Caffeine Cut-off Calculator

Metric	Value	Copy
Bedtime (local)	{{ bedtime_local \|\| '—' }}
Cut‑off (local)	{{ cutoff_time_local \|\| '—' }}
Lead time to bedtime	{{ cutoff_lead_display \|\| '—' }}
Required half‑lives	{{ required_half_lives.toFixed(2) }}
Required decay time	{{ required_time_display \|\| '—' }}
If drink now → safe bedtime	{{ safe_bedtime_if_now_local \|\| '—' }}
If drink now → residual @ bedtime	{{ residual_if_now_display \|\| '—' }}
Status	{{ cutoff_passed ? 'Cut-off passed' : 'OK' }}

Caffeine levels in your body fall over time as your system clears the compound. A caffeine cut off calculator helps you choose a last drink time that supports consistent sleep. You set a bedtime and the tool works backward to a practical cut off you can plan around.

You provide the amount of caffeine in milligrams and a target bedtime, then set your personal half life, a residual threshold, and optional absorption and buffer minutes. The result shows how long the body needs to drop from the dose to the threshold and converts that to a cut off time you can act on.

If you plan to be in bed at 10:00 and you drink a typical 95 mg coffee, you might see that about nine hours of lead time are needed. In that case a mid afternoon drink is fine yet a dinner espresso is likely to push residual caffeine above your target.

Numbers are estimates and people vary, so use them as guidance and adjust as you learn what works for you. This tool provides informational estimates and does not substitute professional advice.

Technical Details:

Caffeine clearance is modeled as an absorption phase to peak followed by exponential decay. The quantities involved are dose D in milligrams (mg), a bedtime instant, half‑life (HL) in hours, a bedtime threshold T_thr in mg, an absorption time, and an optional buffer. Results focus on the decay needed to reach the threshold by bedtime and the corresponding cut‑off time.

The core index is the required number of half‑lives needed to reduce the dose down to the bedtime threshold. That count multiplied by the half‑life yields the required decay duration. Adding absorption and any buffer produces the total lead time to bedtime; subtracting that from bedtime gives the cut‑off.

Status is derived from the current time relative to the cut‑off. If the moment has passed, the status reads “Cut‑off passed”; otherwise it is “OK.” A companion metric shows the residual caffeine at bedtime if you drank now and whether it is at or below your threshold.

\begin{array}{l} HL = \frac{\ln (\frac{D}{T_{thr}})}{\ln 2} \\ t_{decay} = HL \cdot t_{half} \\ t_{lead} = t_{decay} + t_{absorb} + t_{buffer} \\ t_{cutoff} = t_{bedtime} - t_{lead} \end{array}

mg (t) = D \cdot {\frac{1}{2}}^{\frac{t - t_{peak}}{t_{half}}} for t \geq t_{peak}

Symbols and units
Symbol	Meaning	Unit/Datatype	Source
D	Caffeine dose	mg	Input
T_thr	Threshold at bedtime	mg	Input
t_half	Half‑life	h	Input
t_absorb	Absorption time to peak	min	Input
t_buffer	Personal buffer	min	Input
HL	Required half‑lives	count	Derived
t_decay	Required decay time	time	Derived
t_lead	Lead time to bedtime	time	Derived
t_cutoff	Computed cut‑off time	local datetime	Derived
mg(t)	Residual caffeine at time t	mg	Derived

Worked example. Using D = 95 mg, T_thr = 30 mg, t_half = 5 h, t_absorb = 45 min, and t_buffer = 0:

\begin{array}{l} HL = \frac{\ln (\frac{95}{30})}{\ln 2} \approx 1.66 \\ t_{decay} = 1.66 \cdot 5 = 8.3 h \\ t_{lead} = 8.3 + 0.75 + 0 \approx 9.05 h \end{array}

Interpretation: you would plan the last drink roughly nine hours before bedtime so that residual caffeine is at or below 30 mg.

Status interpretation
Status	Condition	Implication	Action cue
OK	now ≤ t_cutoff	Drinking now still reaches your threshold by bedtime.	Proceed or consider a smaller dose for extra margin.
Cut‑off passed	now > t_cutoff	A drink now would exceed the threshold at bedtime.	Delay, reduce dose, or move the bedtime later.

Units, precision & rounding:

Dose and thresholds are in mg; half‑life in hours; absorption and buffer in minutes; times are shown in your local timezone.
Residual mg values display to two decimals; the half‑life count displays to two decimals; durations are rendered as d h m s.

Validation & bounds extracted from the code:

Input validation and limits
Field	Type	Min	Max	Step/Pattern	Error Text	Placeholder/Default
Caffeine dose	Number	0	—	step 0.1	—	95
Target bedtime	Time (HH:MM)	00:00	23:59	step 60; regex `^(\d{1,2}):(\d{2})$`	“Set a bedtime to compute your cut‑off.”	22:00
Date mode	Enum	—	—	`auto` \| `tonight` \| `tomorrow`	—	auto
Half‑life (h)	Number	0.1	—	step 0.1	—	5
Threshold (mg)	Number	0.001	—	step 0.1	—	30
Absorption (min)	Number	0	—	step 1	—	45
Buffer (min)	Number	0	—	step 1	—	0
Drink preset	Enum	—	—	coffee, espresso, tea, cola, chocolate	—	custom

Time & calendrics:

Local timezone is used for all displayed times via locale formatting.
auto mode rolls bedtime to tomorrow if the time for today has already passed; tonight keeps today; tomorrow forces the next day.

I/O formats:

Inputs and outputs
Input	Accepted Families	Output	Encoding/Precision	Rounding
Dose, bedtime, parameters	Numbers; time HH:MM; enums	Cut‑off time; durations; status; residual	Times via locale; mg to 2 decimals	Nearest 0.01 for mg; seconds for durations
—	—	CSV metrics	`Metric,Value` per row	As displayed
—	—	JSON payload	`{ inputs: {...}, results: {...} }`	As computed

Networking & storage behavior:

Processing is browser‑based; no data is transmitted or stored server‑side.
An external charting library is used for the curve; if unavailable, calculations still work and the chart is omitted.

Assumptions & limitations:

Heads‑up Single constant half‑life; no multi‑phase kinetics.
Heads‑up Linear rise to peak during absorption; real absorption can vary.
User‑chosen threshold is a convenience target, not a medical limit.
Model does not account for repeated doses or stacking across the day.
Individual variability can be large; adjust parameters to fit experience.
Local clock and timezone affect displayed times.
Device locale influences decimal separators in numbers.
Chart markers use current time; reloading changes the “Now” line.

Edge cases & error sources:

Empty bedtime yields a warning and no calculation.
Threshold below minimum (0.001 mg) is clamped.
Half‑life below 0.1 h is clamped.
Absorption or buffer below 0 is clamped to 0.
Absorption set to 0 makes peak instantaneous.
Very large doses produce long lead times and extended charts.
Bedtime “tonight” in the past will mark status as passed.
Daylight‑saving transitions can shift clock displays.
System clock drift skews “Now” and status.
Clipboard or download actions can be blocked by permissions.

Privacy & compliance:

No data is transmitted or stored server‑side. Outputs are educational and not medical advice.

Step‑by‑Step Guide:

Plan a caffeine cut‑off that meets your bedtime threshold.

Choose a Drink preset or enter your caffeine dose in mg.
Set your Target bedtime and pick the date mode.
Adjust Half‑life and Threshold to match your preference.
Optionally add Absorption and Buffer minutes.
Read the computed Cut‑off, lead time, and status.

Example: Dose 95 mg, half‑life 5 h, threshold 30 mg, absorption 45 min → lead time ≈ 9.05 h; cut‑off is bedtime minus that lead.

Use the cut‑off as a simple boundary for daily planning.

FAQ:

Is my data stored?

No. Calculations run in your browser, and nothing is sent to a server. Copies and downloads stay on your device.

Privacy note

How accurate is the estimate?

It uses a single half‑life with linear absorption to peak and exponential decay. Individual responses vary, so adjust half‑life and threshold to fit your experience.

Model simplification

What units are used?

Dose and threshold are mg; half‑life is hours; absorption and buffer are minutes; times display in your local timezone.

Units

Can I use it without the chart?

Yes. If the chart library is unavailable, the metrics and cut‑off still compute and display.

Graceful degradation

How do I compute required half‑lives?

Divide dose by threshold, take the logarithm base two, and floor at zero if the ratio is at or below one.

Computation rule

What does “Cut‑off passed” mean?

Current time is later than the computed cut‑off. A drink now would exceed your bedtime threshold unless you delay bedtime or reduce dose.

Status meaning

Do presets lock the dose?

No. Presets simply set the mg value; you can edit it at any time.

Convenience only

Why do times look different on another device?

Time and number formatting follow each device’s locale and timezone. The underlying math is the same.

Locale effect

Troubleshooting:

No cut‑off shown: set a bedtime in HH:MM.
Chart missing: the chart library may be blocked; metrics still work.
Clipboard copy fails: check site permissions for the clipboard.
Download does nothing: allow downloads or pop‑ups for this site.
Unexpected locale formatting: review device language and region.
Status seems off: check device clock and timezone.

If bedtime is blank or invalid, calculations are disabled until you enter a valid HH:MM time.

Advanced Tips:

Tip Calibrate half‑life by back‑solving from days when sleep felt ideal.
Tip Lower the threshold if you notice light sleep near bedtime.
Tip Add a buffer to protect important nights with tight wake times.
Tip Use presets to sanity‑check drink labels against typical values.
Tip Watch the residual metric when experimenting with earlier or later bedtimes.
Tip Revisit parameters during schedule changes, travel, or training cycles.

Glossary:

Half‑life (HL): Time for caffeine to drop to half its current level.
Threshold: Target residual caffeine at bedtime, in mg.
Lead time: Total time needed before bedtime to reach the threshold.
Cut‑off time: Latest drink time that meets the bedtime threshold.
Absorption time: Minutes from drinking to peak caffeine level.
Residual: Remaining caffeine at a specific time, in mg.
Buffer: Extra margin added to the lead time for caution.
Status: OK or Cut‑off passed based on current time vs cut‑off.