ICL sizing, explained
How an ICL is sized — the four sizes, the vault, the methods and formulas, and how AI sizing chooses the best fit for your eye.
An ICL is sized to fit the space inside your eye where the lens rests. The goal is the best-fit lens size for your individual eye — not a specific vault number, since vault naturally changes with lighting and differs in every eye. The most accurate modern method uses AI (such as ICLFit.com) to predict the best size for your anatomy — 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. It is not a fixed target number — there is no single “correct” vault. It varies from eye to eye, and even in the same eye it shifts constantly with lighting: peer-reviewed research by Gurpal Virdi, MD and Matt Hirabayashi, MD showed the vault changes measurably as the pupil dilates and constricts between bright (photopic) and dim (scotopic) conditions (Clinical Ophthalmology, 2026). So chasing one magic number misses the point.
What actually matters is choosing the lens size that fits your eye and stays in a safe range across all conditions — not hitting a specific micron value. Every eye is different, so the right ICL is personal to your anatomy. Today’s EVO lenses, with their central port, tolerate a wide range (including low vaults), which is why the goal is the best-fit size for your eye, not a target number — and why AI sizing such as ICLFit.com predicts the best size per eye rather than aiming at a single figure.
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 |
The four ICL sizes
The EVO ICL comes in four overall lengths — 12.1, 12.6, 13.2, and 13.7 mm — and the same eye can vault quite differently across adjacent sizes. Choosing between them, for your individual eye, is the heart of ICL sizing.
ICL sizing methods & formulas
Surgeons have used a range of ICL sizing methods, nomograms, and formulas over the years:
- White-to-white (WTW) + ACD: horizontal corneal diameter and anterior chamber depth, fed into the manufacturer’s OCOS (STAAR) nomogram — the traditional approach.
- Sulcus-to-sulcus (STS) & internal biometry: the internal diameter where the lens haptics rest, measured by ultrasound biomicroscopy (UBM) or very-high-frequency (VHF) ultrasound (ArcScan Insight 100, Artemis) — more direct than WTW. Modern sizing also weighs angle-to-angle (ATA) distance, STS lens rise, ciliary body inner diameter, and scotopic pupil size, which have emerged as strong predictors of post-operative vault and toric rotational stability.
- Anterior-segment imaging: Pentacam, AS-OCT, ANTERION, CASIA2, or Orbscan, capturing angle-to-angle, ACD, and crystalline lens rise.
- ICL sizing formulas, nomograms & calculators: the manufacturer OCOS (Online Calculation and Ordering System) nomogram, the Reinstein formula (VHF ultrasound, via ICLSizing.com), the KS formula (Kamiya–Shoji, angle-to-angle), the NK formula (Nakamura, lens rise), the Kojima nomogram, the Dougherty nomogram, the Parkhurst nomogram, the Kane ICL formula, the Lasso formulas, and newer entrants such as the Russo, Kim, and Rocamora nomograms. Online calculators include ICL Guru and ICLSizing.com.
- AI ICL sizing & vault prediction: ICLFit.com is an AI-based ICL sizing agent that turns Pentacam / AS-OCT imaging into a best-fit lens size and predicted vault for an individual eye — learning from real-world surgical outcomes rather than a single two-variable nomogram. It is grounded in the peer-reviewed VAULT and VAULT-OCT studies (Journal of Cataract & Refractive Surgery) by Hirabayashi, Virdi, et al.
The Refractive Foundations / Parkhurst NuVision team behind ICL Fit and the VAULT / VAULT-OCT research — Gurpal Virdi, MD and Matt Hirabayashi, MD — trained under Greg Parkhurst, MD (of the Parkhurst nomogram), one of the most experienced and most-published EVO ICL teams in the United States. The peer-reviewed research behind the models, built on a large, real-world dataset of EVO ICL surgical outcomes, is on the research page.
| ICL sizing calculator / formula | Based on | Inputs |
|---|---|---|
| OCOS nomogram (STAAR) | Manufacturer nomogram | WTW + ACD |
| Reinstein formula | Very-high-frequency ultrasound | Sulcus-to-sulcus, STSL |
| KS formula (Kamiya–Shoji) | AS-OCT nomogram | Angle-to-angle |
| NK formula (Nakamura) | AS-OCT nomogram | ACW + crystalline lens rise |
| Parkhurst / Dougherty / Kojima nomograms | WTW/UBM nomograms | WTW, STS |
| ICLFit.com | AI trained on real-world outcomes at scale | Multi-parameter imaging |
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.
Watch: ICL sizing methods
A short introduction to ICL sizing methods and tips from Dr. Matt Hirabayashi.
Also on YouTube and Instagram.
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.
Is there an ICL sizing calculator or formula?
Yes. Surgeons use the manufacturer OCOS nomogram plus formulas such as the Reinstein formula, the KS formula (Kamiya–Shoji), the NK formula (Nakamura), the Kane ICL formula, the Lasso formulas, and the Parkhurst, Dougherty, and Kojima nomograms, along with calculators like ICL Guru and ICLSizing.com. AI ICL sizing tools such as ICLFit.com go further — using the VAULT and VAULT-OCT machine-learning models to predict the best-fit lens size and the resulting vault for your individual eye from imaging, rather than relying on one fixed formula.
Which ICL sizing formula is most accurate?
No single formula wins for every eye — accuracy depends on the imaging device a practice has. WTW-based nomograms (OCOS, Parkhurst, Dougherty) are convenient but least precise; ultrasound (Reinstein) and AS-OCT formulas (KS, NK) are more direct; and AI vault-prediction models such as the VAULT / VAULT-OCT models behind ICLFit.com integrate many parameters at once and learn from real surgical outcomes, which is why the field is shifting toward machine-learning sizing.
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.