Hey r/biohackers,

Long-time lurker, first time posting about something I built. I want to be upfront: this is my app, I’m the developer, and I’m sharing it because I genuinely think the community here would find the science interesting, and give me honest feedback.

# The problem I was trying to solve

Most vitamin D advice is basically “go outside for 15 minutes.” That’s useless. 15 minutes at noon in July produces wildly different amounts than 15 minutes at 4pm in November. I wanted something that actually accounted for the variables that matter.

# What the app actually does

It estimates your vitamin D synthesis session by session using this model:

IU = IU\_max × B × (1 − e\^(−k·f)) × η

Breaking that down honestly:

\- SED (skin erythema dose) is calculated  
from UV index and session time: SED = 0.9 × UVI × hours. This is your actual skin dose.

\- f is your SED normalized against your personal MED range (minimum erythemal dose for your skin type). This feeds a saturating curve, because your body genuinely self-limits vitamin D synthesis after a threshold.

\- B scales for exposed body surface area based on your clothing.

\- η is an efficiency modifier that combines sun angle, skin type (Fitzpatrick scale), and BMI.

**The part I’m most proud of is Sun Zenith Angle:**

When location is available, it calculates the real solar zenith angle using standard solar geometry. This matters a lot because the atmosphere filters UVB (the wavelength that actually drives vitamin D synthesis) at low sun angles. The modifier follows a cos(SZA)\^1.4 curve. Winter morning sun is genuinely not the same as summer noon sun. Most apps ignore this entirely.

# What I’m not claiming

This is an estimate. Vitamin D synthesis is notoriously hard to measure precisely, and individual variation is real. I’m not telling you to stop getting blood tests. This is a tool for awareness and optimizing your sessions, not medical advice.

The science it’s built on:

\- Holick et al. (2007) — vitamin D synthesis guidelines

\- Fitzpatrick (1988) — skin phototype classification

\- McKenzie et al. (2009) — UV radiation and solar zenith angle dependence

\- Wortsman et al. (2000) — decreased bioavailability in obesity

\- Drincic et al. (2012) — volumetric dilution and body fat storage

# Where I’d love your feedback

The biohacking community is exactly the crowd I built this for. I’d genuinely love to know: does the model make sense to you? Are there variables you’d want to see added? Is there research I’ve missed?

The app is called SunMax and it is free. No paywall on the core tracking.

Happy to answer any questions about the methodology. I can get nerdy about this.

I built a free app that estimates your vitamin D synthesis in real time using actual solar geometry. Here’s the real science behind it