Used only to estimate blood alcohol from your drink count, with the Widmark formula. Stays on this device.
Pick a sex and enter a weight.
BAC here is a rough estimate. It ignores food's first-pass effect, body composition, and timing — and is never a measure of whether you're okay to drive.
A drink doesn't hit your bloodstream all at once. It sits in your stomach and seeps into your bloodstream over time, while your liver clears it out at a constant rate. Nightcap tracks those two things — alcohol arriving and alcohol leaving — and draws the curve they make.
The fuller your stomach, the slower a drink seeps into your blood. Each stomach setting is a speed, given as a half-life:
What "half-life" means here. It's how long it takes for half of whatever alcohol is sitting in your stomach to move into your blood — not half a drink, just half of whatever's in there, however much that is. The seeping isn't steady; it starts fast and tapers off. So with a 12-minute half-life, half of what's in your stomach crosses into your blood in the first 12 minutes, then half of what's left in the next 12, and so on — until after about four steps (~50 minutes) it's essentially all in. That tapering is what makes the curve rise smoothly instead of jumping straight up.
Studies show the empty-stomach half-life is close to real measured values, and that food meaningfully slows absorption.
Your liver clears alcohol at a steady, fixed rate — about one drink per hour — no matter how much is in you. Nothing speeds this up: not coffee, not water, not a cold shower. Only time. Studies show this steady, one-drink-per-hour rate is well established.
To show a blood-alcohol estimate, the app uses your sex and weight in the standard Widmark formula — the same math used in forensic and medical settings. Smaller people and women reach a higher BAC from the same drinks, because alcohol spreads into body water and they have proportionally less. The factors it uses are population averages.
The shape of the curve — rising, peaking, then falling — is solid and well-supported. The peak height is the most reliable number. The "fully clear" time is the fuzziest — real clearing is often a bit sooner than shown. Every number here is a population average, so your own body may run faster or slower.
It's a model built on rules of thumb and averages — useful for understanding how your night is trending, never a measurement, and never a way to decide whether you're okay to drive.