Sunscreen Reapplication Interval
Calculator
Results
- Theoretical protection (minutes)
- 300
- Theoretical protection (hours)
- 5
- Practical reapplication interval (minutes)
- 150
- Practical reapplication interval (hours)
- 2.5
- UVB blocked at this SPF (%)
- 96.666666
Beauty and personal care results
| Theoretical protection (minutes) | 300 |
| Theoretical protection (hours) | 5 |
| Practical reapplication interval (minutes) | 150 |
| Practical reapplication interval (hours) | 2.5 |
| UVB blocked at this SPF (%) | 96.666666 |
formula-map diagram
- Theoretical protection (minutes)
- 300
- Theoretical protection (hours)
- 5
- Practical reapplication interval (minutes)
- 150
- Practical reapplication interval (hours)
- 2.5
- UVB blocked at this SPF (%)
- 96.666666
Beauty and personal care relationship
Formula
protection time = SPF × unprotected burn time (a simplification)= 300
Note
This is a simplified model: it applies the mixing ratio, dilution, application rate, growth rate or price you enter to plain arithmetic. Mixing figures use the ratio on your own product's label, because developer, powder and colour ratios differ from brand to brand. Dilutions use C1 x V1 = C2 x V2 and the standard conversion of 20 drops per millilitre; real droppers vary with the oil's viscosity and the dropper itself. Sunscreen amounts use the Du Bois body surface area formula with the application rate you enter, and the SPF figure uses the textbook approximation protection time = SPF x unprotected burn time, which is a simplification: sweat, water, towelling, UV index, altitude, reflection and your own skin all shorten it in practice. Hair, nail and hair-removal timings assume a constant linear growth rate, which varies by person, body area, age, season and health. Costs ignore taxes, sales and waste. Always patch test a new product on a small area first and follow the product label and any professional instructions. This is not dermatological, cosmetic or medical advice; see a dermatologist or a qualified professional for anything affecting your skin, scalp or health.
More in Beauty and personal care
See all →Frequently asked questions
Why do I need to reapply sunscreen every two hours?+
Sunscreen's active ingredients degrade under UV exposure and are physically removed by sweat, water, and contact with clothing or towels, so protection diminishes over time regardless of the labeled SPF; the standard two-hour interval is the general guideline for maintaining consistent protection during continued sun exposure.
Does a higher SPF mean I can reapply less often?+
No, SPF measures the level of protection against UVB while it remains effective, not how long that protection lasts; both a high and low SPF sunscreen need reapplication at similar intervals, generally every two hours, or sooner after swimming or heavy sweating.
How does swimming or sweating change the reapplication interval?+
Water-resistant sunscreens are only tested to maintain their SPF rating for a specific duration, typically 40 or 80 minutes of swimming or sweating, after which they should be reapplied regardless of whether the standard two-hour interval has been reached.
What does this calculator use as inputs to estimate my next reapplication time?+
Typically the time of your last application plus factors like whether you are swimming, sweating heavily, or towel-drying, which shorten the safe interval; the calculator adjusts the standard two-hour baseline downward for these activities.
Is reapplying at the same total thickness as the first application necessary?+
Yes, reapplication should use a similar amount to the initial application to restore the protective film, since a thin top-up layer over degraded sunscreen does not fully re-establish the tested level of protection.