How the Weight Variation Calculator works

Weight variation, also called uniformity of mass, shows whether the units in a batch are close to the average. Twenty tablets are weighed individually, the average is calculated, and each tablet is compared with the limit that applies to that average weight. The calculator does the arithmetic and marks each unit as inside the limit, outside it, or outside twice the limit.

It covers three methods: the BP, Ph. Eur. and IP uniformity of mass test, the 20-tablet weight table from USP <2091>, and the USP <905> acceptance value for 10 or 30 units. An in-process mode compares readings with your own target and tolerance.

Video walkthrough

Deviation and acceptance value

Example: 200 mg tablets, BP limit ±7.5%

BP and Ph. Eur. test on 20 tablets with an average weight of 200 mg. The limit is ±7.5%.

1. Limits = 200 × 0.925 to 200 × 1.075 = 185 mg to 215 mg
2. Twice the limit (±15%) = 170 mg to 230 mg
3. Two tablets weigh 183 mg (−8.5%) and 216 mg (+8.0%). Both are outside the limit, so two units fail the first test.
4. No tablet is outside 170–230 mg, so none exceeds twice the limit.

The batch passes, because no more than two units are outside the limit and none is outside twice the limit. A third tablet at 184 mg would fail the batch.

Limits by method

• Pass rule: no more than 2 of 20 units outside the limit, and none outside twice the limit.
• USP <905>: Stage 1 with 10 units passes at AV of 15.0 or less. Otherwise 20 more units are tested, and Stage 2 passes if the AV of all 30 is 15.0 or less and no unit is outside the 0.75M–1.25M range.
• Scope: the 130 mg / 324 mg table is the one published in USP <2091> for dietary supplement tablets and is widely taught as the classic USP weight limit. For drug products, USP <905> (weight variation or content uniformity, with the acceptance value) applies. Check which method applies to your product in the current monograph, because some dosage forms need content uniformity testing instead.

Mistakes in the weight test

• Comparing each tablet with the target weight instead of the measured average of the 20 tablets.
• Using the BP and Ph. Eur. limit bands for a USP test, or the reverse. The weight cut-offs differ.
• Applying weight variation where content uniformity is required.
• Weighing coated and uncoated tablets together, or mixing mg and g readings.
• Rounding individual readings before averaging.

Standards and references

The checks on this page relate to the documents below. Standards are revised from time to time, so confirm the current edition and follow your own approved procedures.

• USP <905> Uniformity of Dosage Units
• Ph. Eur. 2.9.40, Uniformity of dosage units
• Ph. Eur. 2.9.5, Uniformity of mass of single-dose preparations

Related tool: the Tablet Friability Calculator checks how well tablets resist abrasion and breakage.

Questions on weight variation

How many tablets are tested?

The BP, Ph. Eur., IP and USP tablet weight tests use 20 tablets, weighed one by one.

Can weight variation replace content uniformity?

No. It is an indirect check and only applies where the pharmacopoeia permits it, usually for higher-dose tablets with a high proportion of active.

What is the acceptance value?

A figure combining how far the mean estimated content is from the target and how large the spread between units is. Stage 1 passes at 15.0 or less.

What should I do if a batch fails?

Stop and investigate feeder, die fill, granule flow and tooling, then follow your deviation procedure.

Related tools and guides: Force to Pressure Guide · Weight Variation Test Guide · Tablet Weight Calculator · Compression Reconciliation · Bulk and Tapped Density

By Sajjad Ahmad. For reference and education only; verify limits against your SOPs and the current pharmacopoeia. See the methodology.