Most UV index apps do the same one thing: they show you a number. The differences that matter are where that number comes from, whether the app adapts it to your skin, and whether it does anything once the number is on screen.
This is a comparison by what each app is actually for. Disclosure up front: Sunlit is our app. It is included because it does something the others do not, and the honest limitations are listed alongside the strengths.
First, the accuracy question
Search any forum for UV apps and you will find people reporting that two apps show different numbers for the same place at the same moment. That is expected, and understanding why makes every app on this list more useful.
Almost no consumer app measures UV. Your phone has no UV sensor. What apps display is a model forecast — computed from sun elevation, the overhead ozone column, cloud cover, altitude and aerosols, from sources such as the Copernicus atmosphere service, national weather services or open forecast APIs. Different models, different update intervals and different grid resolutions give different numbers, especially under broken cloud, which is the hardest condition to model.
What follows from that:
- A ±1 disagreement between apps is normal and rarely changes what you should do.
- Clear-sky forecasts are the most reliable; scattered cloud is the least.
- No app knows your microclimate — a reflective white terrace, deep shade or a sea breeze are outside the model.
- An app that shows a number to one decimal place is implying precision it does not have.
Treat any UV figure as a good estimate of intensity, not a measurement of your dose.
SunSmart Global UV — the institutional reference
Developed by the WHO, the World Meteorological Organization, the UN Environment Programme and Australia's ARPANSA, with forecast data from Copernicus. Free, no advertising, no commercial motive.
- Best for: a trustworthy baseline number and clear sun-protection times.
- Strengths: authoritative provenance; global coverage; shows the daily window during which protection is advised.
- Limitations: general public-health advice, not personalised to your skin type beyond broad guidance; no session tracking.
If you want one number you can trust and nothing else, this is the reference implementation.
EPA SunWise — the US government option
The US Environmental Protection Agency's UV index tool, based on the National Weather Service forecast.
- Best for: people in the United States who want the official national forecast.
- Strengths: free, no tracking, straightforward hourly forecast.
- Limitations: US coverage; minimal features beyond the forecast itself.
UVLens — the established consumer app
A long-standing dedicated UV app with hourly forecasts and burn-time estimates based on skin type.
- Best for: people who want more than a raw number but do not need session tracking.
- Strengths: established, widely recommended in sun-safety communities; skin-type-aware burn-time estimates; broad international coverage.
- Limitations: feature set sits between a forecast and a full tracker; check the current store listing for what is free versus paid, since that changes over time.
Your phone's built-in weather app — the zero-effort option
Apple Weather and most Android weather apps display the UV index in the hourly forecast, alongside a general advisory.
- Best for: a quick check before going out.
- Strengths: already installed; no permissions to grant; reliable enough for a yes/no decision.
- Limitations: the number is buried among temperature and rain; no skin-type personalisation; no alerts tied to UV; typically no timing help.
For most people who just want to know whether to bother with sunscreen, this is genuinely sufficient. Do not download an app to replicate something already on your home screen.
Sunlit — for timing an exposure, not just reading a number
Our app. It exists because the raw UV index answers the wrong question: it tells you how fast the sun burns skin, not how long you can be out.
- Best for: deliberate sun sessions — tanning, a long beach afternoon, outdoor sport — where you want a personal time limit rather than a general advisory.
- What it does: reads the live UV index for your exact coordinates from Open-Meteo, applies your Fitzpatrick skin type, the SPF you applied and how much skin is exposed, and turns the result into a timed session with turn-over alerts and sunscreen reapplication prompts. It logs estimated vitamin D per session, and gives a 10-day UV forecast so you can see whether a given day is even worth planning around.
- Limitations, honestly: iPhone and iPad only, no Android. It is a new app, so it has no long track record. Its UV data comes from the same class of forecast model as everything else on this list, and it carries the same accuracy caveats. Some features are behind a subscription.
- Privacy: no account, and the skin profile, session history and vitamin D log stay on the device.
Which one to pick
| If you want… | Use |
|---|---|
| A quick "do I need sunscreen?" check | Your built-in weather app |
| An authoritative, ad-free public-health number | SunSmart Global UV |
| The official US forecast | EPA SunWise |
| Hourly forecasts with skin-type burn times | UVLens |
| A timed session limit personalised to your skin and SPF | Sunlit |
What to look for in any UV app
- Does it state its data source? An app that will not tell you where its forecast comes from is asking for more trust than it has earned.
- Does it ask for your skin type? A burn-time estimate that does not know your Fitzpatrick type is a population average, and the spread from type I to type VI is roughly five-fold.
- Does it account for SPF? And does it handle it honestly — a claim that SPF 50 gives you 50× your time ignores that real-world application delivers a fraction of the lab-tested protection.
- Does it show the curve, not just the peak? "Today's UV: 9" is a single instant near solar noon and tells you nothing about a 08:00 run.
- What does it do with your location? Continuous background location for a UV forecast is more than the job needs.
- Does it invent authority? Sun exposure sits close to health advice. An app that shows precise burn times with no stated methodology and no caveats is overselling a model.
The limit of any of them
No app on this list measures the ultraviolet actually reaching your skin. They forecast intensity at your location and estimate the rest. That is enough to plan around and not enough to justify pushing your limit — which is why every sensible implementation, including ours, points at a fraction of your burn threshold rather than at the threshold itself.
Sunlit is free on the App Store for iPhone and iPad.
Related reading
- What Is the UV Index? The Scale, the Chart and What It Means
- Can You Burn on a Cloudy Day? What Cloud Really Does to UV
- What UV Index Is Good for Tanning? The Honest Answer
General information about sun exposure, not medical advice. UV index values from any consumer app are model forecasts, not measurements of your personal dose, and no exposure level is entirely risk-free. Feature and pricing details of third-party apps change; check the current store listing before deciding. Sources: WHO/WMO/UNEP/ICNIRP UV Index guidance; Copernicus Atmosphere Monitoring Service UV forecast documentation; US EPA SunWise programme.
