Period Calculator
Your next period start dates for the coming cycles, from the first day of your last period, your cycle length and how long your period lasts, with an estimated fertile window for each cycle.
Result…
How it's calculated
Next period = first day of last period + cycle length (then + cycle length again for each cycle) Period ends = start + period length − 1 Ovulation ≈ next period − 14 days · Fertile window = the 5 days before ovulation + ovulation day
Counts forward from the first day of your last period in steps of your cycle length to give the start of each coming period, and adds your period length for the day it should end. Each cycle also shows an estimated ovulation day, counted back 14 days from the next period because the phase after ovulation is fairly steady, and a six-day fertile window ending on that day. These are averages: stress, illness, travel, breastfeeding, hormonal contraception and the years around puberty and menopause all shift cycles, so treat the dates as a guide and never as contraception. For an ovulation estimate with your own luteal phase, use the ovulation calculator.
Your numbers, step by step
About the Period calculator
This period calculator estimates when your next periods will start and end, up to 12 cycles ahead, from the first day of your last period, your usual cycle length and how many days your period lasts. Each cycle also shows an estimated ovulation day and fertile window, so you can plan travel, events or a pregnancy test date, or keep a rough calendar to compare with what actually happens. The method is simple counting: each period starts one cycle length after the last, and ovulation is estimated at 14 days before the next period because the phase after ovulation is fairly steady. Every date in the working is shown so you can check it. Real cycles shift from month to month, so these are estimates, never a method of contraception. For an ovulation estimate with your own luteal phase, use the ovulation calculator.
How to use it
- Enter the “First day of last period”: the first day of bleeding, not spotting.
- Enter your “Cycle length”, counted from the first day of one period to the first day of the next. Use your average if it varies.
- Enter your “Period length” in days.
- Choose how many “Cycles to show”, from 1 to 12.
- Read the next period's start and end, the estimated ovulation and fertile window, and the later start dates.
Worked example
A last period that started on 12 October 2026, with a 30-day cycle and a 5-day period, puts the next period at 12 October + 30 days = 11 November 2026, ending on 15 November. Ovulation is estimated at 14 days earlier, on 28 October (day 16 of the cycle), with a fertile window from 23 to 28 October. The following periods would start on 11 December 2026 and 10 January 2027.
Updated
Questions people ask
How do I work out when my next period is due?
Add your cycle length in days to the first day of your last period. With a 28-day cycle, a period that started on 1 January 2026 points to the next one on 29 January.
What is a normal cycle length?
Cycle length differs from person to person, and it is normal for yours to vary by a few days from month to month. Cycles that change a lot, stop, or are much shorter or longer than your usual are worth raising with a doctor. The calculator accepts 20 to 45 days.
When is ovulation in my cycle?
It is usually estimated at about 14 days before the next period, not 14 days after the last one. In a 30-day cycle that is around day 16.
Why was my period early or late compared with the calculator?
The dates assume every cycle is the same length. Stress, illness, travel, weight change, breastfeeding, hormonal contraception and the years around puberty and menopause can all move a period by days or more.
Can I use this to avoid pregnancy?
No. Calendar estimates are not reliable contraception, because ovulation can come earlier or later than predicted. A doctor, nurse or pharmacist can advise on contraception and on periods that are irregular, very heavy or painful.