Period Calculator

Forecast menstrual start and inclusive end dates across cycles with an interactive calendar and explicit variability scenarios.

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01 / YOUR PERIOD DETAILS

Estimate when your next period may start

The prefilled date is an example. Calendar arithmetic can be exact while biological timing remains uncertain. Predictions are not diagnoses or reliable contraception.

Prediction modeWhen did your most recent period start?First day of bleeding. All later starts are predictions.Usual days between period startsManual central cycle length, 21–45 days.How many days does your period usually last?Assumed duration, 1–10 days. Predicted end = start + duration − 1; both start and end count as bleeding days.

Advanced assumptions & optional period history

Number of future periods to estimateChoose 1–12. Uncertainty generally increases further into the future.Optional cycle-length variation (± days)0–10 days around the central cycle length. History mode uses the wider of this scenario and the recorded shortest/longest intervals.Optional ovulation estimatesSecondary estimates only; no peak-fertility or advanced ovulation analysis is added.Assumed days from ovulation to the next periodExisting calendar assumption, 7–18 days. Optional ovulation markers do not confirm ovulation.

Practical examples

Calculations and history stay in browser memory. No account, upload or saved menstrual history. CSV and clipboard exports happen only when requested.

02 / PERIOD FORECASTS

Estimated dates, not guaranteed dates

Predicted next menstrual period · not a diagnosisOct 29, 2026The next-period date is a central calendar estimate. Actual timing can fall outside every displayed scenario.

Expected next period: Oct 29, 2026
Next period scenario span: No variability entered
Predicted period end · inclusive: Nov 2, 2026
Prediction method: Manual cycle length · unchanged calendar arithmetic
Forecast anchor: Oct 1, 2026 · entered most recent start

Future menstrual starts and inclusive end dates
Forecast	Predicted start	Predicted end	Earliest scenario start	Latest scenario start
1	2026-10-29	2026-11-02	2026-10-29	2026-10-29
2	2026-11-26	2026-11-30	2026-11-26	2026-11-26
3	2026-12-24	2026-12-28	2026-12-24	2026-12-24

Entered starts and predicted periods

Jump within entered history / forecastMonths shown

User-entered period start
Assumed / predicted bleeding days

October 2026

November 2026

December 2026

Selected Oct 1, 2026: Entered start plus assumed bleeding duration · not observed end dates; User-entered period start · not clinically verified; prefilled inputs are examples unless replaced

S marks an entered start only; no historical bleeding ends are inferred. P is assumed duration or predicted bleeding. V is an illustrative start-date range, not continuous bleeding or a confidence interval. Arrow keys move by day/week, including month boundaries; Enter or Space selects a date. Forecast dates are not medical observations.

Calculation method, assumptions & limitations

Cycle / period-duration assumptions: 28 days / 5 days

Next start = entered date + cycle length. For forecast k, central start = 2026-10-01 + k × 28 calendar days.
Ovulation = predicted next start − 14 days. The modeled fertile window is the five preceding days plus that ovulation day; actual fertility may occur outside it.
Period end = predicted start + 4 days (inclusive duration). Variability spans use repeated shortest/longest entered cycles and widen with forecast horizon; they are scenarios, not statistical confidence limits.
All date arithmetic uses integer calendar days in UTC, not local times or 24-hour daylight-saving offsets. Forecast period end = predicted start + duration − 1, so the duration is inclusive. Mathematical date precision does not establish biological certainty. Substantial variation, missed records, hormonal changes and other factors can reduce calendar reliability.

Menstrual forecasts are approximate. Irregular cycles, pregnancy, contraception changes, breastfeeding, illness and other factors can shift dates. Optional ovulation is secondary and cannot confirm fertility or support reliable contraception. This tool makes no diagnosis.

CSV labels entered starts, calculated intervals, predicted inclusive start/end dates, assumptions and limitations separately.

Changes in bleeding patterns, missed records, pregnancy, breastfeeding, hormonal changes or illness can affect timing. This calendar cannot identify the cause of a change; seek appropriate healthcare advice if concerned.

ACOG · fertility-awareness information and calendar limitations

How to use Period Calculator

1
Enter a period start and cycle/bleeding duration
Enter your values locally; no inputs are uploaded.
2
Choose the forecast horizon and optional variability
Check validation feedback before reviewing the browser-generated result.
3
Review predicted dates and understand calendar-model limitations
Review the explanation, copy supported outputs or reset the workspace.

About Period Calculator

Enter a period start and cycle/bleeding duration. Choose the forecast horizon and optional variability. Review predicted dates and understand calendar-model limitations. Read the worked explanation and scope alongside every result.

Benefits

Forecast menstrual start and inclusive end dates across cycles with an interactive calendar and explicit variability scenarios. Interactive controls inspect the calculated model without silently changing submitted assumptions.

Privacy note

Period Calculator inputs and calculations remain in browser memory. No external processing API receives entered dates, measurements, coefficients or financial values. Ordinary website page requests may still be logged. Clipboard access occurs only when you choose Copy Result.

Frequently asked questions

How is the next period forecast?

Predicted start equals the entered first day plus the assumed cycle length. Later central starts use repeated cycles of that same length; this does not detect a menstrual event.

How is the end date counted?

A period lasting five days ends four days after its predicted start. The model uses inclusive duration rather than adding an extra day.

Why does the scenario span widen?

Repeated shortest and longest entered cycle lengths accumulate across the horizon. These spans illustrate assumptions and are not a medically validated probability interval.

Can the calendar also show ovulation?

Optional secondary markers use the next period minus the assumed luteal phase, with the preceding five days as a modeled fertile window. These dates cannot confirm fertility or support reliable contraception.

Why can actual dates differ?

Irregular cycles, pregnancy, breastfeeding, illness, hormonal changes and contraception changes can shift bleeding and ovulation. The calculator makes no diagnosis from a late or early period.

Which dates and forecast lengths are supported?

Inputs and all derived dates must stay within 1900–2100. The horizon is one to twelve cycles; month navigation and the jump selector help inspect the resulting calendar.

Mon	Tue	Wed	Thu	Fri	Sat	Sun
			1S P	2P	3P	4P
5P	6	7	8	9	10	11
12	13	14	15	16	17	18
19	20	21	22	23	24	25
26	27	28	29P	30P	31P

Mon	Tue	Wed	Thu	Fri	Sat	Sun
						1P
2P	3	4	5	6	7	8
9	10	11	12	13	14	15
16	17	18	19	20	21	22
23	24	25	26P	27P	28P	29P
30P

Mon	Tue	Wed	Thu	Fri	Sat	Sun
	1	2	3	4	5	6
7	8	9	10	11	12	13
14	15	16	17	18	19	20
21	22	23	24P	25P	26P	27P
28P	29	30	31