Day of last period start

Cycle length (days)

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General estimate for planning only — not medical advice. Consult a doctor about your specific situation.

Calculation details

Every figure above is calculated locally in your browser from the assumptions shown. No inputs are sent anywhere. See the methodology section below for the formulas used.

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Cycle math showing next period day 43 and ovulation day 29 — A 28-day cycle puts ovulation at day 29 of the example.

Method & verification

How This Tool Works

fact-checked against our calculation methodology

Calculation Method

Next period = Day of last period start + Cycle length

Site methodology

BoringToolsKit methodology and verification standard

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Assumptions to Verify

These tools provide general estimates for planning and education only.
They are not medical advice and not a substitute for a qualified healthcare professional.
Always consult your doctor or another healthcare provider about your specific situation.

Privacy: The calculation runs in this browser. Shared links contain the page address only, not your inputs or results.

A worked example, day by day

Say your last period started on day 15 of a 28-day cycle. The calculator adds the cycle length to the start day: 15 + 28 = day 43, so your next period lands around day 43, roughly 28 days after the last one began.

Ovulation is estimated by counting back 14 days from that next period: 43 - 14 = day 29. The luteal phase (the stretch between ovulation and bleeding) is fairly stable in most people, which is why the calculator holds it at 14 days rather than asking for it. That also means ovulation falls on cycle day 15 of this cycle (29 - 14 = 15 in relative terms), about 14 days after the period started.

The fertile window stretches wider than ovulation day itself: sperm survive up to about 5 days and the egg about 24 hours, so practical fertility spans roughly day 24 to day 30 in this example, peaking at day 29. Shorten the cycle to 24 days and the same math gives next period at day 39 and ovulation at day 25, so a 4-day shorter cycle moves the entire fertile window about 4 days earlier.

Where the estimate drifts

The 14-day luteal phase is an average, not a law. Studies measuring it place the typical range around 11 to 17 days, so a single cycle's ovulation can land several days off the estimate even with a textbook 28-day cycle. At a 14 vs 17 day spread, that is a 3-day error in the ovulation date, and the fertile window widens accordingly.

Cycle length variability compounds the problem. Research tracking untrained app users found only about 13 percent of cycles were exactly 28 days, and cycle-to-cycle variation of 7 to 9 days is common in healthy adults. If you feed the calculator a 28-day average but your actual cycles swing from 25 to 33, the true next-period date can be off by nearly a week in either direction.

Practical use: log at least 3 to 6 recent cycles and use the shortest cycle for the early boundary of your fertile window, since sperm survival makes early shifts the costly ones for avoiding or achieving pregnancy. Hormonal birth control, breastfeeding, and perimenopause can push cycles outside the 15 to 60 day input range entirely, at which point this tool is not the right instrument and cycle-tracking with a clinician is.