Your skin type
Fitzpatrick I to VI, confirmed by you in setup. It sets the UV dose your skin can take before it starts to redden (the MED in the table below), how far Deepen may climb, and the UV index at which Sol starts recommending SPF 50.
Published methodology
How Solmate does the math
Solmate shows its work.
Every window the app gives you comes out of one formula, built from numbers you can check on this page. We publish them because a health number you cannot inspect is a number you should not trust — and because the product only works if it is willing to say enough.
Three commitments hold the math together. The ceilings are deliberately conservative and they do not move. The numbers stay honest even when honest is the less flattering answer. And sunscreen is credited, not trusted: SPF can lengthen the calculation, never past the ceiling, and only ever at a fraction of what the bottle claims. SPF is protection. It is not permission.
Last reviewed September 29, 2026What goes in
Ten things decide your window.
Everything below comes from your setup answers, your plan and diary, your recorded sessions, or live weather for your location. There is no other input, no hidden signal, and nothing here is collected for advertising — your skin-type and sun-sensitivity answers exist to compute this one number and its limits. Your goal shade is left off on purpose: it tracks your progress, but it does not change the dose.
How you burn
Your answer to what a first strong sun day does to you. If you burn easily, Sol recommends SPF 50 instead of 30 two UV points earlier. That is its only route to your window: declaring SPF 15 or more but under the recommendation trims the target by 5%.
The live UV index
Read for your location and converted straight into a rate: the UV index divided by 40 gives erythemally weighted irradiance in watts per square metre. That is how fast your skin is collecting dose right now.
The hours inside your session
The clock decides which forecast hours matter. Solmate takes the highest forecast UV between now and the end of the longest session you could start, and uses whichever is worse — the sky now, or the sky coming.
Recent exposure
Erythemal dose banked over the last 7 days. Each logged exposure counts for no more than one full MED, and the running total can take up to 35% off today's target. A weekly budget stops new sessions once the week's banked dose passes it.
Your plan mode and your age
Base aims each session at 30% of your MED and Maintain at 25%. Deepen starts at 30% and climbs 10% a step, up to a ceiling for your skin type. Under 18, every mode is sized at two thirds of the adult dose.
The SPF you declare
The sunscreen the plan expects to be on your skin. It slows the dose your skin collects, so it lengthens the window, but it is credited at a fraction of the label and can never push a session past the 60-minute ceiling.
What goes on over it
Oil, butter or gel. Oil shortens the session. Butter and gel change the checklist and the order you apply things in, not the clock. None of them can add a minute.
Photosensitivity flags
A medication, a condition, or a history that makes sun exposure riskier. It halves the longest session Solmate will plan, from 60 minutes to 30, raises your burn-risk grade one level, and pins the SPF advice at 50 whatever the sky is doing.
Your skin reports and rest
A Tender diary report means a rest day and a step back on Deepen; a Great one is what lets Deepen step up. The rest you chose between session starts, 48 or 72 hours, has to pass before the next session.
The core numbers
The formula, in full.
Dermatology has a name for the dose of UV that would just begin to redden a given skin: the minimal erythemal dose, or MED. Solmate aims a session at a deliberate fraction of yours, works out how fast today's sun is delivering it, and divides one by the other to get minutes.
- Step one — the target dose
target dose = MED × plan fraction × age factor × recent-exposure factor × SPF compliance factor
Plan fraction is 0.30 on Base, 0.25 on Maintain, and on Deepen 0.30 raised 10% a step up to your type's ceiling. Age factor is 1 for adults and 2/3 under 18. Recent-exposure factor runs 0.65 to 1.00. SPF compliance factor is 0.85, 0.95 or 1.00. Apart from Deepen's earned steps, every one of them is a discount.
- Step two — the rate
dose reaching skin per second = (UV index ÷ 40) ÷ SPF protection
The UV index used here is the worse of the sky now and the highest forecast inside your longest possible session.
- Step three — the minutes
session = the smaller of (target dose ÷ rate ÷ 60) and 60 minutes
Rounded down, never rounded up, and never below one minute on a day that has UV in it at all. The ceiling is 30 minutes if you are photosensitive. When the full target will not fit inside it at the SPF you declared, the app says so instead of pretending it did.
Before any of this runs, three brakes can return zero minutes while the sun is up: skin you reported tender, a rest between sessions that has not passed, and a week's dose budget already spent. On those days the app offers an outdoor log, with a reminder to head into shade, instead of a sized session.
MED and the plan limits, by skin type
The MED figures below come from the only published cohort to measure a sunburn threshold for all six skin types on one protocol, and we take the low end of it — every row is at or below what that cohort measured. The other two columns are ours: how far Deepen may climb, as a share of your MED, and the UV index from which Sol recommends SPF 50 instead of 30.
| Type | Solmate calls it | Typical response to strong sun | MED (J/m²) | Deepen ceiling | SPF 50 from UV |
|---|---|---|---|---|---|
| I | Very fair skin | Always burns, never tans | 200 | 45% | 3+ |
| II | Fair skin | Burns easily, tans minimally | 250 | 55% | 3+ |
| III | Medium skin | Burns moderately, tans gradually | 300 | 60% | 6+ |
| IV | Olive skin | Burns minimally, tans readily | 450 | 65% | 6+ |
| V | Brown skin | Rarely burns, tans easily | 600 | 70% | 8+ |
| VI | Deep skin | Very rarely burns, deeply pigmented | 1,000 | 70% | 8+ |
No session is planned past 60 minutes, whatever the type. If you told us about a medication, a condition, or a history that makes you sun-sensitive, that ceiling is halved to 30 minutes and Sol recommends SPF 50 at any UV. If you burn easily, SPF 50 starts two UV points earlier than the table shows.
Why 30%, and what moves it
Base, where every plan starts, aims a session at 30% of your MED before anything scales it down. Not 100%, not 80% — thirty. That choice is what gives the plan room to be wrong about you: your real MED can be meaningfully lower than the table figure and a session aimed at under a third of it still lands short of redness. Maintain aims lower, at 25%. Deepen climbs from 30% by 10% a step, and only after you report Great skin following a guided session; one Tender report steps it back, two in a row send it to the start, 14 days without a qualifying session restarts it, and it never passes your type's ceiling. Under 18, every one of these is two thirds as much.
How SPF credit works
Solmate credits the square root of the labeled SPF, never more than 6×, and nothing at all below SPF 15. The square root is because the number on the bottle is measured under a far thicker, more even layer than people apply outdoors, and protection falls away steeply as the layer thins; the ceiling is because we are not willing to plan a long sit around a claim of 50× protection.
| Declared SPF | Credited | Why |
|---|---|---|
| Under 15 | None | The dose is computed as though your skin were bare. No credit at all. |
| 15 | 3.9× | The square root of 15. |
| 30 | 5.5× | The square root of 30. |
| 50 | 6× | The ceiling. It is already reached at SPF 36. |
| Above 50 | 6× | Still the ceiling. Nothing above SPF 36 is credited any further. |
Separately, the target dose is scaled by how your declared SPF compares with the SPF we recommend for today: below 15 multiplies it by 0.85, at least 15 but under our recommendation multiplies it by 0.95, and meeting the recommendation leaves it alone. Sunscreen you skip does not make the session longer — it makes the exposure higher than the plan assumed.
How the burn-risk grade is set
The grade is read off the projected dose of the session you are about to run, expressed as a share of your MED. It is the only figure that already accounts for your skin type, the sunscreen you declared and the length of the session at once.
| Projected share of your MED | Grade |
|---|---|
| Under 45% | Low |
| 45% to under 65% | Moderate |
| 65% to under 80% | High |
| 80% and over | Severe |
Three things move a grade off that table, all upward. Photosensitivity raises it one level and names itself as the reason. Oil over less than SPF 15 raises it one level too. And when you declare less than SPF 15 — the same threshold below which we credit no protection — raw UV puts a floor under the grade: UV 3 or higher is at least moderate, UV 8 or higher at least high, UV 11 or higher severe.
Oil, butter and gel
What oil does — and what it never does.
This is policy, not marketing. Products are the one place a tanning app could quietly hand people extra minutes, so the rule is written into the shape of the code: every product coefficient is constrained to the range 0 to 1, and the result is then floored against the session your skin's own limit allows. Even a coefficient tuned wrongly could only ever subtract.
Oil can shorten a session. It can never lengthen one.
There is no arrangement of products, goals or settings that adds a minute to your window. A bottle does not change where your skin reaches its limit, so there is nothing honest to add.
Butter and gel are neutral.
They stay on the checklist because the order you apply things in matters, and because the app should know what is actually on your skin. They do not move the number in either direction.
Oil over little or no SPF: a shorter session and a higher grade.
Any oil trims the session, to 90% of it over SPF 15 or more. With less than SPF 15 underneath, it drops to 60% and the burn-risk grade goes up a level. We do not refuse the session and we do not lecture. We shorten it, we mark it, and we say exactly why.
Oils carry no SPF of their own.
Oil adds no protection, and going on over sunscreen can thin it, so the plan never counts it as protection. Your window stays where your skin's limit and your sunscreen put it, or shorter.
The oil factors are fixed at 0.9 and 0.6, and they apply to the clock, never to the dose the app records, so oil cannot make an exposure read lighter than it was. The direction is not up for tuning: the only change a product is ever allowed to make to a session is a reduction.
The limits
What Solmate does not model.
A methodology page that only lists what a model does is an advert. These are the things the engine genuinely cannot see, each of which means your real exposure can differ from the projection — usually upward.
How much you actually applied
A sunscreen label is measured under a thick, even laboratory application that almost nobody reproduces at the beach. Crediting only the square root of the label is our hedge against that. It is not a measurement of your coverage, and it cannot see the spot you missed.
Water, sweat and towels
Solmate reminds you to reapply every two hours in the sun, and straight away when you tell it you swam, sweated or toweled off. It does not model how long your sunscreen survives any of those, and it does not read a water-resistance rating off the bottle.
Reflected and scattered UV
The reading describes the sky, not the surface you are lying on. Water, sand, snow, concrete and altitude all raise real exposure above the index, and none of them are in the math.
Shade, clothing and glass
The engine assumes exposed skin under open sky for the whole session. An umbrella, a shirt or a window changes your real dose and Solmate has no way to know about it.
Your medication, specifically
The photosensitivity flag halves your ceiling, raises your burn-risk grade one level and pins the SPF advice at 50. That is all it does. It does not know your drug, your dose, or how the two interact — that question belongs with a pharmacist or a clinician.
Your personal MED
The MED table on this page is representative figures per phototype, not a measurement of you. Two people inside one Fitzpatrick type can differ substantially, which is exactly why the target is a fraction of the dose rather than the whole of it.
The tan you already have
Existing colour is tracked for your progress, not credited as protection. A base tan is worth very little sun protection in practice, and Solmate does not pretend otherwise by lengthening your window as you darken.
Everything UV does that is not redness
The weighting we use is the erythema one — the curve for skin turning red. Photoaging, DNA damage and skin-cancer risk do not follow the same curve, and no session length makes UV exposure risk-free.
Vitamin D, precisely
The vitamin D figure in a session summary is an estimate derived from your dose and a rough assumption about how much of you was uncovered. It is encouragement, never a clinical number.
How it's worked out
The UV dose reaching your skin, after your sunscreen's credit (the same one the timer uses), divided by your skin type's MED, times 15,000 IU, times ½ for about half of you uncovered, capped at 10,000 IU.
For example, 20 minutes at UV 6 on type III skin (MED 300 J/m²), where the sky delivers 6 ÷ 40 × 1,200 s = 180 J/m²:
- No sunscreen
- All 180 J/m² reaches the skin.180 ÷ 300 × 15,000 × ½ = 4,500 IU
- SPF 30
- Credited √30 ≈ 5.48, so 180 ÷ √30 ≈ 32.9 J/m² reaches the skin.32.9 ÷ 300 × 15,000 × ½ ≈ 822 IU
The rules we hold
The parts that do not bend.
Numbers can be tuned. These cannot — they are the shape of the product, and changing any of them would make it a different product.
Today's max never moves up
Today's max is the top of the dial, and nothing lifts it: no goal shade, no product, no amount of sunscreen, no group session, and not Extend, which only adds minutes up to it. No session is planned longer than 60 minutes.
The worse sky wins
When the forecast inside your session is higher than the sky right now, the forecast is what we plan around. A rising forecast can only shorten a window, and when it is what shortened yours, the app says so in words.
No UV, no dose
When there is no measurable UV, the engine returns zero minutes and an explanation rather than inventing a dose out of a number too small to tan anybody. The app still lets you time an outing, up to its 60-minute timer limit, and says the reading is near zero.
Risk grades only go up
Photosensitivity and oil over less than SPF 15 each bump the grade one level and name themselves as the reason. Skin under real UV with less than SPF 15 declared has a floor put under its grade. None of these can ever move a grade downward.
One engine, two places
The engine on your phone is generated from the same constants as the one on our server, and they change in the same commit or not at all, so both compute the same dose. The phone then applies what only it knows, like the oil you declared, and those adjustments only shorten a session. Tan anyway, the one way past a rest day, is an adult's explicit choice and stops at a normal day's maximum.
Shorter is always available
The dial opens at today's max and goes down from there, so less is always yours to take. Extend can add minutes back during a guided session, up to today's max and never past it.
Guidance, not medical advice
Solmate keeps exposure limits conservative and SPF in the conversation. It is guidance, not medical advice.
Everything on this page is an estimate applied to a person the app has never examined. UV exposure carries real risk no matter how carefully a plan is followed, and a shorter session is a lower risk, not a safe one — Solmate recommends a window, and it will not call that window safe. Talk to a healthcare professional about skin concerns, medications, or an unexpected reaction, and never let this app be the reason you delay care.
Sources and conventions
Where the constants come from.
- Erythemal weightingAll UV in the engine is erythemally weighted — biologically weighted for its power to redden skin — following the international erythema reference action spectrum standardised by ISO and the CIE. Unweighted irradiance is never used.
- The UV index conversionSolmate uses the WHO and WMO convention that defines the global solar UV index against erythemally weighted irradiance, which makes irradiance in watts per square metre equal to the index divided by 40.
- MED by phototypeThe per-type MED figures sit at the lower end of the confidence interval around the measured means in Shih et al., Journal of Investigative Dermatology 2018;138(10):2244–2252 — to our knowledge the only study to measure a sunburn threshold for all six Fitzpatrick types under a single erythemally weighted protocol. We use the bound rather than the mean deliberately: every figure in our table is at or below what that cohort actually measured. Types I–IV are corroborated by two larger phototype cohorts. Types V and VI rest on one study of six to seven people each, and erythema is harder to see on deeply pigmented skin, so the real threshold there may be lower than published — we size those two rows down accordingly. All of them are population figures, not measurements of an individual, and an individual's MED varies.
- SPF in real useThe square-root credit follows Faurschou and Wulf, British Journal of Dermatology 2007;156(4):716: protection falls steeply as the applied layer thins, and at 1 mg/cm², half the laboratory amount and more than most people use, the effective SPF is about the square root of the label.
- The plan targets, ceilings and budgetsThe plan fractions, Deepen's steps and ceilings, the 60-minute ceiling, the weekly budget, the two-thirds factor under 18 and the 6× ceiling on SPF credit are our own conservative choices rather than published standards. We set them where we did because under-application of sunscreen in the field is well documented, and because a plan should have room to be wrong about a person without hurting them.
- Live UV dataCurrent and forecast UV come from Apple WeatherKit for your location. For the next two days the forecast is hour by hour; after that it gives a daily peak, so the hour-by-hour curve is modelled from the peak and the day's sunrise and sunset, and the app says so where it shows that curve.