Enter your last period start date and average cycle length to see your predicted next period, fertile window, ovulation day, and where you are in your cycle right now — no app, no account, no data stored anywhere.
Your cycle info Free
Day 1 of one period to day 1 of the next. 28 is average; 21–35 is typical.
Today — Cycle Day 1
Menstrual
Enter your last period start date above to see your prediction.
Your predictions
–
Next period
–
Days away
–
Estimated ovulation
–
Fertile window
Cycle overview
Menstrual — your period
Follicular — body prepares to ovulate
Ovulation — egg is released
Luteal — after ovulation, before next period
The white marker shows today's position in your cycle.
Next 6 cycles
Cycle
Period starts
Fertile window
Ovulation (est.)
Predictions assume your cycle stays consistent at the length entered above. The more irregular your cycles, the less reliable predictions further out become — treat cycles 3–6 as rough estimates, not certainties.
Not a method of birth control or a conception guarantee. This tool estimates ovulation using the standard 14-day luteal phase model, which is a population average — your actual ovulation day can shift due to stress, illness, travel, and cycle irregularity. If you're trying to conceive or avoid pregnancy, talk to a doctor about more reliable methods (BBT charting, ovulation predictor kits, or clinical guidance).
Quick answers
How does this calculate ovulation?
It uses the standard model: ovulation typically happens about 14 days before your next period starts (the luteal phase — the time between ovulation and your period — is usually a fairly consistent 12–14 days, even when the first half of your cycle varies). So ovulation day = cycle length − 14.
What is the "fertile window"?
The 5–6 days each cycle when pregnancy is possible: the 5 days before ovulation (sperm can survive that long) plus ovulation day itself (the egg survives about 24 hours after release). It's the same window methods like Flo and Clue use.
My cycles are irregular — is this still accurate?
Less so. This tool assumes a consistent cycle length, so the more your cycles vary month to month, the less reliable the prediction — especially for ovulation timing and cycles further in the future. If you track your actual cycle lengths over a few months, use the average, and treat the fertile window as a wider "roughly this week" estimate rather than exact days.
Why isn't my data saved anywhere?
This tool runs entirely in your browser — nothing you enter is sent to a server, stored in an account, or tracked. Close the tab and it's gone. If you want a running history across cycles, a dedicated app (with an account) will do that; this tool is built for a quick, private, no-signup check.
How to use this tracker
It projects forward from your last period using your own average cycle, so the first input is the one that decides everything else.
Enter the first day of your last period, not the day it ended. Day one is the start of bleeding and it is the anchor for the whole calendar.
Set your average cycle length. If your cycles vary, average the last several rather than using the shortest or the most recent.
Enter how many days you typically bleed, which sets the length of the projected period window.
Read the ovulation estimate and the fertile window. The window is an estimate from a calendar rule, not a measurement of your body.
Look at the six projected cycles as a range to plan around. The further out the cycle, the less the projection means.
A worked example: a 31-day cycle
Last period began November 3, and cycles run 31 days with four days of bleeding. The next period is projected for December 4.
Ovulation is estimated at 14 days before that next period, which is November 20. The fertile window here runs from five days before ovulation through ovulation day, so November 15 to November 20.
The cycle after that begins January 4 if every cycle is also 31 days, and the projection repeats the same arithmetic. That is both the strength and the weakness of a calendar method: it is transparent, and it assumes a regularity your body may not provide.
Five days before ovulation is not arbitrary. Sperm can survive several days in the reproductive tract while the egg is viable for roughly a day after release, which is why a calendar window is wider than the ovulation day itself.
What your results mean
The dates are projections from an average, not observations. The 14-day luteal phase — the stretch from ovulation to the next period — is a convention that fits many cycles but not all, and it is the part of the calculation most likely to be off for an individual.
Cycle length varies for reasons that have nothing to do with a calendar: illness, travel, sleep changes, stress, and significant weight change all move it. A week of disruption can shift a projected date by more than the window is wide.
This is not a contraceptive method and not a pregnancy test. Calendar-based fertility awareness requires more than a start date and an average to be reliable, and using a projection as contraception carries a risk this page cannot quantify for you.
What is worth watching is change. Cycles that were regular and become irregular, bleeding that lasts much longer than usual, or bleeding heavy enough to require changing protection every hour are reasons to speak to a clinician rather than to adjust the cycle length field.
Frequently asked questions
How accurate is the ovulation estimate?
It is a calendar estimate, accurate on average and unreliable for any single cycle. It assumes a 14-day luteal phase, which is typical but not universal, so treat the date as the centre of a window rather than a fact.
What counts as a normal cycle length?
ACOG describes a typical adult cycle as roughly 21 to 35 days, with bleeding of roughly 2 to 7 days. A cycle outside that range is worth discussing with a clinician.
Can I use this to avoid pregnancy?
Not reliably. A projection from one date and one average ignores the daily signals that fertility awareness methods depend on. Use a method with a failure rate you have actually looked up.
Why does the fertile window start before ovulation?
Because sperm survive for several days, so intercourse before ovulation can still lead to conception. The window is drawn to cover that, not just the day of release.
Is my data stored?
No. Everything is calculated in your browser and nothing is saved or transmitted. Closing the page clears the entries.
My cycle length changes every month. What should I enter?
An average of the last several cycles, and read the projections as looser than they look. If the variation is large, a single average hides more than it predicts.
Sources & references
The 14-day luteal estimate is the standard calendar assumption. It is not an ultrasound or a hormone measurement.