Short answer: Hardness measures how much force a tablet resists before fracturing, tested by placing a tablet between the jaws of a hardness tester (a dedicated handheld tester is the practical option for a small lab, versus a benchtop pharma-grade unit) and recording the force at which it breaks — most direct-compression tablets target somewhere in the 4–10 kp range, though the right number depends on your formulation. Friability measures how much a tablet's surface erodes from handling, tested by weighing a sample of tablets, tumbling them in a friabilator for 100 rotations at 25 rpm (4 minutes), and reweighing — a weight loss under 1% is the standard pass threshold. Both tests are quick, and running them on every fill-depth or formulation change catches problems a scale alone won't.
Weight is only half the quality picture — see how to calculate tablet weight for the other half. Two tablets can weigh exactly the same and still differ enormously in whether they'll survive shipping intact or dissolve at the intended rate, which is exactly what hardness and friability are measuring.
Why Test Both, Not Just One
Hardness and friability aren't measuring the same thing, and a tablet can pass one while failing the other:
- A tablet can be hard but still friable if it's hard at the core but has a weak, dusty surface layer — common with insufficient binder even at otherwise-adequate compression force.
- A tablet can be soft but pass a friability test if it's evenly (if weakly) bonded throughout — it survives gentle tumbling but would likely fail a real-world drop or crush test that a hardness tester catches directly.
Testing both after any formulation or fill-depth change is what actually confirms a batch is production-ready, rather than assuming one good result means the other is fine too.
How to Test Hardness
- Pull a random sample from the batch — 10 tablets is a reasonable sample size for a routine check.
- Place one tablet between the tester's jaws, oriented consistently (the same way every time — usually on its edge for a round tablet) so results are comparable tablet to tablet.
- Apply increasing force until the tablet fractures, and record the reading at the moment it breaks. Handheld testers display this directly; either way, the number you want is the force at fracture, not before or after.
- Repeat across the full sample and calculate the average, along with the range (lowest to highest reading) — a wide range matters as much as the average, since it signals inconsistent compression even if the average looks fine.
What the number means: Most direct-compression tablets target roughly 4–10 kp (kilopond, a common hardness-tester unit), but the right target depends on tablet size, intended use, and disintegration requirements — a fast-disintegrating tablet is often formulated toward the softer end of an acceptable range on purpose, since a very hard tablet can dissolve too slowly. There's no single correct number; consistency of the number batch to batch matters more than hitting a specific value.
If hardness is low or inconsistent: this usually traces back to compression force, dwell time, or binder content — see diagnosing capping and lamination if tablets are outright splitting rather than just testing soft, since that's a related but more severe version of the same underlying issue.
How to Test Friability
- Weigh a sample of tablets — commonly around 6.5 g total for tablets under 650 mg each, or 10 whole tablets for heavier ones — and record the starting weight.
- Place the sample in a friabilator, a drum that tumbles tablets by lifting and dropping them repeatedly to simulate handling stress.
- Run the drum for 100 rotations at 25 rpm (roughly 4 minutes).
- Remove the tablets, brush off any loose dust, and reweigh the sample.
- Calculate percent weight loss: (starting weight − ending weight) ÷ starting weight × 100.
What the number means: A weight loss under 1% is the standard pass threshold used across the industry. Above that, tablets are shedding enough material during ordinary handling that packaging, shipping, and shelf life are all likely to suffer — visible chalkiness or dusting in the packaging is often the first real-world sign of a friability problem before anyone runs the formal test.
If friability is high: this points toward the same causes as chipping — excess fines, insufficient binder, or compression force that's too low for the formulation. Since friability and chipping share root causes, a batch that fails friability testing is also likely to show visible chipping in handling even before it's tested formally.
Testing Without a Full Lab Setup
A small lab or first-time producer doesn't need pharma-grade benchtop equipment to get a meaningful result:
- Handheld hardness testers are compact, reasonably priced, and accurate enough for formulation development and routine batch checks — a full digital benchtop tester is more relevant once you're running formal batch-release documentation than for validating a formulation.
- Basic friabilators are similarly available at a small-lab-appropriate scale and price point without needing an industrial QC department behind them.
- Both tests take only a few minutes per batch, which is why it's worth budgeting the equipment and the time into your first production run checklist rather than treating testing as optional overhead — catching a bad batch here is far cheaper than catching it after packaging or shipping.