Menstrual cycle calculator: map your whole cycle
Next period: Friday, November 13
That’s in 18 days.
- Fri, Nov 13 – Tue, Nov 17
- Fri, Dec 11 – Tue, Dec 15
- Fri, Jan 8 – Tue, Jan 12
- Fri, Feb 5 – Tue, Feb 9
- Fri, Mar 5 – Tue, Mar 9
- Fri, Apr 2 – Tue, Apr 6
🔒 Calculated in your browser. Nothing you type is sent or stored. Estimates only: cycles vary.
A menstrual cycle calculator maps the whole cycle, not just the next bleed. From the first day of your last period, your cycle length and your period length, it marks your period days, the follicular phase, the fertile window around ovulation (about 14 days before your next period) and the luteal phase, then repeats the pattern six cycles ahead.
Reading your cycle calendar
Below the dates, the calculator draws the cycle you're in right now as a strip of colored days. Each color is a phase, today is highlighted, and a sun marks the estimated ovulation day. Above it you'll see your next six periods with their expected start and end dates.
| Color on the calendar | Phase | How it's worked out |
|---|---|---|
| Period | Menstrual | Day 1 plus the number of days you usually bleed |
| Follicular | Follicular | From the end of your period to the start of the fertile window |
| Fertile | Ovulatory | Five days before estimated ovulation, ovulation day and the day after |
| Luteal | Luteal | From the end of the fertile window until your next period |
The calendar uses averages, so the edges between phases are approximate. It's for understanding your cycle, not for contraception. Nothing you enter leaves your browser.
What a normal menstrual cycle looks like
A cycle runs from the first day of one period to the day before the next. In adults, anything from 21 to 35 days is considered normal, with bleeding lasting 2 to 7 days and losing roughly 20 to 90 ml of blood (about 1 to 5 tablespoons). Teens can have cycles up to 45 days for the first few years. Very few people hit exactly 28 days every month.
Using the calculator well
- Use your real average. If you have three or four past start dates, the cycle length calculator gives you a better number than 28.
- Check today's phase. The highlighted day tells you roughly where you are. For a closer look at what that phase means, try the phase finder.
- Look for your own rhythm. Once you know which days are luteal, notice whether your sleep, appetite or mood shift then. Many people find their hardest days cluster in the last week.
- Re-run it each month. Enter your newest start date. A calculator can't learn; a tracker can.
What can change your cycle
Cycle length changes mainly when ovulation moves. Stress, illness, travel, intense exercise, eating much less, breastfeeding, coming off the pill, PCOS, thyroid problems and perimenopause can all shift it. If your calendar never seems to match real life, read about tracking irregular cycles.
Lulu fills in the same four phases from your own logged cycles, adapts when your cycle shifts, and shows how you usually feel in each phase from quick daily mood check-ins.
When to talk to a doctor
See a doctor if your cycles are regularly shorter than 21 days or longer than 35, if cycle lengths vary by more than 7 to 9 days, if periods last longer than 7 days or soak a pad or tampon every 1 to 2 hours, or if you bleed between periods or after sex.
Frequently asked questions
How do I calculate my menstrual cycle?
Count the days from the first day of one period to the day before the next. Do this for a few cycles and average them for a reliable number.
What is a normal menstrual cycle length?
21 to 35 days in adults, and 21 to 45 days in the first years after periods start. See how long is a normal cycle.
Is a period calendar the same as a period tracker?
A calendar shows a fixed pattern from one average. A tracker updates as you log, so it can follow your real cycle as it changes.
Which day of the cycle is ovulation?
Usually about 14 days before your next period. In a 32-day cycle that's around day 18.
Sources
- NHS: Periods
- Cleveland Clinic: Menstrual cycle
- ACOG: Your First Period
- Bull JR et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. npj Digital Medicine (2019)
This page is for information only and isn’t medical advice. If something worries you, please talk to a doctor.
Keep reading
Period calculator: when is my next period?Free period calculator: enter the first day of your last period and your cycle length to see your next 6 periods. Private, instant, nothing is stored.
What phase of my cycle am I in? Free phase finderWhat phase of my cycle am I in? Enter your last period to see your cycle day, today's phase and how people often feel in it. Private and instant.The 4 phases of the menstrual cycle, in colorThe 4 phases of the menstrual cycle explained simply: what your hormones do in each, how long each lasts, and how mood and energy usually change.
Your menstrual cycle day by day (day 1 to 28 and beyond)Your menstrual cycle day by day: what happens to hormones, body and mood on cycle day 1, day 14, day 21 and every day in between, plus longer cycles.How long is a normal menstrual cycle?How long is a normal menstrual cycle? Anywhere from 21 to 35 days in adults. What's average, how much variation is fine, and when to see a doctor.