BYTETOOLS

Period Calculator

Predict your next period start and end dates from your last period, cycle length and period duration, shown on a shaded calendar.

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What is the Period Calculator?

A period calculator predicts future menstrual start dates by adding whole multiples of your average cycle length to the first day of your last period, then marking your usual number of bleeding days from each of those start dates.

  • Predicts up to 12 cycles of period start and end dates
  • Colour-coded calendar with period, fertile window and ovulation day
  • Shows today's cycle day and days until the next period
  • Warns clearly if your period length exceeds your cycle length
  • Plain date table alongside the calendar view
  • 100% client-side — nothing is uploaded, stored or tracked

How to use the Period Calculator

  1. 1

    Enter the first day of your last period.

  2. 2

    Set your average cycle length and how many days your period usually lasts.

  3. 3

    Choose how many cycles ahead you want to predict.

  4. 4

    Read the shaded calendar — rose is your period, green is the fertile window.

  5. 5

    Check the table for exact predicted start and end dates.

About the Period Calculator

The ByteTools Period Calculator predicts when your next several periods are likely to start and finish. Enter the first day of your last period, your average cycle length and how many days you usually bleed, and it lays the predictions out on a month-by-month calendar with period days, the fertile window and the estimated ovulation day all shaded.

It also tells you which cycle day you are on today and how many days remain until the next period is due, which is handy for planning a holiday, a race or an event. You can predict anywhere from one to twelve cycles ahead, and every predicted date is also listed in a plain table you can read at a glance.

All the maths happens in your browser — your dates are never uploaded or stored anywhere. These predictions are estimates only, not medical advice and not a diagnosis. Cycles shift for all sorts of ordinary reasons, so use the calendar as a rough guide and see a doctor about anything unusual.

Frequently asked questions

How do I calculate my next period date?

Add your average cycle length to the first day of your last period. If your last period started on the 3rd and your cycles run 29 days, the next one is due around the 1st of the following month. This tool does that repeatedly so you can see several cycles at once.

How accurate is a period predictor?

It is only as good as the assumption that every cycle is the same length, and most are not. Large tracking studies find that cycle length commonly varies by five days or more between consecutive cycles even in healthy people, so a prediction two or three months out can easily be a week off. The first predicted period is always the most reliable one on the list.

What is a normal cycle length?

Roughly 21 to 35 days in adults is generally considered normal, with about 28 days as the population average. Cycles are often longer and more variable in the first years after menarche and in the run-up to menopause.

Why is my period late?

Stress, illness, travel across time zones, hard training, weight change, new medication, thyroid problems, PCOS, perimenopause and pregnancy can all delay a period. A calculator cannot tell you which of those applies. If your period is more than a week late, or you have missed several, take a pregnancy test if that is possible and speak to a doctor.

Does the calculator work if my cycles are irregular?

It will still produce dates, but they will be much less useful. With irregular cycles the best you can do is enter your average length and treat every prediction as a wide window rather than a date. Tracking a few real cycles first will give you a better average to feed in.

Is my cycle data private?

Yes. The calculator runs entirely in your browser. Nothing you enter is sent over the network, saved to a server or shared with anyone.

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