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Menstrual Cycle Calculator
Verified formula Updated Jun 2026 Private — runs on your device
Verified formula
Private
Next period expected
29 Jan 2025
- Following period
- 26 Feb 2025
For general information only — not financial, tax, legal or medical advice. Verify before you rely on it.
How to use the Menstrual Cycle Calculator
The Menstrual Cycle Calculator works out your next period expected, along with 1 related figure in an instant. Enter first day of last period and average cycle length and the result updates as you type — it is free, needs no sign-up, and runs entirely in your browser so your figures stay private.
- Pick the first day of last period.
- Enter the average cycle length.
- Read off your next period expected, together with following period — the calculator updates automatically, with no button to press.
Formula
The Menstrual Cycle Calculator uses the formula:
Next period expected = First day of last period + Average cycle length
Worked example
For example, with first day of last period of 2025-01-01 and average cycle length of 28 days, the next period expected is 29 Jan 2025.
| First day of last period | 2025-01-01 |
|---|---|
| Average cycle length | 28 days |
| Next period expected | 29 Jan 2025 |
|---|---|
| Following period | 26 Feb 2025 |
Results are estimates for educational use, not professional advice.
Key terms explained
- Mean
- The average of a set of numbers — their sum divided by how many there are.
Frequently asked questions
Add your average cycle length to the first day of your last period. A 28-day cycle starting 1 January gives a next period around 29 January.
Around 28 days is typical, but anywhere from 21 to 35 days is common and normal. Use your own average for accuracy.
Cycle length varies with stress, illness, travel and hormones, so the predicted date is an estimate, not a guarantee.
Count from the first day of one period to the first day of the next. That number of days is your cycle length.
The Menstrual Cycle Calculator uses the formula: Next period expected = First day of last period + Average cycle length. For example, with first day of last period of 2025-01-01 and average cycle length of 28 days, the next period expected is 29 Jan 2025.
Pick the first day of last period. Enter the average cycle length. Read off your next period expected, together with following period — the calculator updates automatically, with no button to press.
Related guides
Menstrual Cycle Table: Next period expected by Average cycle length
Reference table of next period expected for Menstrual Cycle across a range of average cycle length values — exact, engine-computed figures you can read off at a glance.
How to Calculate Menstrual Cycle: Formula, Steps & Examples
Learn how to calculate Menstrual Cycle — the formula explained step by step, with worked examples and a free calculator to check your answer.