Period Calculator

Understand your menstrual cycle and predictions

How this period calculator works

Enter the first day of your last period, your average cycle length, period length, and a luteal phase estimate. The tool predicts your next period, estimates ovulation as the next period minus the luteal phase, and highlights the fertile window. A 3D‑style doughnut chart shows how your cycle time divides into menstruation, follicular phase, ovulation, and luteal phase.

Cycle phases explained

• Menstruation: Shedding of the uterine lining; commonly 3–7 days.
• Follicular phase: From the end of menses to ovulation; follicles develop and estrogen rises.
• Ovulation: Release of an egg; typically a short 1‑day event.
• Luteal phase: Post‑ovulation; progesterone rises, preparing the uterus for implantation.

Ovulation and fertile window

Fertility generally peaks in the two to three days before ovulation and on ovulation day. Sperm can survive several days, so planning across a window—rather than a single date—improves timing and reduces stress.

Irregular cycles and tips

• Average over months: Use the mean of your last 3–6 cycles to stabilize predictions.
• Track signs: Cervical mucus changes and ovulation predictor kits (OPKs) help refine the window.
• Recalculate: Update inputs when your average cycle length shifts.

Global format and accessibility

The calculator uses your locale setting to display dates and is fully responsive for mobiles, tablets, laptops, and desktops. No sliders, just clear fields and accessible labels.

Safety and limitations

This tool offers educational estimates and does not replace personalized medical advice. Medications, postpartum cycles, conditions, and stress can alter timing. Consult a qualified professional for medical concerns.

Frequently asked questions

• HOW TO calculate my next period? Enter your last period’s first day and average cycle length. The calculator adds your cycle length to the LMP to predict the next start.
• HOW TO estimate ovulation with this tool? It subtracts the luteal phase (commonly 14 days) from the predicted next period date.
• HOW TO find my fertile window? Choose a 5–7 day window; by default it's the five days before ovulation plus ovulation day.
• HOW TO use this with irregular cycles? Average your last 3–6 cycles and recalculate monthly. Combine with OPKs or cervical mucus tracking to refine timing.
• HOW TO change date formatting? Use the Display locale selector for your preferred regional format.