Track your menstrual cycle. Enter your last period date to predict upcoming periods, ovulation, and your fertile window.
days
days
Next Period
Thursday, July 23, 2026
In 28 days
Upcoming Periods1st: Thursday, July 23, 2026 2nd: Thursday, August 20, 2026 3rd: Thursday, September 17, 2026
Ovulation DateWednesday, July 8, 2026
Fertile WindowFriday, July 3, 2026 to Thursday, July 9, 2026
Cycle Length28 days
This calculator is for educational purposes. Cycle lengths vary; typical ranges are 21–35 days. Ovulation typically occurs 12–16 days before your next period. Consult a healthcare provider for medical concerns.
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What is the Period Calculator?
A period calculator is a tool that predicts upcoming menstrual periods and identifies key dates in your cycle based on your last menstrual period (LMP) and cycle length. Most cycles last 21–35 days, with ovulation typically occurring around day 14 of a 28-day cycle. The fertile window—the days when pregnancy is possible—spans about 5 days before ovulation through 1 day after.
How it works
Enter the first day of your last period and your average cycle length (default 28 days). The calculator then forecasts the next three period start dates, pinpoints ovulation (roughly cycle day 14), and shows your fertile window (typically 5 days before ovulation to 1 day after). The wider the fertile window is tracked the higher the chance of conception if unprotected intercourse occurs.
Next Period = Last Menstrual Period + Cycle Length (in days); Ovulation ≈ Cycle Length ÷ 2
The next period is simply the last period date plus your cycle length. Ovulation occurs roughly halfway through your cycle (day 14 in a 28-day cycle). The fertile window extends approximately 5 days before ovulation through 1 day after, since sperm can survive up to 5 days and an egg is viable for about 12–24 hours.
Examples
Input
Result
Notes
Last period: 22 July 2026, Cycle length: 28 days
Next period: 19 August 2026; Ovulation: 5 August 2026; Fertile window: 31 July – 6 August 2026
A standard 28-day cycle; ovulation on day 14; fertile window is 6 days centred on ovulation.
Last period: 1 July 2026, Cycle length: 32 days
Next period: 2 August 2026; Ovulation: 16 July 2026; Fertile window: 11 July – 17 July 2026
A longer 32-day cycle shifts ovulation to day 16; fertile window extends further out.
Last period: 10 June 2026, Cycle length: 24 days
Next period: 4 July 2026; Ovulation: 22 June 2026; Fertile window: 17 June – 23 June 2026
A shorter 24-day cycle brings ovulation earlier (day 12); periods are closer together.
How to use the Period Calculator
Enter the first day of your last menstrual period in the date field.
Enter your average menstrual cycle length (typical range 21–35 days; default is 28).
Optionally, enter your average period duration (typical range 2–7 days; default is 5).
Click or tap the input fields and the calculator updates in real time.
View the predicted next period date prominently displayed.
Check the stats section to see the next three period dates, ovulation date, and fertile window.
Benefits
Predict periods weeks or months ahead—never be caught off guard.
Identify your ovulation window for fertility planning or contraception awareness.
Track patterns over time to spot irregularities and discuss with a healthcare provider.
Free, private, and instant—no sign-up, no ads, no data storage.
Works on any device; supports both metric and standard date formats.
Empowers you to take ownership of your reproductive health and wellness.
Tips & common mistakes
Common mistakes
Assuming every cycle is exactly 28 days; real cycles vary 21–35 days and individual variation is normal.
Forgetting that ovulation dates are estimates based on cycle length; confirm with ovulation tests or ultrasound if precise timing is critical.
Ignoring that cycle lengths can shift due to stress, travel, illness, weight changes, or contraception; recalculate if patterns change.
Using a single cycle's duration as your 'true' average; track 3–6 cycles to get an accurate baseline.
Tips
Record your period start and end dates for 2–3 months to establish your true average cycle length; don't rely on a single cycle.
If your cycles are irregular, use the range (e.g., 24–35 days) and check predictions monthly rather than assuming one fixed length.
Ovulation typically occurs 12–16 days before the next period; use this as a sanity check against the calculated date.
If planning pregnancy, consider ovulation tests (LH kits) or tracking basal body temperature to confirm ovulation; this calculator is an estimate.
Frequently asked questions
How accurate is this period calculator?
The calculator is accurate for estimating average cycles but assumes regular, predictable periods. If your cycle varies by more than ±3 days, predictions are less reliable. Ovulation tests or ultrasound give certainty when timing is critical.
What if my cycle is irregular?
Track 3–6 cycles to find your average length, then use that. If cycles still vary widely (e.g., 24–35 days unpredictably), consult a doctor; irregular cycles can indicate hormonal imbalances or other health factors worth investigating.
Can I rely on this for birth control?
The calendar method (predicting the infertile days) is less reliable than barrier methods, hormonal contraception, or IUDs. Use this calculator for awareness, not as your sole contraception. Consult a healthcare provider for reliable options.
What is the fertile window?
The fertile window is the 5–6 day period when pregnancy is possible: roughly 5 days before ovulation through 1 day after. Sperm can survive up to 5 days; an egg is viable for ~24 hours. Unprotected intercourse during this window increases conception risk.
When exactly do I ovulate?
Ovulation typically occurs 12–16 days before your next period. This calculator estimates it at roughly the midpoint of your cycle, but real ovulation varies. Ovulation predictor kits, basal body temperature tracking, or ultrasound confirm the exact day.
Does the calculator account for hormonal contraception?
No. Hormonal birth control (pill, patch, implant) suppresses ovulation and often shortens or lightens periods. If you use hormonal contraception, your natural cycle doesn't apply; this calculator is designed for unmedicated cycles.