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Pregnancy & Baby
Period Calculator — Predict Your Next 6 Periods & Fertile Days
The simple calendar math that turns one period date into a six-cycle forecast
Caught off guard by your period? A period calculator fixes that with a little calendar arithmetic. Give it three things you already know — the first day of your last period (LMP), your average cycle length, and how many days your period lasts — and it projects the start and end dates of your next six periods, plus the estimated ovulation day inside each one. It's the same model the Mayo Clinic describes for a menstrual cycle: a cycle runs from the first day of one period to the day before the next, and the next period is simply the last one plus your average cycle length.
Here's the core idea. A menstrual cycle has two phases split by ovulation. The luteal phase — ovulation to next period — is remarkably steady at about 14 days for most people, while the follicular phase before ovulation stretches or shrinks to make cycles longer or shorter. That's why ovulation is best estimated by counting backward from the next period, not forward from the last: ovulation ≈ next period start − 14 days. On a 28-day cycle that lands on day 14, on a 32-day cycle closer to day 18, and on a 24-day cycle around day 10.
How the six-cycle forecast is built:
- Period 1 start = LMP + cycle length
- Period 2 start = Period 1 start + cycle length
- …and so on through Period 6
- Period end = start + (period length − 1)
- Estimated ovulation = each period start − 14 days
Worked example. Your LMP was June 1, 2026, your cycles average 28 days, and your period runs 5 days. The tool predicts your next period starting June 29, then July 27, August 24, and so on — six dates in all — each lasting through day five, with ovulation for the first upcoming cycle estimated around June 15.
A quick boundary note so you use the right tool: this page owns period prediction — when will my period come? If your real question is when am I most fertile?, the dedicated Ovulation Calculator maps the full fertile window (the five days before ovulation plus the day itself), and the Safe Period Calculator covers the rhythm method. This one stays focused on your bleed dates.
This forecast is for general planning and is informational only — it is not medical advice and is not a reliable form of birth control. Cycles shift with stress, travel, illness, weight changes, perimenopause, and hormonal birth control, so treat every date here as an estimate, not a guarantee. If your periods are very irregular, suddenly change, or stop, talk to a healthcare provider.
📖 See also: Ovulation and Your Fertile Window, Explained
Calculator
Fill in the fields and click "Calculate" for instant results.
📰 Formula
• Next period start = first day of last period (LMP) + average cycle length • Each later period = previous predicted start + cycle length (repeated for 6 cycles) • Period end date = start date + (period length − 1 day) • Estimated ovulation in a cycle = that period's start − 14 days • Cycle = first day of one period to the day before the next period
📰 Formula
• Next period start = first day of last period (LMP) + average cycle length • Each later period = previous predicted start + cycle length (repeated for 6 cycles) • Period end date = start date + (period length − 1 day) • Estimated ovulation in a cycle = that period's start − 14 days • Cycle = first day of one period to the day before the next period
🧪 Worked examples
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Example 2
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Example 3
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Example 4
⚠️ Common mistakes
- Entering the day your period ended instead of the first day it started.
- Counting cycle length from the end of one period to the start of the next — it's start to start.
- Assuming ovulation is always on day 14, even when your cycle is much longer or shorter.
- Trusting the predictions as contraception — they are estimates, not a reliable way to avoid pregnancy.
- Expecting exact dates from irregular cycles, where averages don't predict well.
💡 Tips
- Use the first day of bleeding as your LMP — that's day 1 of the cycle.
- Track three or four cycles and average their lengths for a more accurate forecast.
- If your cycle length swings by more than a few days month to month, treat predictions as loose ranges.
- Cycle length is start-of-period to start-of-next-period, not the gap between periods.
- Stress, illness, travel, and big weight changes can move a period — re-run the tool with each new actual start date.
Embed this calculator on your site
Copy the code below and paste it into the HTML of your site or blog.
<iframe src="https://www.calcnimbus.com/embed/period-calculator" width="100%" height="500" frameborder="0" style="border:1px solid #eee;border-radius:12px"></iframe>
❓ Frequently asked questions
How does a period calculator predict my next period?
It adds your average cycle length to the first day of your last period to find the next start date, then repeats that to project six cycles ahead. Each period's length is added on to show its end date. The result is a forecast for general planning, not medical advice.
How is the estimated ovulation day calculated?
The luteal phase between ovulation and the next period is fairly fixed at about 14 days, so ovulation is estimated as the predicted next-period start minus 14 days. On a 28-day cycle that's around day 14, but it shifts later for longer cycles and earlier for shorter ones. It's an estimate, not a confirmed ovulation date.
What's the difference between cycle length and period length?
Cycle length is the number of days from the first day of one period to the day before the next period starts — typically 21 to 35 days. Period length is just how many days the bleeding itself lasts, usually 2 to 7 days. The calculator needs both to forecast start and end dates.
Can I rely on this calculator as birth control?
No. These are statistical estimates based on your past cycles, and ovulation can shift, so the predicted fertile and non-fertile days are not reliable contraception. Sperm can survive several days and ovulation timing varies. Use a proven contraceptive method if you're trying to avoid pregnancy.
What if my periods are irregular?
Averages predict poorly when cycles swing widely from month to month, so treat the dates as rough ranges rather than fixed days. Tracking several cycles and using a representative average helps a little. Persistent irregularity is worth discussing with a healthcare provider.
How do I find my average cycle length?
Mark the first day of bleeding for several cycles, count the days from each start to the next start, and average those numbers. For example, cycles of 27, 29, and 28 days average to 28. The more cycles you include, the steadier the average.
Why does ovulation move when my cycle is longer or shorter?
The follicular phase before ovulation is what varies in length, while the luteal phase afterward stays near 14 days. So a longer cycle mostly means a longer run-up to a later ovulation, and a shorter cycle means an earlier one. Counting back 14 days from the next period captures that shift automatically.
Does this calculator work if I'm on hormonal birth control?
Not reliably. Hormonal contraceptives like the pill, patch, or IUD control or suppress your natural cycle, so the bleeding you have may be a withdrawal bleed on a fixed schedule rather than a true period. Follow your method's packaging or your provider's guidance instead.
How many periods does it predict?
It projects the next six period start dates from your last period, each with its end date and an estimated ovulation day for the cycle. Six cycles covers roughly the next five to seven months for typical cycle lengths. Re-enter your real start date each month to keep the forecast accurate.