Period Calculator
Your next period is due one full cycle length after the first day of your last one. This calculator projects your next two cycles.
Example: For a 28-day cycle with a last period starting 1 January 2026, ovulation falls around 15 January 2026, the fertile window runs 10 January 2026 to 15 January 2026, and the next period is due 29 January 2026.
Enter the first day of your last period to see your estimate.
These are estimates from calendar dating only. Irregular cycles make them unreliable.
Find an OB-GYN near youHow this is calculated
Next period dates are projected by adding your average cycle length to the first day of your last period, then adding it again for the following cycle. Cycle lengths are clamped to 21â35 days. Predictions drift if your cycle length varies.
This tool is for information only. It does not diagnose, treat, or replace advice from a licensed clinician. Reviewed against ACOG guidance.
Frequently Asked Questions
When is my next period due?
One full cycle length after the first day of your last period. Count from the first day of bleeding, not from the day it stopped. The calculator adds your average cycle length to that date, then adds it again so you can see two cycles ahead.
What counts as day 1 of my period?
Day 1 is the first day of proper bleeding, not light spotting beforehand. Getting this right matters because every other date on this page is counted from it. If you spot for a day or two before the flow properly starts, count from the first day of real flow.
Why is my period late?
A late period has many possible causes and pregnancy is only one of them. Stress, illness, weight change, travel and disrupted sleep can all delay ovulation, and a later ovulation pushes the period back with it. If pregnancy is possible, test from the first day of the missed period. See an OB-GYN if your periods are repeatedly late or stop altogether.
How many days apart should my periods be?
Most cycles run between 21 and 35 days, measured from the first day of one period to the first day of the next. This calculator clamps entries to that range because predictions outside it are not meaningful. Cycles consistently shorter or longer than that range are worth raising with an OB-GYN.
Why do my predicted dates keep shifting each month?
Because cycle length varies naturally from month to month while the prediction uses a single average. Any date this far out is an estimate that drifts. Logging the first day of several real periods and using your own average gives a closer result than assuming a textbook cycle length.
Can I use period predictions to plan trips or events?
Yes, for rough planning, as long as you leave a few days of buffer on either side. The output is an average, not a schedule your body has agreed to. Do not use it to avoid pregnancy â see the safe days calculator for why calendar timing fails at that.