About the Period Calculator
A period calculator predicts your next several period start dates from your last period and typical cycle length. Tracking cycles helps with planning, identifies patterns, and makes it easier to notice when something changes.
The formula
Next period = last period start + cycle lengthCycle length is measured from the first day of one period to the first day of the next. A typical cycle runs 21 to 35 days, with 28 days as the commonly cited average.
How to use this calculator
- 1Enter your Last Period Start (Year). The field starts at
2026, which you can overwrite. - 2Enter your Month (1–12). The field starts at
8, which you can overwrite. - 3Enter your Day. The field starts at
1, which you can overwrite. - 4Enter your Cycle Length (Days). The field starts at
28, which you can overwrite. - 5Enter your Period Duration (Days). The field starts at
5, which you can overwrite. - 6Read the result straight away — it recalculates as you type, so there is no button to press. Use Share to copy a link that reopens the page with your exact numbers filled in.
Worked example
| Input | Value |
|---|---|
| Last Period Start (Year) | 2026 |
| Month (1–12) | 8 |
| Day | 1 |
| Cycle Length (Days) | 28 |
| Period Duration (Days) | 5 |
Result
Period 1: Aug 29, 2026 – Sep 2, 2026
Period 2: Sep 26, 2026 – Sep 30, 2026
Period 3: Oct 24, 2026 – Oct 28, 2026
Days Until Next Period: 17
Annual Periods: ~13.0
Those are the values the page loads with, so you can reproduce this result yourself and then change one field at a time to see what drives the outcome.
Understanding your result
Cycle length varies naturally, and a few days of variation between cycles is entirely normal. Prediction assumes consistency, so accuracy improves considerably once you have tracked several cycles and can use your own average rather than a generic 28 days.
Cycles are sensitive to a great deal: stress, illness, travel across time zones, significant weight change, intense exercise, and hormonal contraception all shift timing. Persistent irregularity, cycles consistently shorter than 21 or longer than 35 days, or a sudden change in a previously regular pattern are worth discussing with a clinician.
Things worth knowing
- Track several cycles and use your own average. Personal data beats the 28-day assumption.
- Note flow, pain, and symptoms alongside dates. Patterns are often more informative than timing alone.
- Cycles are frequently irregular in the first years after menarche and again approaching menopause.
- Very heavy bleeding, severe pain, or bleeding between periods should be assessed by a clinician.
- Missing periods with no pregnancy, or cycles outside the 21 to 35 day range, warrant medical review.
Frequently asked questions
What is a normal cycle length?+
Anywhere from 21 to 35 days in adults, with 28 as an average rather than a norm. Variation of a few days between cycles is entirely normal.
Why is my period late?+
Common causes include stress, illness, travel, weight change, intense exercise, and hormonal contraception — as well as pregnancy. An occasional late period is usually unremarkable, but a persistent change warrants review.
How accurate are period predictions?+
Reasonably accurate for regular cycles, within a day or two. Irregular cycles are much harder to predict, and accuracy improves once you have tracked several months of your own data.
When should I see a doctor about my periods?+
If cycles are consistently shorter than 21 or longer than 35 days, if you miss periods without being pregnant, if bleeding is very heavy or prolonged, or if pain interferes with daily life.