Calculator
What the Fertility Calculator does
Fertility Calculator: calculates Est. Ovulation Day, Fertile Window and Est. Next Period from First Day of Last Period and Average Cycle Length. Example: first day of last period 2026-07-01 and average cycle length 28 days gives est. ovulation day 2026-07-15.
The inputs are First Day of Last Period and Average Cycle Length; the output is Est. Ovulation Day, Fertile Window and Est. Next Period. There is no submit step; the numbers move as you adjust them.
It is mostly used by anyone curious about a reading. Population formulas give you a starting point, not a verdict about your body.
No sign-up, no data collection, no processing queue, just code running locally.
The field reference, the method and a worked example using the default inputs follow the tool itself.
What do the Fertility Calculator fields mean?
The Fertility Calculator uses 2 inputs. The starting figures are ordinary, not edge cases, so they make a sensible baseline.
| Field | What to enter | Default |
|---|---|---|
| First Day of Last Period | pick a calendar date | 2026-07-01 |
| Average Cycle Length | measured in days; accepts 21 to 40 | 28 |
How does the Fertility Calculator work?
Each output is derived from the inputs above in a single pass; there is no hidden state carried between runs, so the same inputs always give the same calculation.
If something is off you are told which field, not given a generic failure.
Worked example
These are the values the Fertility Calculator loads by default, and the result it produces from them. The defaults produce the result shown here; change any of them and the answer moves.
| First Day of Last Period | 2026-07-01 |
|---|---|
| Average Cycle Length | 28 days |
| Est. Ovulation Day | 2026-07-15 |
|---|---|
| Fertile Window | 2026-07-10 → 2026-07-16 |
| Est. Next Period | 2026-07-29 |
| Note | Estimated fertile window 2026-07-10 to 2026-07-16. Not contraception: cycles vary. |
How to use it
- Select First Day of Last Period from the date picker.
- Fill in Average Cycle Length in days (21 to 40).
- It runs on every change, so there is no moment where the screen is out of date.
- Read Est. Ovulation Day, Fertile Window, Est. Next Period and Note in the results panel.
- If you need a record, Copy Link saves the inputs and Print produces a clean page.
What this tool does not do
- Body composition, ethnicity and age all shift what a "normal" result means for an individual.
- Population-level formulas cannot account for medical conditions, medication or pregnancy.
- Intermediate steps are rounded only for display; if you need full precision, work from the source figures.
- A plausible-looking answer from the wrong input is still the wrong answer: verify the fields first.
Frequently asked questions
Enter Average Cycle Length in days. Each field states its unit next to the input, and outputs are labelled the same way.
Average Cycle Length accepts 21 to 40 days. The range is enforced before the calculation runs, so a bad entry never reaches the formula.
With the values loaded when the page opens, est. ovulation day comes out as 2026-07-15. Edit any input and that figure moves with it.
No. There is no upload step, no database, and nothing to delete afterwards.
No cost and no account. Use it as often as you like.
It is a screening estimate, not a diagnosis. The formulas are standard, but they describe averages and you are not an average. Use it as a starting point for a conversation with a professional.