- 6Find the lightest and heaviest tablets and compare them with the limits for the average weight.
Notes for analysts
- Pharmacopoeias differ in detail: check the current IP, Ph. Eur. or USP requirement that applies to your product. USP <905> uses weight variation as part of uniformity of dosage units, with a different calculation.
- Capsules: weigh each intact capsule, empty it, weigh the shell and use the content weight; IP limits are ±10% below 300 mg and ±7.5% at 300 mg or more.
- Handle tablets with forceps to avoid moisture pick-up and contamination.
- Record all 20 individual weights; do not record only the average.
Method parameters (typical)
| Number of units | 20 tablets |
|---|---|
| Balance | Analytical balance, readability 0.1 mg, with each tablet above the balance minimum weight |
| Limit: average weight 80 mg or less | ±10% |
| Limit: more than 80 mg and less than 250 mg | ±7.5% |
| Limit: 250 mg or more | ±5% |
| Rule | Not more than 2 tablets outside the % limit, and none outside twice the % limit |
System suitability & sequence
| Parameter | Limit |
|---|---|
| Balance daily check | Passed on the day of testing |
| Minimum weight | Each tablet weight above the balance's working minimum weight |
Typical sequence
- 1Balance daily check
- 2Weigh tablets 1 to 20 individually
- 3Calculate average and % deviations
Calculation with a worked example
Where
- Wi
- Weight of an individual tablet (mg)
- Σ Wi
- Total weight of the 20 tablets (mg)
- Average
- Average tablet weight (mg)
Given
- Individual weights (mg)
- 604.2, 610.5, 598.7, 607.9, 601.3, 612.4, 596.8, 605.6, 608.1, 603.0, 599.5, 611.2, 606.4, 602.7, 597.9, 609.3, 604.8, 600.6, 607.1, 603.5
- Σ Wi
- 12,091.5 mg
- Heaviest tablet
- 612.4 mg
- Lightest tablet
- 596.8 mg
Working
- Average = 12,091.5 ÷ 20 = 604.575 mg (reported as 604.6 mg)
- Average is 250 mg or more, so the limit is ±5%: 574.3 mg to 634.8 mg (twice the limit, ±10%: 544.1 mg to 665.0 mg)
- Heaviest: (612.4 − 604.575) ÷ 604.575 × 100 = +1.29%
- Lightest: (596.8 − 604.575) ÷ 604.575 × 100 = −1.29%
- No tablet deviates by more than 5%
Average weight = 604.6 mg; individual deviations from −1.29% to +1.29%. Complies (limit ±5%).
Try it with your numbersWhat result passes
- Not more than 2 of the 20 tablets deviate from the average weight by more than the percentage limit, and no tablet deviates by more than twice that limit.
- Average weight within the range stated in the product specification (for example target ± 5%), if one is set.
Validation parameters (ICH Q2(R2))
| Parameter | Typical requirement |
|---|---|
| Approach | Compendial test; no method validation needed beyond balance qualification |
| Balance | Calibrated; daily check passed; minimum weight below the lightest tablet |
Run this method in LimsPulse: specifications and STPs under version control, results calculated and checked against limits, and a signed Certificate of Analysis.