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Knowing when your next period is due helps you plan — travel, events, trying to conceive, or just having supplies ready. The prediction is simple arithmetic once you know two things: when your last period started and how long your cycle usually runs.
Enter those two values and the tool projects your next three expected period start dates.
Period Calculator: step-by-step worked example
- First day of last period
- September 30, 2026
- Cycle length (days)
- 29
- 1
Next period
Period₁ = Last period + Cycle
September 30, 2026 + 29 = October 29, 2026
- 2
Second period
Period₂ = Last period + 2 × Cycle
September 30, 2026 + 58 = November 27, 2026
- 3
Third period
Period₃ = Last period + 3 × Cycle
September 30, 2026 + 87 = December 26, 2026
- Last periodSeptember 30, 2026
- Period 1October 29, 2026
- Period 2November 27, 2026
- Period 3December 26, 2026
Good to know: The estimate assumes every cycle is the same length, while shifts of ±2–3 days are normal. Enter the average of your last 3–6 cycles. See a gynaecologist if cycles are shorter than 21 days, longer than 35 days or very irregular.
Period Calculator — how it works
Cycle length is the key number
Your cycle length is counted from the first day of one period to the first day of the next — not the number of bleeding days. The average is about 28 days, but anything from roughly 21 to 35 days is considered normal for adults.
| Prediction | Calculation |
|---|---|
| Next period | Last period + cycle length |
| Following period | Last period + 2 × cycle length |
| Third period | Last period + 3 × cycle length |
Regular vs irregular
These projections assume your cycle is fairly consistent. Many people vary by a few days month to month, which is normal. Larger swings, very short or very long cycles, or skipped periods can have many causes — from stress and travel to hormonal conditions — and are worth discussing with a clinician if they’re a change for you.
Tracking makes it sharper
A single cycle length is a starting point; the more months you track, the better your personal average becomes. If your recent cycles have run 26, 29 and 27 days, using ~27 will predict better than assuming a textbook 28.
Worked example
If your last period started on 1 March 2026 and your average cycle is 28 days, your next period is expected around 29 March, the one after around 26 April, and the third around 24 May. If you actually average 26 days, the same last-period date shifts every prediction earlier — next period around 27 March, then 22 April, then 18 May — which is why using your own average matters.
If your period started today (12 October 2026): next periods by cycle length
| Cycle | Next period | 2nd period | 3rd period |
|---|---|---|---|
| 21 days | 2 November 2026 | 23 November 2026 | 14 December 2026 |
| 24 days | 5 November 2026 | 29 November 2026 | 23 December 2026 |
| 26 days | 7 November 2026 | 3 December 2026 | 29 December 2026 |
| 28 days | 9 November 2026 | 7 December 2026 | 4 January 2027 |
| 30 days | 11 November 2026 | 11 December 2026 | 10 January 2027 |
| 32 days | 13 November 2026 | 15 December 2026 | 16 January 2027 |
| 35 days | 16 November 2026 | 21 December 2026 | 25 January 2027 |
A cycle runs from the first day of one period to the first day of the next; 21–35 days is normal.
Frequently asked questions
How is my next period date predicted?+
By adding your average cycle length to the first day of your last period. Cycle length is measured from the first day of one period to the first day of the next, so if your last period started on the 1st and you average 28 days, the next is expected around the 29th.
What is a normal cycle length?+
For adults, roughly 21 to 35 days is typical; the average is about 28. Teenagers and people approaching menopause tend to have more variable cycles. What matters most is what’s normal and consistent for you.
Why are my periods irregular?+
Cycle length can shift with stress, travel, illness, exercise, weight change, breastfeeding, contraception and hormonal conditions such as PCOS or thyroid problems. Occasional variation is normal; a persistent change from your usual pattern is worth raising with a healthcare provider.
How accurate is a calendar prediction?+
It’s only as regular as your cycles are. For consistent cycles it’s usually within a day or two; for irregular cycles it’s a rough guide. Tracking several months and using your real average improves accuracy.
Can I use this to plan for or avoid pregnancy?+
It shows when to expect your period, and pairs well with an ovulation calculator for the fertile window. But calendar predictions are not reliable contraception on their own — they can’t catch a cycle where ovulation shifts. For contraception, use a proven method.
Sources
About this page
- Publisher:
- Kalkoro — Teknolojig Bilişim Teknolojileri A.Ş., İstanbul
- Written by:
- Kübra Yıldız — Editor (Health, nutrition, pregnancy, education, exam scores)
- Reviewed by:
- Furkan Tur — Editor
Parameters come from official sources; the last update date is shown at the bottom of the page.
Spotted a wrong result? Use the "Report a bug" button at the top of the page; every report is reviewed.
How we calculate and our accuracy policy →
This tool is for general information only and is not medical diagnosis, treatment or advice. Consult a physician for decisions about your health.
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