Who makes OvaCalc, and how the numbers are worked out

Who

I am Roman Šterba. I live in Prague, run a small e-commerce business, and build single-purpose calculators on the side. This is one of them. I am not a doctor, a midwife or a fertility specialist, and nothing on this site is medical advice. What I do is read the studies, get the arithmetic right, and show the answer without asking you to sign up first.

If a number here is wrong, I want to know. Write to [email protected] and I will fix it.

Why another cycle calculator

Most calculators add 14 days to the first day of your last period and call that ovulation. That is only right on a 28-day cycle. The second half of the cycle, from ovulation to the next period, is the steady part at about 14 days. The first half is what stretches and shrinks. So every tool here finds your next period first and counts back from it, and the due date calculator shifts the standard formula by the same amount.

The other difference is the odds. The fertile window is six days, but the chance of conceiving is not the same on each of them. The home page shows the day-by-day figures from the study that measured them, so you can see why the last two days matter most.

What each calculator does

• Ovulation: your next period is the last one plus your cycle length. Ovulation is 14 days before that. The fertile window is the five days before ovulation plus the day itself.
• Period: repeats your cycle length forward six times and marks the week before each period.
• Conception: counts back from a due date, a last period or a scan to the day of ovulation, and shows the six days in which sex could have led to it.
• Due date: the last period plus 280 days, shifted by the difference between your cycle length and 28.
• IVF due date: the transfer date plus 266 days, minus the age of the embryo on the day of transfer. From egg retrieval it is retrieval plus 266.
• Reverse due date: counts back 280 days from a due date to the last period and 266 to conception. From a birth date it uses the week the baby came at.
• Pregnancy test: hCG first appears 6 to 12 days after ovulation and a home test needs a few days more, so both the earliest useful test day and the day a negative can be trusted are counted from ovulation, not from a missed period.

Where the numbers come from

• Wilcox AJ, Weinberg CR, Baird DD. Timing of Sexual Intercourse in Relation to Ovulation. New England Journal of Medicine 1995;333:1517–21. The six-day window and the odds on each day of it.
• Wilcox AJ, Baird DD, Weinberg CR. Time of Implantation of the Conceptus and Loss of Pregnancy. New England Journal of Medicine 1999;340:1796–9. When hCG first appears after ovulation.
• American College of Obstetricians and Gynecologists. Methods for Estimating the Due Date. Committee Opinion No. 700, 2017. Dating rules, and why an early ultrasound overrides them.

Each calculator names the source it rests on at the bottom of its own page.

What is recorded about you

Nothing that identifies you. There is no account, no cookie and no advertising tracker. The site keeps a count of page views per day and which page was viewed, with no IP address attached, so I can tell whether anyone is using it. The dates you enter stay in your browser. "Copy link to this result" puts them into the link you copy, and nowhere else.

Contact

[email protected]. Corrections, questions, and requests for a calculator that does not exist yet are all welcome.

Last reviewed 10 September 2026.