What is Period Calculator?
The period calculator is an online health tool based on the standard cycle model. Enter your last menstrual period (LMP), cycle length (default 28 days, range 21-45), and period length (default 5 days, range 2-10). It computes using the rule that ovulation occurs about 14 days before the next period: next period = LMP + cycle length; ovulation day = next period − 14 days; fertile window = 5 days before ovulation through 1 day after (sperm can survive ~5 days in the body, an egg survives ~24 hours after release). It also shows your current phase (menstrual / follicular / fertile / luteal) and a visual cycle timeline. All calculation happens locally in your browser — nothing is uploaded. Use it for cycle tracking and conception planning reference; results are estimates, not medical advice.
▶Use Cases5 scenarios
- ◆Tracking and predicting your next period date
- ◆Estimating ovulation and the fertile window when trying to conceive
- ◆Knowing which phase of the cycle you are in
- ◆Checking whether your cycle looks regular
- ◆Learning how ovulation timing works
▶How to Use5 steps
- 1Pick your last menstrual period (LMP) date
- 2Adjust the cycle length (21-45 days, default 28)
- 3Adjust the period length (2-10 days, default 5)
- 4Read the next period, ovulation day, fertile window, and current phase
- 5Use the timeline bar to see the full cycle layout
▶Features8 features
- ✓Free
- ✓No signup
- ✓Custom cycle and period length
- ✓Next period and ovulation prediction
- ✓Fertile window calculation
- ✓Current phase detection
- ✓Visual cycle timeline
- ✓100% local, nothing uploaded
▶FAQ7 questions
What is a period calculator?
An online tool that predicts your period and ovulation. Input: last period date, cycle length, period length. Output: next period date, ovulation day, fertile window, and current cycle phase. Used for cycle tracking and conception planning. All calculation runs locally in your browser.
How is the next period date calculated?
Next period = LMP + cycle length. Example: LMP August 1 with a 28-day cycle gives a predicted next period of August 29.
How is the ovulation day estimated?
Using the standard model that ovulation occurs about 14 days before the next period: ovulation day = next period date − 14 days. Actual timing varies by person.
Why is the fertile window 5 days before to 1 day after ovulation?
Sperm can survive about 5 days in the body and an egg about 24 hours after release, so the 5 days before through 1 day after ovulation is the highest-conception window.
What cycle lengths are supported?
21-45 days, default 28. Most adult cycles fall between 21-35 days; persistently irregular cycles are worth a doctor visit.
Can I rely on this for contraception?
No. Cycles shift with stress, sleep, and health; calendar-based contraception has a high failure rate. Use other methods and consult a doctor.
Is my data uploaded?
No. Everything is calculated locally in your browser — your dates never leave your device.