Flo Period Calculator

Ignored when you supply a run of recent cycle lengths below.
Optional. Enter the lengths of your last few cycles to average them and assess regularity.
Next period starts
2026-09-10
Period and Menstrual Cycle Calculator
Next period ends
2026-09-14
Next period starts on a
Thursday
Days until the next period
4
Cycle day today
25
Current phase
luteal
What is happening
Luteal phase: the corpus luteum produces progesterone until the next period begins.
Estimated ovulation
2026-08-27
Ovulation cycle day
15
Fertile window opens
2026-08-22
Fertile window closes
2026-08-27
Cycle length used
28
Cycle frequency
normal frequency (24-38 days)
Due date if you conceive this cycle
2027-05-20
Worked out from Munro MG et al., FIGO classification system (PALM-COEIN) for abnormal uterine bleeding, System 1 normal limits and 2 other sources.
The period entered is 8 whole cycle(s) in the past, so figures are for the cycle beginning 2026-08-13.
Calendar prediction assumes the next cycle resembles the last. Stress, illness, travel and hormonal contraception all shift ovulation, and the period date moves with it.
An estimate from calendar arithmetic, not a clinical assessment. Only about 4% of births occur on the estimated due date; ultrasound dating by a clinician supersedes menstrual dating when they disagree.

How this is worked out

5 steps
  1. Using the stated 28-day cycle.
  2. Next period = 2026-08-13 + 28 days = 2026-09-10, running 5 day(s) to 2026-09-14.
  3. Ovulation is one luteal phase before that: 2026-09-10 - 14 = 2026-08-27, cycle day 15.
  4. The fertile window is the six days ending on ovulation: 2026-08-22 to 2026-08-27.
  5. On 2026-09-06 you are on cycle day 25, in the luteal phase.
Predicted cyclestable
cycleperiodStartperiodEndfertileWindowStartovulationDate
12026-09-102026-09-142026-09-192026-09-24
22026-10-082026-10-122026-10-172026-10-22
32026-11-052026-11-092026-11-142026-11-19
42026-12-032026-12-072026-12-122026-12-17
52026-12-312027-01-042027-01-092027-01-14
62027-01-282027-02-012027-02-062027-02-11

Results are general information, not financial, tax, legal, medical or engineering advice.