Estimate when you're likely to ovulate and your most fertile days, from the first day of your last period and your usual cycle length.
days
From the first day of one period to the day before the next. 28 is average.
Pick the first day of your last period.
Not medical advice and not a form of contraception: a calendar can only estimate ovulation, and cycles vary from month to month, so it can't tell you which days are safe. It can't tell you if you're fertile, either. See a doctor if your periods are irregular, very painful or heavy, or if you've been trying to get pregnant for a year (6 months if you're over 35, as ACOG advises).
How the dates are worked out
Your next period is expected one cycle length after the first day of your last period.
Ovulation is estimated at 14 days before that next period (ACOG). In a 28-day cycle that's day 14; in a 32-day cycle, day 18.
The fertile window runs from 5 days before ovulation to 1 day after.
If you conceive in a cycle, the estimated due date follows Naegele's rule, adjusted for your cycle length, as in our pregnancy due date calculator.
Worked example
Your last period started on 20 September and your cycle is 28 days. The next period is due on 18 October, so ovulation is likely around 4 October and the fertile window runs from 29 September to 5 October. A pregnancy from this cycle would be due around 27 June 2027. With a 32-day cycle, ovulation moves to about 8 October.
Signs that you're ovulating
Cervical mucus becomes clear, thin, slippery and stretchy, like raw egg white.
Basal body temperature rises slightly after ovulation and stays up until your period (it confirms ovulation after it happens).
An ovulation test turns positive when it detects the hormone surge that comes 24 to 48 hours before ovulation.
In the days just before and on the day you ovulate. ACOG explains that sperm can live about 3 (sometimes up to 5) days and the egg about 24 hours, so pregnancy is possible from 5 days before ovulation until 1 day after. The chance is highest in the 2 to 3 days before ovulation.
How accurate is an ovulation calculator?
It's an estimate. It assumes ovulation about 14 days before your next period, which is the average, but the NHS says it usually happens 10 to 16 days before, and it can shift from month to month. Ovulation tests (which detect the LH surge 24 to 48 hours before ovulation), changes in cervical mucus and basal body temperature give a better picture.
Can I use this to avoid getting pregnant?
No. A calendar estimate isn't reliable contraception: cycles vary, and sperm can survive for days. If you don't want to get pregnant, talk to a doctor, nurse or pharmacist about contraception that suits you.
What if my cycles are irregular?
Use your average cycle length, but treat the dates as rough. The NHS says cycles of 21 to 35 days are normal for adults. If your cycles are very irregular, or you've been trying to conceive for a year (6 months if you're over 35), see a doctor.