Safe period calculator
Given the first day of your last period, average cycle length and period length, project up to 12 cycles of period dates, ovulation days, fertile windows and relative safe days, plus your current cycle day, phase and whether today falls in a calendar-method safe period.
Runs in your browserEvery computation happens in your browser — your data never leaves this device.
Cycle settings
Calendar method: ovulation = cycle start + (cycle length − 14); the fertile window runs from 5 days before to 1 day after ovulation, everything else is a relative safe day.
Result
Enter the last period start, cycle length and period length to list the dates for each cycle.
What this tool does
- Planning a pregnancy: see the ovulation days for the next few months so appointments, travel and workload can be arranged around them.
- Understand how long a “safe period” really is — short cycles often have no safe stretch after the period at all.
- Unsure of your cycle length? Run 26, 28 and 30 days and compare how far the ovulation day drifts.
- Print a cycle calendar to take to an appointment so you and your clinician share the same timeline.
Example
Input
Last period 2024-01-01, average cycle 28 days, period 5 days, project 3 cycles, today 2024-01-14
Output
Cycle 1: period 2024-01-01 to 2024-01-05, ovulation 2024-01-15, fertile window 2024-01-10 to 2024-01-16, relative safe days 2024-01-06 to 2024-01-09 (4 days) and 2024-01-17 to 2024-01-28 (12 days); today is cycle day 14, inside the fertile window
Ovulation = cycle start + (cycle length − 14) with a fixed 14-day luteal phase. Safe days are the calendar-method remainder, not a medical conclusion.
Frequently asked questions
How are “safe days” defined here?
Every date that is neither inside the fertile window nor inside the period is marked as a relative safe day. The fertile window spans five days before ovulation to one day after, because sperm survive about five days and an egg stays fertilisable for roughly 24 hours. It is a calendar estimate, not a guarantee against pregnancy.
Why do short cycles have no safe days after the period?
With a 21-day cycle, ovulation lands on day 7 and the fertile window opens on day 2, which overlaps the five-day period completely, so the fertile window starts the moment the period ends. Shorter cycles leave less room; below 21 days the calendar method stops being meaningful.
Why does every cycle use the same length?
The calendar method has to assume a constant cycle to project future dates. Real cycles wobble by a couple of days, so projections further out drift more. If you have logged recent period starts, run the calculation with your shortest and longest cycle and take the union of the two fertile windows.
Where does “today” come from?
To stay independent of the system clock and time zone, today is an explicit input: the page fills in the current date on load and you can change it to test any date. All date maths uses whole calendar days, so the same day gives the same answer in every time zone.
Can I use this for contraception?
Not on its own. The method depends on regular cycles and a fixed ovulation day, while stress, poor sleep, illness and travel all shift it — typical-use failure rates run as high as 12–24% per year. Use condoms or hormonal contraception to avoid pregnancy, and combine this with ovulation tests or basal body temperature if you are trying to conceive.
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