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Period calculator: next period dates

Next period date and the following 5 predicted periods, with each cycle's fertile window and ovulation day, from your last period and average cycle length.

更新日 検証済みの例：5

= 2026-09-19
days
From the first day of one period to the first day of the next.
days
その他の設定
days
Days from ovulation to the next period; 14 is the textbook value.
試す
Next period starts
Saturday, 17 October 2026
Next period starts: Saturday, 17 October 2026
Next period ends
Wednesday, 21 October 2026
Days until the next period
18
Cycle day today
11
Cycle phase today
Follicular phase, fertile window
Next estimated ovulation
Saturday, 3 October 2026
Next fertile window, from
Monday, 28 September 2026
Next fertile window, to
Sunday, 4 October 2026
Calendar predictions assume equal cycles and can't confirm ovulation; they are not reliable contraception on their own.

Your next period should start on Saturday, 17 October 2026, 18 days from now, and last until Wed 21 Oct 2026. Today is cycle day 11 (follicular phase, fertile window), and the next estimated ovulation is Sat 3 Oct 2026.

Next 3 cycles: periods, fertile windows and ovulation

Period: 19 Sep 2026 to 24 Sep 2026Fertile: 28 Sep 2026 to 5 Oct 2026Period: 17 Oct 2026 to 22 Oct 2026Fertile: 26 Oct 2026 to 2 Nov 2026Period: 14 Nov 2026 to 19 Nov 2026Fertile: 23 Nov 2026 to 30 Nov 2026Period: 12 Dec 2026 to 17 Dec 2026Today29 SepOvulation3 OctOvulation31 OctOvulation28 Nov

Your 28-day cycle

Day 11of 28
Period5 dOther follicular days4 dFertile window7 dLuteal phase12 d
Next 6 periods 行数：6
#Fertile from排卵日Fertile toPeriod startsPeriod ends
128 Sep 20263 Oct 20264 Oct 2026Sat 17 Oct 202621 Oct 2026
226 Oct 202631 Oct 20261 Nov 2026Sat 14 Nov 202618 Nov 2026
323 Nov 202628 Nov 202629 Nov 2026Sat 12 Dec 202616 Dec 2026
421 Dec 202626 Dec 202627 Dec 2026Sat 9 Jan 202713 Jan 2027
518 Jan 202723 Jan 202724 Jan 2027Sat 6 Feb 202710 Feb 2027
615 Feb 202720 Feb 202721 Feb 2027Sat 6 Mar 202710 Mar 2027
計算方法 S

  1. Next period

    Sat 19 Sep 2026+28 d=Sat 17 Oct 2026\text{Sat 19 Sep 2026} + 28\ \text{d} = \text{Sat 17 Oct 2026}

  2. Period end

    Sat 17 Oct 2026+(5−1) d=Wed 21 Oct 2026\text{Sat 17 Oct 2026} + (5 - 1)\ \text{d} = \text{Wed 21 Oct 2026}

  3. Ovulation before that period

    Sat 17 Oct 2026−14 d=Sat 3 Oct 2026(cycle day 15)\text{Sat 17 Oct 2026} - 14\ \text{d} = \text{Sat 3 Oct 2026}\quad(\text{cycle day } 15)

    Counted back from the period, because the luteal phase varies less than the days before ovulation.

  4. Later periods

    periodk=Sat 19 Sep 2026+k×28 d,k=1,…,6\text{period}_k = \text{Sat 19 Sep 2026} + k \times 28\ \text{d},\quad k = 1, \dots, 6

    The 6 predictions assume every cycle lasts 28 days.

  5. Today

    Tue 29 Sep 2026 is cycle day 11: follicular phase, fertile window.

Period calculator: next period datesについて

The next period is expected one average cycle length after the first day of the last one, and each later period another cycle length on. A period of 5 days that started on 5 January 2026, in a 28-day cycle, points to periods starting on 2 February, 2 March, 30 March and so on. Ovulation in each cycle is placed 14 days before the period that ends it, with a fertile window from 5 days before ovulation to 1 day after.

People use it to plan around the next few periods and to see which phase of the cycle they are in today. The calendar layout is built for periods; the ovulation calculator on this site covers fertile days in more detail for anyone trying to conceive.

Predictions assume every cycle is the same length. In 612,613 app-recorded cycles (Bull and colleagues, 2019) the standard deviation of one person's cycle lengths averaged 2.6 days, so each later prediction is less certain than the one before.

計算例

Period on 5 Jan 2026, 28-day cycle, 5 days

First day of last period
2026-01-05
Average cycle length
28 days
Period length
5 days
Periods to predict
6
Next period starts
2026-02-02
Next period ends
2026-02-06
Days until the next period
18
Days late
該当なし
Cycle day today
11
Cycle phase today
Follicular phase, fertile window
Next estimated ovulation
2026-01-19
Next fertile window, from
2026-01-14
Next fertile window, to
2026-01-20

照合元：Python 3.8 datetime with today 2026-01-15: LMP + 28 d; period end + 4 d; ovulation 2026-02-02 − 14 d; window −5/+1 d (Wilcox 1995)

30-day cycle, this cycle's ovulation already past

First day of last period
2025-12-20
Average cycle length
30 days
Period length
5 days
Periods to predict
6
Next period starts
2026-01-19
Days until the next period
4
Cycle day today
27
Cycle phase today
Luteal phase
Next estimated ovulation
2026-02-04
Next fertile window, from
2026-01-30
Next fertile window, to
2026-02-05

照合元：Python 3.8 datetime: 2025-12-20 + 30 d; ovulation 2026-01-05 has passed, so the next is 2026-02-18 − 14 d = 2026-02-04

Late period

First day of last period
2025-12-10
Average cycle length
28 days
Period length
5 days
Periods to predict
6
Next period starts
2026-01-07
Days late
8
Days until the next period
該当なし
Cycle day today
37
Cycle phase today
Period overdue

照合元：Python 3.8 datetime: 2025-12-10 + 28 d = 2026-01-07, 8 days before 2026-01-15

Edge: period started today

First day of last period
2026-01-15
Average cycle length
28 days
Period length
5 days
Periods to predict
6
Next period starts
2026-02-12
Days until the next period
28
Cycle day today
1
Cycle phase today
Period
Next estimated ovulation
2026-01-29

照合元：Python 3.8 datetime: 2026-01-15 + 28 d; ovulation 2026-02-12 − 14 d

よくある質問

How do I calculate my next period date?

Add your average cycle length to the first day of your last period. Cycle length is counted from the first day of one period to the first day of the next, so a period that started on 5 January with a 28-day cycle points to 2 February. For an average, add up the lengths of your last few cycles and divide by the number of cycles.

What is a normal menstrual cycle length?

21 to 35 days in adults, according to ACOG, with bleeding that usually lasts up to 7 days. Only a minority of cycles are exactly 28 days: in 612,613 cycles logged in a fertility app, 13 % were 28 days long and the mean was 29.3 days. Cycles regularly shorter than 21 days or longer than 35 are worth mentioning to a clinician.

How many days late can a period be?

A few days either way is within the usual variation. Across 612,613 cycles from 124,648 app users, Bull and colleagues (2019) found the standard deviation of each person's cycle lengths averaged 2.6 days, rising to 3.1 days at age 45. A period 2 or 3 days after the predicted date is therefore common. If a period is late and pregnancy is possible, a pregnancy test is the usual next step.

When am I most fertile in my cycle?

In the 6 days ending on ovulation day, which in a 28-day cycle is around cycle day 15. Wilcox and colleagues (1995) found pregnancies only followed intercourse in that window. The ovulation calculator on this site gives those dates for the next few cycles and lets you change the luteal phase length.

「Period calculator: next period dates」の精度はどのくらいですか？

精度は入力値と計算方法の前提に依存します。十進演算には有効数字50桁を使いますが、推定、数値計算手法、元データの精度はそれより低い場合があります。表示の丸め処理でこれらの制約がなくなるわけではありません。 独立した出典の解答と照合した計算例：5。 例えば、「Period on 5 Jan 2026, 28-day cycle, 5 days」はPython 3.8 datetime with today 2026-01-15: LMP + 28 d; period end + 4 d; ovulation 2026-02-02 − 14 d; window −5/+1 d (Wilcox 1995)と照合しています。

この計算方法の出典は何ですか？

ACOG — Abnormal uterine bleeding (FAQ): normal cycle 21–35 days, bleeding up to 7 days; Bull JR et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ Digit Med 2019;2:83; Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517–21.

この計算機について

periodk=LMP+k×cycle,ovulationk=periodk−luteal,fertilek=[ovulationk−5, ovulationk+1]\text{period}_k = \text{LMP} + k \times \text{cycle},\qquad \text{ovulation}_k = \text{period}_k - \text{luteal},\qquad \text{fertile}_k = [\text{ovulation}_k - 5,\ \text{ovulation}_k + 1]

出典

  1. ACOG — Abnormal uterine bleeding (FAQ): normal cycle 21–35 days, bleeding up to 7 days
  2. Bull JR et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ Digit Med 2019;2:83
  3. Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517–21

情報提供のみで、医療上の助言ではありません。これらの式は集団を表すもので、個人の状態を評価するものではありません。結果に基づいて行動する前に医療専門家へ相談してください。

出典と照合済み

この計算機には、独立した出典の解答を使った計算例が 5 件あります。テストに組み込まれており、ここでも実行できます。

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