How the EtG calculation formula actually works

The exponential decay formula

Like many substances the body eliminates at a rate proportional to how much is currently present, EtG concentration over time is commonly modeled as exponential decay: C(t) = C0 × 0.5^(t / t½). Here C(t) is the estimated concentration at time t, C0 is the starting (peak) concentration, t is hours elapsed, and t½ is the half-life—the time it takes for the concentration to fall by half.

This is standard first-order elimination math, the same shape used across pharmacokinetics. Nothing about the formula itself is controversial or unique to EtG.

Where the half-life number comes from

Studies following EtG concentrations after controlled drinking generally describe a urinary elimination half-life in the range of roughly 2 to 3 hours, though individual results vary with hydration, kidney function, and the assay used. That range is narrow enough that reasoning about it in a formula is defensible—use the low end (faster decline) and the high end (slower decline) as bounds rather than picking one number and presenting it as exact.

The hard part: you cannot formula your way to C0

Half-life describes the shape of the decline. It says nothing about where the curve starts. Peak urinary EtG after drinking depends on dose, absorption speed, food in the stomach, body composition, sex, hydration at the time, and individual enzyme activity—factors that can each shift a result several-fold. Published dose-ranging studies show wide scatter in peak concentration even among people who drank comparable amounts.

That is why you will see calculators online that confidently state something like '4 drinks produces roughly 20,000 ng/mL peak EtG.' That specific number is not measured from your body; it is an assumption the calculator's author picked. Running it through a correct decay formula does not make the output more accurate—it just adds false precision on top of an invented starting point. Our quantitative EtG levels guide covers why concentration-to-dose conversions are unreliable in more depth.

A worked example, done honestly

Say a quantitative lab report shows 4,000 ng/mL. Using the formula with a 2-hour half-life, that sample would model to roughly 500 ng/mL at about 8.6 hours later; with a 3-hour half-life, the same starting point takes about 13 hours to reach 500 ng/mL. That 4.4-hour spread, from the half-life range alone, is the honest uncertainty even when the starting number is real. Starting from a guessed peak instead of a measured one widens that uncertainty considerably further.

Our EtG half-life calculator runs this exact formula interactively so you can enter a known or hypothetical concentration and see the range yourself.

What this formula is—and isn't—good for

• Good for: understanding why EtG declines the way it does, and projecting a real, known concentration forward using a defensible half-life range.
• Good for: interpreting two sequential quantitative results from the same person to sanity-check the decline rate.
• Not good for: converting a number of drinks into a specific peak concentration.
• Not good for: producing a single exact hour at which a test will turn negative.

Frequently asked questions