The homepage does not ask your skin type. It shouldn't. The UV index is the sky: erythemally weighted irradiance, scaled so 1 is a slow tap and 8 is a fast one. Your neighbor standing next to you has the same 8.

What changes is how many minutes that 8 takes to reach a just-perceptible reddening on your unprotected skin. The index is the rate. Phototype is the bucket. Mixing them up is how a Moderate day becomes a free pass, or a Very High day becomes someone else's problem.

Two meters, one outing

WHO defines the UV index with k_er = 40 m²/W. One Standard Erythemal Dose (SED) is 100 J/m² of that same weighted UV. Hold the index still and the conversion is blunt:

About 0.9 SED per hour for each index point.

A steady UV 8 is then 7.2 SED per hour. Same number for every skin type.

ICNIRP's indicative unadapted MED—the dose for a just-perceptible erythema, not a safe budget—is a different table. Without a recent tan:

• Phototypes I–II — about 2 SED
• III–IV — about 7 SED
• V — about 10 SED
• VI — about 15 SED

They also publish ranges: type I always sunburns below 2 SED; type VI's indicative line sits above 10 SED. Those figures are not a prescription for your arms. They are the spread.

At a steady 8, 2 SED arrives in about 17 minutes. 7 SED in about an hour. 15 SED in about two hours. Same sky. Different clocks.

Our SPF calculator is the planning version of that split. Pick a Fitzpatrick type (I–VI: always burns and never tans, through deeply pigmented). It divides a skin-type factor by the UV you feed it—live for this location, or a number you type. Type I uses 67 minutes at UV 1; type II uses 100. At a live 8 that is about 8 minutes unprotected versus about 13. Type VI lands near an hour. Those factors are outdoor-planning estimates, not a lab MED measured on you. They sit on the conservative side of ICNIRP's unadapted I–II number. Treat them that way.

The trap is reading the sky as the skin

Three everyday misses:

1. "I don't burn, so the index is for other people." Cancer Council: all skin types can be damaged by too much UV. Types V and VI still need care even when they rarely redden. Eyes do not get a phototype discount. AAD: darker skin has a lower skin-cancer risk; it can still develop. They recommend protection for everyone, not only for people who pink.
2. "The calculator gave me 40 minutes, so I have 40 free minutes." EPA's UV Index guide tells communicators not to report "time to burn" as a safe window. People hear a permit. Redness is also late—labs score the burn 16–24 hours later. Feeling fine at minute 39 is not an all-clear. See the burn shows up after you leave.
3. "We're under the same 4, so we have the same day." Hours at UV 4 can outdose minutes at UV 8 is how long the meter runs. This is whose bucket fills first. A type I at 4 can outrun a type IV's first hour at the same 4. The index did not change. The minutes did.

WHO still wants cover from 3 up. That line does not wait for your phototype.

Use both numbers

1. Read the live UV index—current, today's peak, next few hours. That is the shared rate.
2. Open the SPF calculator and pick the closest Fitzpatrick match. If you usually burn and barely tan, you are not a IV.
3. Treat unprotected minutes as how fast redness can arrive, not a slot to stay bare. EPA's point: the index is not a safe-tanning clock. Cover before the estimate, not after it.
4. Protect from 3 anyway. What UV index requires sunscreen is the WHO line for the sky, not a fair-skin special.
5. If the calculator then multiplies by SPF, remember the film. The bottle SPF assumes a thicker coat than most people put on. Protected time is an upper bound. The tool already says so.

A tan from last week is not a new phototype. ICNIRP's "with adaptation" column is three weeks of tanning without erythema, and even then the bump is limited. Do not upgrade yourself to VI because a stretch of outdoor days went well.

Sources

WHO defines the UV index with k_er = 40 m²/W and treats 3 as the protection line in the UV index Q&A. SED arithmetic uses the CIE standard erythemal dose (100 J/m²). ICNIRP's indicative MED ranges and unadapted values (2 / 7 / 10 / 15 SED) are in Protecting Workers from Ultraviolet Radiation (Tables 1–2); they are not a personal timer. Fitzpatrick's I–VI types are a burn/tan response scale, not a race chart: Fitzpatrick 1988, Arch Dermatol. EPA warns against presenting "time to burn" as a safe period in A Guide to the UV Index. Cancer Council: all skin types can be damaged; V and VI still need care; eyes regardless of phototype — Check for signs of skin cancer. AAD recommends protection for everyone and notes darker skin's lower skin-cancer risk is not zero: The latest in sun protection.