A regular menstrual cycle is the body's monthly blueprint, and once you know how to read it you can plan around it — whether you are trying to conceive, trying to avoid pregnancy naturally, simply tracking your health, or preparing for a baby. This menstrual cycle calculator takes the first day of your last period (LMP) and your average cycle length, then projects your next period, your estimated ovulation day, your fertile window, and your estimated due date if the cycle resulted in conception.
The math behind the calculator is the same model clinicians use: ovulation happens roughly 14 days before the next period begins, sperm can survive inside the reproductive tract for up to about 5 days, and a full-term pregnancy lasts about 280 days (40 weeks) from the last menstrual period. Enter your numbers below and every date on the result card updates instantly.
How to Use
- Set your Average Cycle Length — the number of days from the first day of one period to the day before the next. The default is 28, but anywhere from 21 to 35 days is considered normal. If your cycles vary, use the average of your last 3–6 cycles.
- Enter the Last Period Start Month (1 = January through 12 = December) when your most recent period actually began.
- Enter the Last Period Start Day as the calendar day of that month.
- Enter the Last Period Start Year as a four-digit year.
- Read the result card: your next period start date, the estimated ovulation day, the fertile window (the five days leading up to and including ovulation), and the estimated due date.
The most common mistake is counting the day your period ends instead of the day it started. The first day of flow — not spotting, not the day after — is day 1 of your cycle. If you are unsure, track 2–3 cycles on paper or in a free app before relying on the projection for family planning.
The Cycle Math in Plain English
A menstrual cycle has two phases separated by ovulation. The follicular phase (before ovulation) varies a lot between people and even between cycles for the same person — this is why cycle lengths differ. The luteal phase (after ovulation) is far more stable, typically close to 14 days.
Because the luteal phase is the steadier piece, the standard clinical estimate works backward from the next period:
- Ovulation day = LMP + (cycle length − 14 days). On a 28-day cycle that is day 14; on a 32-day cycle it is day 18; on a 24-day cycle it is day 10.
- Fertile window = ovulation day minus 5 days through ovulation day. Sperm can live roughly 3–5 days waiting for the egg, and the egg itself is fertile for about 12–24 hours after release.
- Next period = LMP + cycle length.
- Estimated due date = LMP + 280 days (Naegele's rule, the standard 40-week clinical pregnancy dating).
For example, with an LMP of October 1 and a 28-day cycle: ovulation lands on October 15, the fertile window runs October 10 through October 15, the next period is expected on October 29, and a conception in this cycle would put the estimated due date at July 8 next year.
Examples
Example 1 — The textbook 28-day cycle. Your last period started on October 1, and your cycles average 28 days. The calculator projects ovulation around October 15, a fertile window from October 10 to October 15, your next period on October 29, and an estimated due date of July 8 of next year. Roughly 80% of people who track will land within a day or two of these dates once their cycles are regular.
Example 2 — A shorter 21-day cycle. If your last period started October 1 and your cycles run 21 days, ovulation moves earlier to October 8, the fertile window opens October 3 and closes October 8, and your next period is expected October 22. Note that ovulation is earlier even though the luteal phase is still about 14 days.
Example 3 — A longer 35-day cycle. With a 35-day cycle starting October 1, ovulation shifts out to October 22, the fertile window runs October 17 through October 22, and the next period lands around November 5. Longer cycles do not mean a longer pregnancy — the due date still uses LMP + 280 days, though an early ultrasound will often refine it.
Example 4 — Trying to conceive. Many people buy a basal-body-temperature thermometer or an ovulation predictor kit that costs between $15 and $40 to confirm the ovulation date this calculator estimates. The calculator narrows the search to about six days; the kit then confirms the surge in luteinizing hormone 24–36 hours before the egg is released.
FAQ
How does this menstrual cycle calculator work?
It takes the first day of your last period (LMP) and your average cycle length, then applies the standard clinical model: ovulation is estimated at LMP plus (cycle length minus 14 days), the fertile window is the five days before ovulation through ovulation day, the next period is LMP plus cycle length, and the estimated due date is LMP plus 280 days (Naegele's rule).
What is a normal menstrual cycle length?
Most adults have cycles between 21 and 35 days. Adolescents and people approaching perimenopause may see wider variation. Cycles shorter than 21 days or longer than 35 days that persist over several months are worth discussing with a clinician.
When am I most fertile?
Your most fertile days are the five days before ovulation plus ovulation day itself, because sperm can survive up to about five days in the reproductive tract while the egg remains viable for only 12–24 hours. This calculator highlights that window as your fertile window.
How accurate is the ovulation prediction?
It is an estimate, not a guarantee. Ovulation timing can shift cycle to cycle even when periods look regular, and cycle-length assumptions break down after about the 6th cycle on hormonal birth control. For conception planning, confirm the estimated day with ovulation predictor kits or basal body temperature tracking; for avoiding pregnancy, calendar methods alone are roughly 76–88% effective with typical use.
How is my due date calculated?
Clinicians use Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. This assumes a regular 28-day cycle and ovulation on day 14. If your cycles are consistently longer or shorter than 28 days, your clinician may adjust the date, and an early ultrasound around 8–12 weeks is the most accurate way to refine it.
Why does my cycle length not match the 28-day default?
The 28-day figure is a population average, not a requirement. Normal cycles range from 21 to 35 days, and it is common for one cycle in five to differ from your usual length by several days. Enter your own average (from the last 3–6 cycles) rather than forcing 28, because the ovulation and fertile-window dates shift directly with your cycle length.
Can I use this calculator to prevent pregnancy?
The calendar rhythm method is the least effective form of fertility awareness, with around a 22% typical-use failure rate because cycles shift unexpectedly. If avoiding pregnancy is the goal, use a barrier method, hormonal contraception, or a medically monitored fertility-awareness method with formal instruction.
Disclaimer
This menstrual cycle calculator is provided for informational purposes only and is not medical advice. Dates are estimates based on the inputs you enter and the standard LMP-plus-cycle-length model; actual ovulation, menstruation, and conception dates vary between people and between cycles. The tool is not a contraceptive device and should not be used to diagnose, treat, prevent, or manage any health condition. Always consult a qualified healthcare provider with questions about your cycle, fertility, pregnancy, or missed periods, and seek prompt care for severe pain, very heavy bleeding, or a positive pregnancy test.