Period Calculator
Predict your next periods, fertile days and ovulation from the start of your last period, your cycle length and how long your period lasts.
Results
| Item | Value |
|---|
Next six periods
| Period | Starts | Ends about | Fertile window |
|---|
Quick Answer
Your next period usually starts one cycle length after the first day of your last period. If your last period started on March 1, 2026 and your cycle is 28 days, your next period should start on Sunday, March 29, 2026. Ovulation is likely about 14 days earlier, around Sunday, March 15, 2026.
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Table of Contents
- Period Calculator
- Quick Answer
- Related Calculators
- How This Period Calculator Works
- How to Use the Period Calculator
- Core Formulas
- Key Terms
- Worked Example
- Real-Life Examples
- Cycle Length vs Period Length
- What Can Change Your Cycle
- Common Mistakes to Avoid
- Frequently Asked Questions
- References & Sources
How This Period Calculator Works
Enter the first day of your last period, your usual cycle length and how many days your period lasts. The calculator adds your cycle length to predict each next period, and estimates ovulation and the fertile window for each cycle. It lists your next six periods so you can plan ahead.
Quick explanation: a cycle runs from the first day of one period to the first day of the next.
- Average cycle: about 28 days, but 21 to 35 days is normal for adults.
- Period length: usually 2 to 7 days.
- Ovulation: often about 10 to 16 days before the next period.
How to Use the Period Calculator
- Pick the first day of your last period.
- Enter your average cycle length. Track a few cycles to find it.
- Enter how many days your period usually lasts.
- Press Calculate.
- Read your next period, ovulation estimate and the next six periods.
Core Formulas
- Next period: first day of last period + cycle length
- Period end: period start + period length − 1
- Ovulation: next period − 14 days
- Fertile window: 5 days before ovulation to ovulation day
Key Terms
- Menstrual cycle: the monthly cycle from the first day of one period to the first day of the next.
- Period: the days of menstrual bleeding.
- Ovulation: the release of an egg, usually about 2 weeks before the next period.
- Fertile window: the days when pregnancy is most likely.
- Luteal phase: the time from ovulation to the next period.
Worked Example
Your last period started on March 1, 2026. Your cycles are 30 days and periods last 5 days.
- Next period = March 1 + 30 days = Tuesday, March 31, 2026
- It should end about Saturday, April 4, 2026
- Ovulation is around Tuesday, March 17, 2026
Real-Life Examples
- Planning a trip: see if a vacation in the next few months falls on your period.
- Being prepared: with a 28-day cycle from March 1, your next three periods start Mar 29, Apr 26 and May 24.
- Trying to conceive: the fertile window shows the best days. Our Ovulation Calculator has more detail.
- Noticing a late period: if your period is a week or more late, consider a pregnancy test.
Cycle Length vs Period Length
- Cycle length: days from the start of one period to the start of the next, often 21 to 35 days.
- Period length: days of bleeding, usually 2 to 7 days.
- Both matter: cycle length predicts when the next period starts, and period length predicts when it ends.
What Can Change Your Cycle
- Stress and illness: can delay ovulation and your period.
- Weight changes and exercise: big changes can affect cycles.
- Hormonal birth control: changes bleeding patterns, so predictions may not apply.
- Life stage: cycles are often irregular after periods start and before menopause.
Common Mistakes to Avoid
- Counting from the last day of bleeding. Cycle day 1 is the first day of your period.
- Using one cycle only. Average a few cycles for a better estimate.
- Treating predictions as exact. Cycles naturally vary by a few days.
- Using it as birth control. Calendar predictions are not reliable enough.
- Ignoring big changes. See a doctor if your cycle suddenly changes or stops.
Frequently Asked Questions
Add your cycle length to the first day of your last period.
The NHS says 21 to 35 days is normal, with an average of about 28 days.
Around 2 to 7 days, according to the NHS.
Stress, illness, weight changes, exercise, pregnancy and some medicines can delay a period.
Usually about 10 to 16 days before your next period.
It is less likely but possible, especially with short cycles.
Count the days from the first day of one period to the first day of the next, for a few months, and average them.
Some variation is normal. See a doctor if your cycle often changes by more than about a week, or periods stop.
Hormonal birth control changes bleeding, so this calculator may not apply.
It is accurate for regular cycles. Real cycles may vary by a few days.
From the day your period is due, if you might be pregnant.
Enter the average of your last few cycles. If the gap between your shortest and longest cycle is large, treat the predicted dates as a rough range rather than exact days, and consider tracking with ovulation tests.
Yes, but cycles can take a few months to settle after stopping hormonal birth control. Track a couple of natural cycles first for a better estimate.
For very heavy or painful periods, bleeding between periods, or periods that stop for 3 months or more.
References & Sources
| Source | Used For | Link |
|---|---|---|
| NHS, Periods and fertility in the menstrual cycle | Normal cycle and period lengths, and ovulation timing. | https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/ |
| ACOG, Fertility awareness-based methods | The menstrual cycle and fertile days. | https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning |