# Period Calculator _Your next period is due one cycle length after the last one began. See your next three periods and your ovulation day._ - Source: https://proba.baby/en/tools/period-calculator - Language: English - Last verified: v1.0.4 · 7 Sep 2026 - Last updated: 2026-10-11 ## How it works The short answer: add your cycle length to the first day of your last period. If your last period started on 1 March and your cycle is 28 days, your next period is due on 29 March. The calculator does this for the next three periods and shows how long each one should last. Enter the first day of your last period, your usual cycle length (the days from the first day of one period to the first day of the next) and how many days you usually bleed. You also see this cycle's estimated ovulation day and fertile window. If the expected day has passed, the page says so and points you to the right test day. You can print the forecast or save it as a PDF, and copy a link that brings the same dates back. Periods are rarely exact. A few days early or late is common, and stress, illness, travel, weight change and breastfeeding can all move them. FIGO counts cycles of 24 to 38 days as typical; the NHS calls cycles shorter than 21 days or longer than 35 days irregular. These are calendar estimates: they are not contraception and not a diagnosis. ## The math Next period = first day of your last period + cycle length. The two after it are one more cycle length each. Each period is shown as lasting the number of days you entered. This is the rule of the ProbaBaby app's Planning engine, and the calculator is tested against the app's own functions on every cycle length from 20 to 45 days. Ovulation for the same cycle = cycle length − 14 cycle days (an average 14-day luteal phase), and the fertile window is the five days before it plus the day itself (Wilcox 1995). Below 21 days the estimate is marked as unreliable; outside FIGO's 24 to 38 days the cycle is marked as atypical. A period counts as late here once the expected first day has passed. That is a calendar fact only: it does not say why a period is late. ## FAQ **How do I calculate my next period?** Add your cycle length to the first day of your last period. A 28-day cycle that started on 1 March gives 29 March; a 32-day cycle gives 2 April. Count your cycle from the first day of one period to the first day of the next, and use your average if it varies. **What is a normal cycle length?** FIGO, the international federation of gynaecologists, counts 24 to 38 days as typical, and up to 7 to 9 days of difference between your shortest and longest cycle as regular. The NHS calls cycles shorter than 21 days or longer than 35 days irregular. Around 28 days is the average, not a target. **How long should a period last?** Usually 2 to 7 days, the NHS says, with about 20 to 90 ml of blood lost, roughly 1 to 5 tablespoons. FIGO counts bleeding for more than 8 days as prolonged. If your periods are very heavy or long, see your doctor: help is available. **My period is late. Am I pregnant?** Pregnancy is one possible reason, and a test can tell you. Most pregnancy tests work from the first day of a missed period (NHS). Stress, illness, weight change, exercise, breastfeeding and some contraception can also delay a period. If you have missed 3 periods in a row and are not pregnant, see your doctor. **Can I use this to avoid pregnancy?** No. A period calculator is not contraception. Ovulation moves from cycle to cycle, and sperm can survive for up to 7 days (NHS), so there is no day a calendar can promise is safe. Talk to your doctor, midwife or a sexual health service about contraception. **When should I see a doctor about my periods?** If you have missed 3 periods in a row and are not pregnant, your periods last more than 8 days, you bleed between periods or after sex, or your periods are suddenly much heavier or more painful. Very heavy bleeding, or bleeding with severe pain, dizziness or fainting, needs your emergency number straight away. ## Sources - Munro MG et al. FIGO systems for normal and abnormal uterine bleeding, 2018 revisions. Int J Gynaecol Obstet 2018;143:393-408: https://europepmc.org/article/MED/30198563 - NHS: Periods: https://www.nhs.uk/conditions/periods/ - NHS: Periods and fertility in the menstrual cycle: https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/ - NHS: Irregular periods: https://www.nhs.uk/conditions/irregular-periods/ - NHS: Missed or late periods: https://www.nhs.uk/conditions/stopped-or-missed-periods/ - NHS: Heavy periods: https://www.nhs.uk/conditions/heavy-periods/ - ACOG: Abnormal Uterine Bleeding: https://www.acog.org/womens-health/faqs/abnormal-uterine-bleeding - Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517-21: https://europepmc.org/article/MED/7477165 --- ProbaBaby (listed on the App Store as “Pregnancy & Baby Tracker - PB”) is a free iPhone, iPad and Apple Watch app for pregnancy, newborn and postpartum tracking, built for twins, in 32 in-app languages, with an optional AI baby-face prediction that computes Mendelian genetics (Punnett squares) from the traits you enter before any image is generated. Tracking, tools, PDF reports and family sharing are free; only AI predictions use credits — 5 starter credits cover your first Scientific prediction — and no subscription is required. Requires iOS 26 or later. Not available on Android.