Period Next Date Online Calculator

See your likely next period date, ovulation estimate, and fertile window based on your recent cycle. This is just a reference tool — not medical advice and not birth control.

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Period & Next Date

Quick estimates for next period, ovulation & fertile window. Not for contraception.

First day of your last period

Average cycle length (days)

Usual period length (days)

Luteal phase (optional — 14 typical)

Irregular range (± days, optional)

Tip: add an **Irregular range** (e.g., 3–5) to get date windows.

At a glance

Cycle day today

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Days until next period

—

Late threshold

—

Predictions

Next period—

Flow days—

Ovulation (estimate)—

Fertile window—

PMS window (estimate)—

Fertile window ≈ 5 days before ovulation + ovulation day.

If cycles are often outside 21–35 days or periods are very heavy/painful, consider talking to a clinician.

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What to enter

First day of your last period — the day bleeding started (cycle day 1).
Average cycle length — the number of days from one period’s day 1 to the next (many adults fall between 21–35 days).
Usual period length — how many days you typically bleed.
Luteal phase (optional) — days from ovulation to the next period. If you don’t know it, the common estimate is 14 days.
Irregular range (optional) — how much your cycle typically varies (e.g., ±3). The results will show windows instead of single dates.

What you’ll get

Cycle day today — what day of the cycle you’re on right now.
Days until next period — a number or range. If it says “today,” your predicted window is here.
Late threshold — a “grace” date set a few days after the latest predicted start; helpful if you’re wondering when a period is considered late for this cycle.
Next period — expected start date (or window).
Flow days — your usual duration applied to the prediction.
Ovulation (estimate) — calculated as “next period date minus your luteal phase.” It’s an estimate, not a confirmation.
Fertile window — most likely fertile days (the five days before ovulation plus ovulation day).
PMS window — a broad timeframe in the late luteal phase when symptoms often appear.

Tips for better estimates

Track a few months and update your average and variability as you learn your pattern.
If you know your personal luteal length, enter it — ovulation estimates align better.
Cycles can shift with stress, illness, travel, intense training, weight changes, stopping/starting contraception, thyroid issues, PCOS, and more.

Trying to conceive?

Use the fertile window and ovulation estimate for timing.
For higher confidence, combine this estimate with signs like cervical mucus changes, ovulation predictor kits (OPKs), or basal body temperature tracking.

When to check in with a clinician

Very heavy bleeding (e.g., soaking a pad/tampon every 1–2 hours for several hours).
Severe pain that isn’t helped by usual measures.
Cycles that often fall outside 21–35 days, or sudden, persistent changes in your pattern.
Bleeding between periods, after sex, or after menopause.
Missed periods (not pregnant) or ongoing irregular cycles.

About your data

This calculator runs entirely in your browser. Your entries stay on this page and clear when you reload or close it.

Reminder: Predictions are based on averages and may not match your exact timing. Use this to plan and to notice trends — not to prevent pregnancy or diagnose health conditions.

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