Period Calculator
Your next period date, from one cycle length.
FREE - NO SIGN-UP - RUNS ON YOUR DEVICE
Your cycle dates stay on your device
- Nextperiod date
- Ovulikely day
- Windowfertile days
- 0data uploaded
What you can count on
- 1cycle ahead
- The next date, not a year of them
- Rerun it when the cycle shifts
- 3dates returned
- Next period, ovulation, fertile window
- All from the same two inputs
- 0uploads
- Dates stay in this tab
- No history and no account
Period Calculator at a glance
- Input
- Last period's first day and length
- Output
- Next period, ovulation, window
- How far ahead
- The next cycle, not a calendar
- Tracking
- No history, no account
- Health notice
- Estimate, not medical advice
- Processing
- Runs locally in your browser
- Cycle length accepted
- 21–45 days
- How the dates are found
- Ovulation −14, window 6 days
- Price
- Free
- Account required
- No
How to use Period Calculator
- 1
Enter the last start date
The first day of bleeding, not the last.
- 2
Give the cycle length
In days, from one start to the next. Use the length that is usual for you.
- 3
Read the dates
The next period, the ovulation estimate and the fertile window appear together and move as you edit.
Frequently asked questions
Does it track my cycle month to month?
No. It works out the next dates from the two values you type. Nothing is saved, so each time you use it, the last start date goes in again.
My cycle is irregular. Will this work?
It uses the length you give as if it repeated. With a cycle that varies, treat the dates as a rough marker and expect them to move.
Can I rely on the fertile window to avoid pregnancy?
No. It is a calendar estimate from an assumed cycle, not a contraceptive method. A qualified health professional can advise on that properly.
Are my dates saved anywhere?
No. They are used for the arithmetic in this tab and are gone when the page closes. Nothing is sent to a server.
What a cycle estimate can and cannot say
Give the first day of the last period and how long your cycle usually runs. The next period date comes back with the ovulation day and the fertile window around it.
- The next date is the last start date plus the length you gave, which is why the length matters more than anything else.
- Ovulation is placed relative to the next period rather than the last one, so a changed length moves it too.
- Cycles vary with illness, travel and stress, and a calendar has no way of knowing about any of that.
- Anything you need to be certain about belongs with a clinician rather than an estimate from two numbers.
Full specification
| Input | The first day of the last period, and your usual cycle length in days. |
|---|---|
| Output | The next period date, the estimated ovulation day and the fertile window. |
| How far ahead | One cycle. Enter the new date each time rather than reading a calendar of months. |
| Tracking | No history, no account, and no pattern learned over time. Your dates stay in the tab. |
| Health notice | This result is an estimate for general information and is not medical advice. |
| Processing | All calculations run locally in the current browser tab. |
| Cycle length accepted | 21 to 45 days. Anything outside that comes back as a message rather than a set of dates. |
| How the dates are found | Ovulation is 14 days before the next period. The fertile window runs from 5 days before it to 1 day after — six days. |
| Price | Free |
| Account required | No |
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