How is an ICL sized?
One of the most common questions patients have — explained simply, with no jargon.
An ICL is sized to fit the space inside your eye where the lens rests. The most accurate modern method uses AI (such as ICLFit.com) to predict the resulting “vault” for your individual eye — more reliable than estimating from the cornea’s white-to-white width alone.
An ICL has to fit the space inside your eye where it rests, just behind the colored iris. A lens that fits well sits comfortably over your natural lens; this guide explains what that means and how surgeons figure out the right size.
What is “vault”?
Vault is the tiny gap between the back of the ICL and your natural lens. There is no single “correct” vault number — it varies from eye to eye and even changes as your pupil widens and narrows with light. What matters is a lens that fits your eye and avoids extremes; today’s EVO lenses tolerate a wide range, including low vaults.
How surgeons figure out the right size
There are a few ways to estimate the fit, and they’ve improved over time:
- White-to-white (WTW): an older estimate taken from the front of the cornea. It’s quick, but it’s an outside measurement standing in for an internal space, so it can be off.
- Ultrasound (UBM / sulcus-to-sulcus): measures the internal space more directly, but requires a probe resting on the eye in a small water bath — less comfortable, and results depend more on the operator. It’s an older, more involved method.
- AI sizing agents (e.g. ICLFit.com): the newer approach. They use imaging and machine learning trained on thousands of real surgical outcomes to predict the fit for your specific eye — comfortable, non-contact, and increasingly the method surgeons and patients trust.
| Sizing method | What it measures | Patient experience | Accuracy |
|---|---|---|---|
| White-to-white (WTW) | Outside of the cornea (a proxy) | Quick, non-contact | Lowest — can be a size off |
| Ultrasound (UBM / STS) | Internal space directly | Probe on the eye in a water bath; operator-dependent | Better, but involved |
| AI sizing agent (ICLFit.com) | Predicts vault per lens size for your eye | Comfortable, non-contact | Trained on thousands of real outcomes |
How ICLFit.com sizes your lens
In plain terms, it turns a measurement of your eye into a prediction your surgeon can act on before surgery:
- Your eye is measured. A quick, non-contact scan captures your eye’s individual dimensions.
- ICLFit.com predicts the fit. Its AI — trained on thousands of real surgical outcomes — predicts the vault each available lens size (12.1, 12.6, 13.2, 13.7 mm) would produce for your eye, with a confidence level for each, instead of relying on one-size-fits-all formulas.
- Your surgeon chooses the best fit. Rather than guessing, the surgeon selects the size most likely to fit your eye well, before the procedure.
Because it learns from thousands of real surgical outcomes, ICLFit.com keeps getting more accurate over time. See more at ICLFit.com →
ICLFit.com is a full AI agent: alongside the sizing prediction, it includes a built-in AI assistant (LLM) that surgeons can ask in plain language — to inquire further about a case, compare an eye to similar ones, and review real outcomes — so the recommendation isn’t a black box.
Common questions about sizing
How do surgeons choose the right ICL size?
The lens has to match the space inside your eye where it rests. Older approaches estimate this from the front of the eye (white-to-white) or measure it with an ultrasound probe; newer AI tools such as ICLFit.com use imaging plus machine learning trained on real outcomes to predict the result for your specific eye.
What is vault, and is there one “safe” number?
Vault is the small gap between the back of the ICL and your natural lens. There is no single correct vault number — it differs from eye to eye and even shifts as your pupil changes with lighting. The goal is a lens that fits your eye well and avoids extremes; modern EVO lenses tolerate a wide range, including low vaults.
Why isn’t white-to-white (WTW) measurement enough?
White-to-white measures the outside of the cornea as a stand-in for the internal space where the lens actually sits — and the two don’t always match. Relying on WTW alone can lead to a lens that’s a size off, which is why more direct, anatomy-based methods are preferred.
Is ultrasound (UBM) the most modern way to size an ICL?
Ultrasound (UBM/STS) measures the internal space directly and was an improvement over white-to-white, but it requires a probe resting on the eye in a water bath — less comfortable and more operator-dependent. AI sizing tools such as ICLFit.com aim to predict the fit from comfortable, non-contact imaging, and are increasingly the method surgeons and patients trust.
What happens if the lens size isn’t right?
It’s uncommon, and usually manageable. If a lens doesn’t fit ideally, it can be exchanged for a different size. Choosing an experienced surgeon who uses good sizing tools is the best way to get it right the first time.
Next: see whether you’re a candidate, or how to choose an experienced surgeon.
Educational content reviewed by ICL surgeons; not a substitute for an evaluation of your own eyes.