How to use the Period Calculator
The period calculator predicts the dates of your next three periods.
What you need to enter
- First day of last period
- Cycle length (days)
- Period length (days)
The formula
Next period = last period + cycle length; end = start + period length − 1.
Worked example
Using the default values in the calculator:
- First day of last period: 2026-10-13
- Cycle length (days): 28
- Period length (days): 5
| Period 1 | Tue, 10 Nov 2026 – Sat, 14 Nov 2026 |
| Period 2 | Tue, 08 Dec 2026 – Sat, 12 Dec 2026 |
| Period 3 | Tue, 05 Jan 2027 – Sat, 09 Jan 2027 |
Tips for better results
- Cycle lengths vary by a few days.
- Track symptoms.
Common mistakes to avoid
- Treating predictions as exact.
- Ignoring unusual changes that need medical advice.
Please note: Health calculators and guides give general estimates, not medical advice. Talk to a doctor or other qualified professional about your own health. See our Disclaimer.