About OvulationCalc

OvulationCalc is a free ovulation and fertile-window calculator at ovulation-calc.com, with three guides on how a cycle is timed and how people read an estimate. J B runs it alone, from New York. There is no clinic, no medical staff, and no account to sign up for. The calculator is free to use. Google ads on the calculator and the guides are what support the site. The about, contact, privacy, and terms pages do not carry ads.

What the calculator does

You enter the first day of your last period, an average cycle length, and a luteal-phase length. You can show one, three, or six months. The page then shows:

• An estimated ovulation day.
• A fertile window covering the five days before that day and ovulation day itself.
• The next period, one cycle length after the date you entered.
• An estimated due date, 266 days after the estimated ovulation day, if conception were to occur then.
• A month grid. Period days are shaded for five days starting on the date you entered. Fertile days are green. Ovulation day is marked in magenta. Today is outlined.

The same arithmetic is what the guides discuss in words. The guides are explanations. They are not a care plan. Open the calculator to change the inputs yourself.

How the estimate is made

Cycle length is the number of days from the first day of one period to the first day of the next. The slider runs from 21 to 35 and starts at 28. Luteal length is the number of days from ovulation to the next period. The slider runs from 10 to 16 and starts at 14. Ovulation day is counted as:

first day of the last period + (cycle length − luteal length)

On the defaults, that is day 14 of a 28-day cycle. A 32-day cycle with a 14-day luteal phase lands on day 18. The fertile window is that ovulation date and the five days before it. Each later month on the calendar repeats the same lengths. The page does not adjust them if one month was different, because it has no log of other cycles.

The five shaded period days are not an input. Many periods are shorter or longer than five days. The shading is only a way to see where the cycle count starts.

The date field cannot be set in the future. On each load it starts at today’s date. Entries stay in the page while you use it. OvulationCalc does not store them, and the script does not use localStorage. Pages that show ads still contact Google, which the privacy policy describes.

Educational purpose

OvulationCalc is an educational calculator, not a clinician and not a contraceptive. Results are illustrations of a calendar method and the numbers you typed. They do not confirm that ovulation happened, and they are a poor tool for avoiding pregnancy. Cycle length moves. Sperm can survive for several days. A home ovulation test, basal temperature, or cervical mucus can disagree with the calendar, which is why the tracking guide compares those methods instead of treating the calendar as the last word.

Nothing on this site is medical advice, a diagnosis, or a promise about fertility. Decisions about trying to conceive, irregular bleeding, or birth control belong with a qualified clinician. The guides link to public pages from the CDC, ACOG, the NHS, and similar sources so you can read those organizations yourself. Those links are not a medical review, and OvulationCalc does not have a medical reviewer.

Contact

Corrections, privacy requests, and other notes go to contact@ovulation-calc.com. J B reads that mailbox. Details are on the contact page.