Flo Period Calculator
Flo period calculator works out Next period starts and Next period ends from First day of your last period, Average cycle length and Days of bleeding. The default figures on this page give next period starts of 2026-10-08; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
5 steps
- Using the stated 28-day cycle.
- Next period = 2026-09-10 + 28 days = 2026-10-08, running 5 day(s) to 2026-10-12.
- Ovulation is one luteal phase before that: 2026-10-08 - 14 = 2026-09-24, cycle day 15.
- The fertile window is the six days ending on ovulation: 2026-09-19 to 2026-09-24.
- On 2026-09-14 you are on cycle day 5, in the menstrual phase.
Predicted cyclestable
| cycle | periodStart | periodEnd | fertileWindowStart | ovulationDate |
|---|---|---|---|---|
| 1 | 2026-10-08 | 2026-10-12 | 2026-10-17 | 2026-10-22 |
| 2 | 2026-11-05 | 2026-11-09 | 2026-11-14 | 2026-11-19 |
| 3 | 2026-12-03 | 2026-12-07 | 2026-12-12 | 2026-12-17 |
| 4 | 2026-12-31 | 2027-01-04 | 2027-01-09 | 2027-01-14 |
| 5 | 2027-01-28 | 2027-02-01 | 2027-02-06 | 2027-02-11 |
| 6 | 2027-02-25 | 2027-03-01 | 2027-03-06 | 2027-03-11 |
Its 5 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
3 sources
What the flo period calculator calculates
Predicts upcoming period start and end dates, reports the current cycle day and phase, derives the fertile window, and assesses cycle regularity against the FIGO normal ranges.
How to use this flo period calculator
Enter First day of your last period, Average cycle length, Days of bleeding and Luteal phase length. Next period starts updates as you type. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind Next period starts, Next period ends and Next period starts on a.
The method follows Munro MG et al., FIGO classification system (PALM-COEIN) for abnormal uterine bleeding, System 1 normal limits, ACOG Committee Opinion No. 651: Menstruation in Girls and Adolescents - Using the Menstrual Cycle as a Vital Sign and Wilcox AJ, Dunson D, Baird DD. The timing of the "fertile window" in the menstrual cycle.
What you can compare
Change one input at a time to see how it affects Next period starts, Next period ends and Next period starts on a. The inputs you can vary are First day of your last period, Average cycle length, Days of bleeding, Luteal phase length, Cycles to predict and Recent cycle lengths.
Understanding Next period starts, Next period ends and Next period starts on a
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.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
See something wrong? Report a problem with this calculator
Frequently asked questions
How to tell if Flo your period is over?
Predicts upcoming period start and end dates, reports the current cycle day and phase, derives the fertile window, and assesses cycle regularity against the FIGO normal ranges. Open “How this is worked out” beside the result to see each step for your own figures.
How accurate is Flo at predicting periods?
The flo period calculator follows the stated method, but the result is only as reliable as First day of your last period, Average cycle length, Days of bleeding, Luteal phase length and Cycles to predict and the assumptions shown. Check the working and sources before relying on it for an important decision.
What is the monthly cost of Flo?
On the figures this page opens with, the next period starts is 2026-10-08. No single number fits everyone: it follows First day of your last period, Average cycle length and Days of bleeding, so enter your own above and the answer updates as you type.
What information do I need for the flo period calculator?
You need First day of your last period, Average cycle length, Days of bleeding, Luteal phase length, Cycles to predict, Recent cycle lengths and Today. The page opens with default figures that give next period starts of 2026-10-08, so you can see the scale before entering your own.
What does this flo period calculator show?
It shows Next period starts, Next period ends, Next period starts on a, Days until the next period, Cycle day today, Current phase, What is happening and Estimated ovulation, with the working and any warnings for your entries.
Can I check how the flo period calculator result was calculated?
Yes. The “How this is worked out” section shows the calculation steps, and the sources section identifies the references used for the method.