The simplest way to understand the three is as a scale, based on how much of the tooth is damaged and needs rebuilding. A filling is the smallest repair, used for minor to moderate decay, and it is placed and set straight into the tooth in one visit. An inlay is the next step up, a solid piece custom-made to fit inside the tooth when the damage is a bit too large for a filling to hold well.
An onlay is larger again, covering one or more of the raised points on the chewing surface when more of the tooth is gone. So the real difference is not the purpose, since all three restore a damaged tooth, but the size of the problem each one solves and the way each is made. Think of them as rungs on a ladder, with a filling at the bottom and a crown at the very top.
Where each sits on the restoration ladder
Dentists aim to save as much natural tooth as possible, so they match the repair to the damage rather than always jumping to the biggest fix. Running up the ladder, a filling comes first for small cavities, an inlay handles moderate damage within the tooth, an onlay covers larger damage that reaches the cusps, and a crown caps the whole tooth when very little sound structure is left.
Inlays and onlays fill the important middle ground, stronger and more protective than a filling but far more conservative than a full crown, which is why they are sometimes called partial crowns. Knowing this scale helps the whole thing make sense, because the choice between them is really about how far up the ladder your particular tooth sits.
What a filling is
A filling is the everyday repair most people have had at some point. When a tooth has a cavity, the dentist removes the decay and packs the space with a soft material, usually tooth-coloured composite resin, which is then hardened with a curing light and shaped to match the tooth. The whole thing is done directly in the mouth in a single appointment, which is why a filling is called a direct restoration.
Fillings are quick, affordable, and ideal for small to moderate cavities. Their limit is size. When a cavity gets large, a filling has less healthy tooth to bond to and can be more likely to crack or come loose under chewing pressure over time. The early warning signs that a tooth may need one are covered in detail in the guide to the symptoms of a cavity, and the various options and costs are set out on the dental filling page.
What an inlay is
An inlay steps in when a cavity is too big for a reliable filling but the tooth does not need full coverage. Rather than being packed in soft and set in the mouth, an inlay is a single solid piece, made from porcelain, composite, or sometimes gold, custom-crafted to fit precisely into the hollow within the biting surface of the tooth, sitting neatly between the raised cusps without covering them.
Because it is made outside the mouth and then bonded in, an inlay is called an indirect restoration. Traditionally that meant taking an impression and having the piece made at a lab over a couple of visits, with a temporary in between, though many clinics can now mill one in a single visit using in-clinic scanning and milling. The payoff for the extra process is strength. A solid, precisely fitted inlay restores more of the tooth's original chewing strength than a large filling and tends to last longer in a bigger cavity.
What an onlay is
An onlay is the big sibling of the inlay. It works the same way, as a custom-made solid piece bonded onto the tooth, but it covers more ground. Where an inlay sits within the cusps, an onlay extends over one or more of those cusps, rebuilding the raised edges of the chewing surface when they are too damaged or worn to keep.
This makes an onlay the choice when a tooth is significantly broken down, perhaps from a large old filling that has failed or a crack, yet still has enough healthy structure that a full crown would mean removing more than necessary. By replacing only the damaged parts and protecting what remains, an onlay rebuilds and strengthens the tooth while keeping more of the natural tooth than a crown would. When even an onlay is not enough and the whole tooth needs wrapping, the next step is a dental crown.
The distinctions that matter most
A few practical differences separate the three, beyond simply how much tooth is involved.
How they are made. A filling is direct, built straight into the tooth in one visit. Inlays and onlays are indirect, made as a solid piece and then bonded in, which can mean two visits or one, depending on whether the clinic mills them in-house.
Strength and lifespan. Fillings are perfect for smaller cavities but can struggle in large ones. Inlays and onlays are stronger and tend to last longer where a lot of tooth needs rebuilding, because they are made from solid, hard-wearing material.
How much tooth is kept. This is the heart of it. The approach is always to remove and cover as little as possible, so a filling, inlay, onlay, or crown is chosen to preserve the most natural tooth for the level of damage present.
Cost and visits. A filling is the most economical and the fastest. Inlays and onlays cost more because they are custom-made, and may involve more than one appointment, though they often prove durable in the situations that call for them.
How the dentist decides
The decision comes down to a simple question the dentist answers by examining the tooth and any x-rays: how much healthy tooth is left, and how much needs rebuilding? Small damage gets a filling. Moderate damage that a filling cannot reliably hold gets an inlay. Larger damage reaching the cusps gets an onlay. And a tooth with very little sound structure left gets a crown. The aim throughout is to repair the tooth with the most conservative option that will last, keeping as much of your own tooth as possible. An in-person assessment is the only way to know which rung your tooth sits on, since it depends entirely on the state of that specific tooth.
Frequently asked questions
Is an inlay or onlay better than a filling? Not better in general, just suited to different amounts of damage. A filling is the right, more economical choice for small to moderate cavities. An inlay or onlay is better when a cavity is large, because a solid, custom-made piece restores more strength and tends to last longer than a big filling that might crack or loosen over time.
Do inlays and onlays take more than one visit? Traditionally yes, because they are custom-made, which usually meant an impression, a temporary, and a second visit to bond the finished piece. Many clinics now use in-clinic scanning and milling that can produce an inlay or onlay in a single appointment, so it depends on the technology your clinic uses.
