Short answer: An excipient is any ingredient in a tablet that isn't the active ingredient — the material that does the actual work of turning loose powder into a tablet that fills to the right weight, holds together, and behaves correctly once swallowed. Every direct-compression formulation needs some combination of five functional categories: fillers (bulk), binders (hold it together), disintegrants (break it apart again), lubricants (release it from the die), and glidants (keep the powder flowing). Most active ingredients need a small dose of several of these, not one "magic" ingredient.
If you're staring at a formulation sheet for the first time, the terminology alone can be the hardest part — five categories, often overlapping in what a single ingredient actually does. This is the plain-language version: what each category does, why it exists, and roughly how much of it shows up in a typical formula.
Fillers (Bulking Agents)
Most active ingredients are only needed in milligram doses — far too little powder to fill a die cavity and compress into a handleable tablet on their own. A filler (also called a diluent) is the bulk ingredient that makes up the rest of the tablet's weight so there's enough material for the press to actually compress.
Common fillers: microcrystalline cellulose (MCC), dicalcium phosphate, lactose, mannitol. Microcrystalline cellulose is the default starting point for most direct-compression formulas because it's also self-binding — it does filler and binder duty at once. Dicalcium phosphate is denser and flows better, which is why it's often blended alongside MCC to balance compressibility against flow.
Typical use level: anywhere from 10% to 90%+ of total tablet weight — fillers usually make up the majority of the tablet by weight, since they're filling the gap between the active dose and a workable tablet size.
Binders (Adhesives)
A binder holds powder particles together under compression so the tablet doesn't crumble the moment it's ejected from the die. Some fillers (MCC, for one) are self-binding at normal use levels; formulations with a lot of poorly-compressible active ingredient often need an added binder on top.
Common binders: povidone (PVP), HPMC, pregelatinized starch, and — in wet granulation, which is a separate process from direct compression — a binder solution sprayed onto powder before it's dried and compressed. See direct compression vs. wet granulation if you're not sure which approach applies to you.
Typical use level: 2–10% for most direct-compression binders, though this varies a lot by how compressible the active ingredient itself is.
Disintegrants
A disintegrant does the opposite job of a binder: it makes the tablet fall back apart once it's in the body, so the active ingredient is exposed and can dissolve. Without a disintegrant, a well-compressed tablet can take an uncomfortably long time to break down, or fail to break down fully.
Common disintegrants: croscarmellose sodium, crospovidone, sodium starch glycolate. These work by swelling rapidly on contact with moisture, physically pulling the tablet apart from the inside. Our ReadyBlend Fast-Disintegrating blend is built around a superdisintegrant specifically for tablets that need to break apart in under 60 seconds. For the full mechanics, see tablet disintegrants explained.
Typical use level: 1–5% is typical for a standard-release tablet; fast-dissolve and orally-disintegrating formats run higher.
Lubricants
A lubricant reduces friction between the compressed tablet and the die wall, so the tablet ejects cleanly instead of sticking, capping, or tearing on the way out. This is a mechanical function, not a formulation-strength one — too little lubricant causes sticking and picking; too much can weaken the tablet and slow disintegration.
Most common lubricant: magnesium stearate, used in the overwhelming majority of direct-compression formulas. See tablet lubricants explained for exact use-level guidance and what to reach for if you need a magnesium-stearate alternative.
Typical use level: 0.25–2% — a small enough amount that over-blending is a more common mistake than under-dosing.
Glidants (Flow Agents)
A glidant reduces friction between powder particles, not between the powder and the die — its job is keeping the blend flowing freely into the die cavity so every tablet fills to the same weight. Poor powder flow is one of the most common causes of weight variation across a batch; see powder flowability and tablet compression for how to test for it.
Most common glidant: colloidal silicon dioxide (fumed silica). See our silicon dioxide product page for exact dosing.
Typical use level: 0.25–1%, added last in the blending sequence so it coats the other particles rather than clumping.
Putting It Together: A Simple Starting Formula
| Category | Function | Typical range | Example |
|---|---|---|---|
| Filler | Bulk / fill weight | 10–90% | Microcrystalline cellulose |
| Binder | Hold tablet together | 2–10% | Povidone, or self-binding filler |
| Disintegrant | Break tablet apart | 1–5% | Croscarmellose sodium |
| Lubricant | Clean ejection | 0.25–2% | Magnesium stearate |
| Glidant | Powder flow | 0.25–1% | Colloidal silicon dioxide |
For a worked starting ratio and how to adjust it against your first test batch, see how to make a direct-compression powder — or skip the formulation work entirely with a pre-balanced blend like ReadyBlend, which already combines these five roles.