Crown vs Onlay vs Large Filling — Which Is Right for You?
When a tooth is heavily broken down or decayed, you may have more than one option. This guide explains the differences between a large filling, an onlay, and a full crown to help you decide with your dentist.
Why does a damaged tooth need more than a simple filling?
When a tooth has a small area of decay or damage, a straightforward filling is usually the right answer. But when a large portion of the tooth structure is missing — through extensive decay, a broken cusp, a cracked tooth, or a failing old filling — a simple filling may not be strong enough to hold up to the forces of chewing.
In these situations, your dentist may discuss one of three approaches:
A large direct filling — placed chairside in a single visit using composite resin
An onlay (indirect restoration) — a custom-made piece that covers part of the tooth, made in a laboratory
A full crown (cap) — a laboratory-made covering that fits over the entire visible tooth
Each option has genuine advantages and real limitations. The right choice depends on how much of your natural tooth remains, where the tooth is in your mouth, and other clinical factors your dentist will assess.
Option 1 — Large direct filling (composite resin)
A direct filling is placed in a single appointment. Your dentist removes any decay, shapes the cavity, and builds up the tooth using layers of tooth-coloured composite resin, which are hardened with a blue curing light.
Benefits:
Completed in one visit — no waiting for a laboratory
No need for a temporary restoration
Tooth-coloured and natural-looking
Less tooth removal than a crown in most cases
Most affordable option — typically £120–£300 privately (NHS Band 2: £75.30 where available). The exact cost will be confirmed on your individual treatment plan. Fees vary between practices and depend on the complexity of your case.
Drawbacks and risks:
Large composite fillings are more likely to fracture or wear over time compared to laboratory-made restorations
Shrinkage during setting can sometimes cause sensitivity or gaps at the margins
Not ideal when very little natural tooth structure remains — the filling has nothing strong to bond to
May need replacing sooner — average lifespan of a large composite filling is 5–10 years
Can be technique-sensitive — quality varies depending on tooth anatomy and moisture control
Approximate lifespan: 5–10 years for a large filling, though smaller fillings often last longer.
Option 2 — Onlay (indirect restoration)
An onlay is a custom restoration made in a dental laboratory, designed to cover one or more of the biting cusps of a back tooth while preserving as much of the remaining natural tooth as possible. It sits on top of and around part of the tooth — like a partial crown.
Onlays are made from porcelain (ceramic), gold alloy, or composite resin. Ceramic onlays blend in naturally; gold onlays are extremely durable.
Benefits:
More conservative than a full crown — less natural tooth needs to be removed
Stronger and more durable than a large direct filling
Custom-made for a precise fit and accurate bite
Porcelain onlays look very natural
Typically last 10–20 years with good care
Drawbacks and risks:
Requires two appointments — preparation at the first, fitting at the second (usually 1–2 weeks apart)
A temporary restoration is placed between appointments, which can occasionally come off
More expensive than a direct filling — typically £600–£1,200 privately. The exact cost will be confirmed on your individual treatment plan. Fees vary between practices and depend on the complexity of your case.
If the tooth is very heavily broken down, an onlay may not provide enough coverage — a crown may be needed instead
Small risk of sensitivity after preparation
Approximate lifespan: 10–20 years, and sometimes longer with good oral hygiene.
Option 3 — Full crown (cap)
A crown fits over the entire visible part of a tooth like a thimble over a finger, providing full coverage and protection. To fit a crown, your dentist prepares the tooth by reducing it on all sides, takes an impression, and the crown is made in a laboratory. A temporary crown is worn in the meantime.
Crowns are made from all-ceramic (porcelain), ceramic-fused-to-metal, gold alloy, or zirconia — a very strong white ceramic. Your dentist will discuss which material suits your tooth's position and function.
Benefits:
The strongest protection for a heavily broken down or root-treated tooth
Full coverage prevents further fracture of a weakened tooth
Highly durable — typically last 10–20+ years
Modern ceramic crowns look very natural
Usually recommended after root canal treatment on back teeth to reduce the risk of fracture
Drawbacks and risks:
Requires significant removal of natural tooth structure — this is irreversible
Two appointments required, with a temporary crown in between
Higher cost — typically £800–£1,500 privately (NHS Band 3: £319.10 where available). The exact cost will be confirmed on your individual treatment plan. Fees vary between practices and depend on the complexity of your case.
Small risk of sensitivity or nerve irritation after preparation; in rare cases, root canal treatment may subsequently be needed
If the crown fails, more options remain if some natural tooth is preserved; this depends on how much tooth was left before crowning
Porcelain can chip or crack if subjected to very heavy forces
Approximate lifespan: 10–20 years or more, depending on the material chosen and oral hygiene.
Key factors that guide the decision
Your dentist will weigh up several things when recommending the most suitable option:
How much natural tooth remains — the more tooth structure that is intact, the more conservative the restoration can be
Whether the tooth has had root canal treatment — root-treated back teeth almost always need a crown to prevent splitting
The position of the tooth — back teeth (molars and premolars) take heavier biting forces than front teeth
The extent and pattern of the damage — cracks, multiple missing cusps, or a very large existing filling all influence the decision
Your bite — if you grind your teeth (bruxism), stronger materials and full coverage may be recommended
Your long-term goals — a crown is a larger investment but may be the most durable solution if the tooth is heavily compromised
In some cases, your dentist may explain that a large filling is not recommended — not because it costs less, but because it genuinely carries a higher risk of the tooth fracturing. Protecting a weakened tooth properly can avoid more complex (and expensive) treatment later.
What if the tooth cannot be saved at all?
In some situations, a tooth may be too damaged or decayed to restore predictably with any of the options above. Your dentist will discuss this honestly with you. If the tooth cannot be saved, the options shift to extraction followed by replacement — which is covered in our separate leaflet, Replacing a Missing Tooth — Your Options.
Your dentist should always explain the realistic prognosis of any restoration before you commit to treatment.
Questions to ask your dentist
Use these questions to guide your discussion:
How much natural tooth structure is still sound?
Is an onlay a realistic option for this tooth, or is a crown necessary?
What is the risk if I choose the more conservative (cheaper) option?
Has this tooth had root canal treatment, and does that change the recommendation?
Which material would you recommend for the crown or onlay, and why?
What is the realistic lifespan of each option in my specific situation?
If I have the filling now, what is the likelihood I will need a crown later anyway?
NHS charges are set nationally and reviewed annually. Private fees are a guide — your dentist will confirm the exact cost on your treatment plan before you agree to proceed. NHS availability varies by area and not all treatments are available on the NHS. Your dentist will discuss the best option for your specific case.
This leaflet is for general information only and does not replace professional dental advice. It is intended to support — not replace — the discussion with your dentist about your individual options, risks, and treatment plan. Your dentist will confirm what is suitable for your specific circumstances before you agree to any treatment.