How does our THC calculator work?

TL;DR

The peak concentration ($C_0$) is estimated from the absorbed dose and an apparent distribution volume, then decays via a (optional) one- or two-phase exponential curve. Heavy consumers add a second, slower tail to account for depot THC.

1 · How much THC reaches the blood?

Grams × THC percentage × bioavailability:

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absorbedMg = grams * 1000 * (thcPercent / 100) * bioavailability(route)

2 · Peak concentration ($C_0$)

Divide the absorbed mass by the apparent distribution volume ($V_d$). Rule of thumb: $(V_d \approx 2{}3\,\text{L kg}^{-1})$.

Example: 8 mg absorbed, 70 kg body weight → $(C_0 \approx 50\,\text{ng ml}^{-1})$.

3 · Elimination: One vs. two compartments

The THC curve falls biphasically:

1. Fast phase $(T_{1/2} \approx 1\text{–}2\,\text{h})$
2. Slow phase $(T_{1/2} \approx 20\text{–}30\,\text{h})$

The two-phase model is

with $(s = 0{}15)$, $(k_1 = \ln 2 / 1{}5\,\text{h}^{-1})$, $(k_2 = \ln 2 / 24\,\text{h}^{-1})$.

4 · Inputs

Required

• Grams of cannabis
• THC percentage
• Hours since consumption
• Route of administration (joint, edible, …)

Optional (improves accuracy)

• Body weight / body fat
• Consumption frequency

5 · Legal thresholds (Germany)

• 3.5 ng/ml → administrative offence (§ 24a StVG)
• >1.0 ng/ml for novice drivers (<21 years)
• Criminal offence (§ 315c StGB) once driving impaired, no matter the level

The calculator highlights when you cross below these lines.

6 · Worked example

0.3 g @ 20% THC smoked in a bong → ~18 mg absorbed. $C_0 \approx 112\,\text{ng ml}^{-1}$.

• <3.5 ng/ml after ≈ 4 h
• <1.0 ng/ml after ≈ 7 h

7 · Disclaimer

• These values are guidelines. Genetics, liver function, chronic use, or just a heavy meal can shift everything. Licence or job on the line? Play it safe and add extra time.
• I am not a physician or toxicologist. I work hard to ground the calculator in research, but I am not a pharmacology expert.

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