How to use the Period Calculator (Next Period Date)
- Enter the first day of your most recent period.
- Enter your usual cycle length, counted from the first day of one period to the first day of the next.
- Enter how many days your period usually lasts.
- Read the predicted dates for the next three periods.
Formula used
Next period start = first day of last period + cycle length. Each later period adds another cycle length. The end date is the start plus the period length minus one day.
Worked example
Here is the Period Calculator (Next Period Date) worked through with the example values shown above, so you can check each step by hand.
| Inputs | |
|---|---|
| First day of last period | 2026-10-15 |
| Cycle length | 28 days |
| Period length | 5 days |
| Result | |
| Next period | 12 Nov 2026 |
| Next period ends around | 16 Nov 2026 |
| The following period | 10 Dec 2026 |
| And the one after | 7 Jan 2027 |
How the prediction works
The next period is expected one cycle length after the first day of the last one. With a last period starting on 15 October 2026 and a 28-day cycle, the next is expected on 12 Nov 2026, lasting until about 16 Nov 2026 for a 5-day period. The one after that is on 10 Dec 2026, and then 7 Jan 2027.
A longer cycle moves everything later. With a 32-day cycle the next period is 16 Nov 2026, and a period starting on 20 December with a 30-day cycle falls on 19 Jan 2027.
What is a normal cycle
A typical cycle is around 28 days, but cycles between about 21 and 35 days are common in adults, and the length can differ from month to month by a few days. Periods last from about 2 to 7 days for most people. Cycles are often longer and less regular in the first years after periods start and in the years before menopause.
Stress, illness, travel, changes in weight or exercise, contraception and hormonal conditions can all shift a period. A late period does not always mean pregnancy, and a prediction cannot tell you.
Ovulation and fertile days
Ovulation usually occurs about 14 days before the next period starts, not 14 days after the last one starts, so in a longer cycle it falls later. That is why predicting from cycle length matters. Use the ovulation calculator on this site to estimate your fertile window.
This calculator is not a method of contraception. Cycle-based predictions are not reliable enough to prevent pregnancy.
When to see a doctor
Consider speaking to a doctor if your periods suddenly become irregular, stop for three months or more without pregnancy, are unusually heavy or painful, last longer than a week, or occur more often than every 21 days.
Keeping a record of start dates, flow and symptoms helps them. Use the average of your last few cycles as your cycle length for a better prediction.
Frequently asked questions
How is cycle length counted?
From the first day of one period to the day before the first day of the next, so a 28-day cycle starts again on day 29 counting the first day as day 1.
How accurate is the prediction?
It is best for regular cycles. If your cycle varies, expect the actual date to differ by several days.
What if my cycle length changes?
Use the average of your last three to six cycles.
Can I use it to avoid pregnancy?
No. Cycle predictions are not a reliable contraception method.
Why is my period late?
Stress, illness, weight change, exercise, hormones and pregnancy are some causes. If worried, take a test or ask a doctor.
How long should a period last?
Most periods last 2 to 7 days. Enter your own usual length.
This calculator gives estimates for information only. Actual figures from a bank, employer, tax authority or doctor may differ. It is not financial, tax, legal or medical advice.