Quick answer
The next period is estimated as the first day of the last period plus the average cycle length, ovulation about 14 days before it, and the fertile window from 5 days before ovulation to 1 day after. From 5 January 2025 with a 28-day cycle: next period 2 February, ovulation about 19 January, fertile window 14 to 20 January.
Enter the first day of your last period, your average cycle length and period length. The calculator returns the next two period start dates, an estimated ovulation date and a fertile window.
Related tools: Payback Period Calculator: Time to recover an investment from steady cash flow. Period to Frequency Calculator: Frequency from period, or period from frequency. Age Calculator: Calculate your exact age in years, months and days.
How to use the Period Calculator
- Pick first day of last period.
- Enter average cycle length (days) (a whole number, between 20 and 45). It starts at 28.
- Enter period length (days) (a whole number, between 1 and 10). It starts at 5.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| First day of last period | A calendar date | — |
| Average cycle length | A whole number, in days | ≥ 20, ≤ 45, starts at 28 |
| Period length | A whole number, in days | ≥ 1, ≤ 10, starts at 5 |
Formula and method
Formula
next period = last start + cycle length; ovulation ≈ next period − 14 days; fertile window = ovulation − 5 to + 1 days
IEEE-754 double precision; shown to the decimals indicated.
A calendar estimate assuming a regular cycle and a 14-day luteal phase; it is not a method of contraception.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: First day of last period = 2025-01-05; Average cycle length = 28; Period length = 5.
| Result | Value |
|---|---|
| Next period starts | 2025-02-02 |
| Next period ends | 2025-02-06 |
| Estimated ovulation | 2025-01-19 |
| Fertile window | 2025-01-14 to 2025-01-20 |
| Following period starts | 2025-03-02 |
Example 2
Inputs: First day of last period = 2025-03-20; Average cycle length = 32; Period length = 4.
| Result | Value |
|---|---|
| Next period starts | 2025-04-21 |
| Next period ends | 2025-04-24 |
| Estimated ovulation | 2025-04-07 |
| Fertile window | 2025-04-02 to 2025-04-08 |
| Following period starts | 2025-05-23 |
Example 3
Inputs: First day of last period = 2025-06-01; Average cycle length = 24; Period length = 6.
| Result | Value |
|---|---|
| Next period starts | 2025-06-25 |
| Next period ends | 2025-06-30 |
| Estimated ovulation | 2025-06-11 |
| Fertile window | 2025-06-06 to 2025-06-12 |
| Following period starts | 2025-07-19 |
The first line is the headline result (next period starts). Below it the period Calculator lists next period ends, estimated ovulation, fertile window and following period starts, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
How the dates are estimated
Cycle length runs from the first day of one period to the first day of the next. Ovulation typically happens about 14 days before the next period, so a 28-day cycle ovulates around day 14 and a 32-day cycle around day 18. Sperm can survive several days, so the fertile window starts earlier than ovulation itself.
Estimates, not predictions
Cycles vary from month to month, stress and illness can shift ovulation, and this tool cannot confirm either. It is not a contraception method and not medical advice; irregular cycles or health concerns should be discussed with a clinician.
How the result changes with period length
Holding every other input at the first worked example, this table changes only period length. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Period length | Next period starts | Next period ends | Estimated ovulation |
|---|---|---|---|
| 3 | 2025-02-02 | 2025-02-04 | 2025-01-19 |
| 5 | 2025-02-02 | 2025-02-06 | 2025-01-19 |
| 10 | 2025-02-02 | 2025-02-11 | 2025-01-19 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- First day of last period: left empty shows "Enter first day of last period." beside that field and no result.
- Average cycle length: entering "10" shows "Average cycle length must be at least 20." beside that field and no result.
Choosing between the Period Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Payback Period Calculator](/business/payback-period/) | initial investment and annual cash flow | Payback period |
| [Period to Frequency Calculator](/converters/period-to-frequency/) | convert and value | Frequency (Hz) |
| [Age Calculator](/time/age-calculator/) | date of birth | Your age |
| [Time Duration Calculator](/time/time-duration-calculator/) | start time and end time | Duration |
| [Hours Calculator](/time/hours-calculator/) | start time and end time | Hours worked |
Background: dates and times
Date and time results depend on the conventions a tool uses: whether the end date counts, whether weekends and holidays are skipped, how months of different lengths are handled and whether time zones are applied. This page states its convention, and the worked examples show it.
Dates use the Gregorian calendar and calendar days unless the page says otherwise; check deadlines against the rule in your contract or regulation.
Assumptions and limitations
- Cycles vary; not medical advice
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for first day of last period and average cycle length to confirm this.
Frequently asked questions
How long is a normal cycle?
Commonly 21 to 35 days, with 28 as the textbook average.
What if my cycle is irregular?
Use your average, but treat the dates as rough.
Can I use this to avoid pregnancy?
No. Predictions of fertile days are unreliable on their own.
What does the Period Calculator calculate?
Predicted next period, ovulation and fertile window from your last period. It returns Next period starts, Next period ends, Estimated ovulation, Fertile window and Following period starts.
What formula does the Period Calculator use?
next period = last start + cycle length; ovulation ≈ next period − 14 days; fertile window = ovulation − 5 to + 1 days. A calendar estimate assuming a regular cycle and a 14-day luteal phase; it is not a method of contraception.
What do I need to enter in the period Calculator?
You enter First day of last period, Average cycle length (days) and Period length (days).
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Cycles vary; not medical advice.
What is an example of a period result?
With first day of last period 2025-01-05, average cycle length 28 and period length 5, the next period starts is 2025-02-02.
Where does the method come from?
The method follows: Standard calendar-based fertility awareness estimates (e.g. ACOG patient education).
Related calculators
- Payback Period Calculator — Time to recover an investment from steady cash flow.
- Period to Frequency Calculator — Frequency from period, or period from frequency.
- Age Calculator — Calculate your exact age in years, months and days.
- Time Duration Calculator — Elapsed time between two clock times, including across midnight.
- Hours Calculator — Hours worked between a start and end time, minus breaks.
- Time Card Calculator — Total hours, overtime and gross pay from daily start and end times.
Sources
- Standard calendar-based fertility awareness estimates (e.g. ACOG patient education)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.