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.

Updated 27 September 2026Checked against 5 worked examples

Next period starts

Thursday, 15 October 2026

Next period ends: Monday, 19 October 2026
Days until the next period: 18
Cycle day today: 11
Cycle phase today: Follicular phase, fertile window
Next estimated ovulation: Thursday, 1 October 2026
Next fertile window, from: Saturday, 26 September 2026
Next fertile window, to: Friday, 2 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 Thursday, 15 October 2026, 18 days from now, and last until Mon 19 Oct 2026. Today is cycle day 11 (follicular phase, fertile window), and the next estimated ovulation is Thu 1 Oct 2026.

Next 3 cycles: periods, fertile windows and ovulation

Your 28-day cycle

Period5 dOther follicular days4 dFertile window7 dLuteal phase12 d

Next 6 periods (6 rows)

#	Fertile from	Ovulation	Fertile to	Period starts	Period ends
1	26 Sep 2026	1 Oct 2026	2 Oct 2026	Thu 15 Oct 2026	19 Oct 2026
2	24 Oct 2026	29 Oct 2026	30 Oct 2026	Thu 12 Nov 2026	16 Nov 2026
3	21 Nov 2026	26 Nov 2026	27 Nov 2026	Thu 10 Dec 2026	14 Dec 2026
4	19 Dec 2026	24 Dec 2026	25 Dec 2026	Thu 7 Jan 2027	11 Jan 2027
5	16 Jan 2027	21 Jan 2027	22 Jan 2027	Thu 4 Feb 2027	8 Feb 2027
6	13 Feb 2027	18 Feb 2027	19 Feb 2027	Thu 4 Mar 2027	8 Mar 2027

How it's calculated S

Next period
$\text{Thu 17 Sep 2026} + 28\ \text{d} = \text{Thu 15 Oct 2026}$
Period end
$\text{Thu 15 Oct 2026} + (5 - 1)\ \text{d} = \text{Mon 19 Oct 2026}$
Ovulation before that period
$\text{Thu 15 Oct 2026} - 14\ \text{d} = \text{Thu 1 Oct 2026}\quad(\text{cycle day } 15)$
Counted back from the period, because the luteal phase varies less than the days before ovulation.
Later periods
$\text{period}_k = \text{Thu 17 Sep 2026} + k \times 28\ \text{d},\quad k = 1, \dots, 6$
The 6 predictions assume every cycle lasts 28 days.
Today
Sun 27 Sep 2026 is cycle day 11: follicular phase, fertile window.

About the period calculator

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.

Worked examples

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: not applicable
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

Checked against: 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

Checked against: 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: not applicable
Cycle day today: 37
Cycle phase today: Period overdue

Checked against: 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

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

Questions

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.

How accurate is the period calculator?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 5 worked examples whose answers come from independent sources; for example, “Period on 5 Jan 2026, 28-day cycle, 5 days” is checked against 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).

Where does the method come from?

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.

About this calculator

\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]

Sources

For information only, not medical advice. These formulas describe populations, not you; talk to a clinician before acting on a result.

Checked against references

5 worked examples with independently sourced answers ship with this calculator. They run in the test suite; you can run them here too.

Next 3 cycles: periods, fertile windows and ovulation

Your 28-day cycle

Next period

Period end

Ovulation before that period

Later periods

Today