Health & Fitness · Pregnancy
Period Calculator — Next Period Dates and Cycle Tracker
Predict your next period from the first day of your last one and your usual cycle length, see the next six period dates with how long each lasts, today's cycle day, and your likely fertile window and ovulation for each cycle.
A ready-made version of the Ovulation Calculator — Fertile Window and Next Periods.
How this is calculated
Next period = last period + cycle length · Ovulation = next period − luteal phase (≈ 14 days) · Fertile window = ovulation − 5 days to ovulation day
- Luteal phase
- Days between ovulation and the next period — fairly constant for each person
Ovulation happens about two weeks before your next period, not two weeks after the last one, so in a 32-day cycle it's around day 18. Sperm can survive up to five days and an egg about one day, which makes the five days before ovulation and ovulation day itself the fertile window; the two days before ovulation carry the highest chance.
Assumptions & sources
- Regular cycles of the length entered; calendar dates only.
- 1Timing of the 'fertile window' in the menstrual cycle: day specific estimates from a prospective study (Wilcox, Dunson & Baird) — BMJ 2000;321:1259
- 2Ovulation and the fertile window — University of California, San Francisco
Calculator ID HLTH-CYCLE-001/period · version 1.0.0
Common questions
How do I work out my next period?
Add your cycle length to the first day of your last period: 28 days after 1 March is 29 March.
Is a 35-day cycle normal?
Adult cycles of 21 to 35 days are usually considered normal; see a doctor if yours change suddenly or are very irregular.
Why was my period late?
Stress, illness, travel, weight change and pregnancy can all delay a period. Take a pregnancy test if it's late and pregnancy is possible.