Period Calculator
Project your next several periods and cycle phases from your history.
About Period Calculator
A menstrual cycle begins on the first day of bleeding and ends the day before the next period. Tracking your average cycle length lets you project future periods and fertile windows, which is useful for planning, recognising late periods, or identifying cycle irregularities.
Formula
Next period
Next period start = LMP + cycle length
Worked example
With the default values below, Period Calculator returns Next period starts of Jul 29, 2025. Change any input above and every figure here is recalculated in your browser.
Inputs used
- First day of last period
- 2025-07-01
- Average cycle length (days)accepts 21–45
- 28
- Typical period duration (days)accepts 1–10
- 5
- Periods to projectaccepts 1–12
- 6
Results
- Next period starts
- Jul 29, 2025
- Next period ends
- Aug 2, 2025
- Cycle length
- 28 days
- Period duration
- 5 days
Step by step
- 1LMP= Jul 1, 2025
- 2Cycle length= 28 days
- 3Next period= Jul 29, 2025 – Aug 2, 2025
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or fees or charges into account.
- Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
- This calculator is an estimate based on average cycle patterns. It is not a reliable contraceptive method. Consult a healthcare provider for personalised reproductive health advice.
- This is an estimate for general information and not professional medical advice. See the full disclaimer for the limits that apply to this kind of calculator.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
This calculator is an estimate based on average cycle patterns. It is not a reliable contraceptive method. Consult a healthcare provider for personalised reproductive health advice.
Frequently Asked Questions
What is a normal cycle length?
A typical menstrual cycle is 24–38 days, with 28 days being the textbook average. Cycles shorter than 21 days or longer than 45 days may warrant investigation by a healthcare provider.
Why is my period late?
Many factors can shift ovulation — and therefore the next period — including stress, illness, significant weight change, intensive exercise, travel, hormonal contraceptives, or pregnancy. One late period is rarely cause for alarm, but a consistent pattern of irregular cycles is worth discussing with a doctor.
How do I use this for family planning?
This calculator can help you identify fertile windows for conception or, if combined with OPKs and BBT charting, as part of fertility awareness-based methods for avoiding pregnancy. Calendar methods alone have a higher failure rate than hormonal contraceptives.