Key takeaways
- Predicts your ovulation date and the six-day fertile window from your last period.
- Surfaces a 14-day fertility heatmap with daily fertility scores from 0 to 100, peaking on the day before ovulation and the day of ovulation.
- Includes a PCOS-aware mode that uses a 13-day luteal phase, widens the buffer, and flags cycle data suggestive of anovulation.
- Calculations run on your device. Results are shareable as a URL — nothing is stored on a server.
- Pairs naturally with the Period Calculator (cycle prediction) and the Safe Period Calculator (post-fertile window for fertility awareness).
How it works
Like all calendar ovulation calculators, HerCalc starts from a single observation: in a typical cycle, ovulation happens 14 days before your next period — not 14 days after the previous one. We compute the projected next period (LMP + cycle length), subtract the luteal phase (14 days standard, 13 in PCOS mode), and that gives you your projected ovulation date.
The fertile window is six days: five days before ovulation (the longest sperm survive in favorable cervical mucus) plus the day of ovulation itself (the egg's 12–24 hour viability window). We render this as a 14-day heatmap so you can see which days are peak, which are approaching peak, and which are post-peak.
Daily fertility scores are an approximation of relative conception probability, calibrated against the Wilcox et al. (1995, NEJM) cohort:
- Day of ovulation: peak (100)
- Day before ovulation: also peak (88)
- Two days before: high (88)
- Three days before: medium (70)
- Four days before: lower (50)
- Five days before: lowest fertile (30)
- Day after ovulation: small tail (55) — egg still viable
Outside the window, fertility drops to functionally zero. If your goal is conception, prioritize intercourse on days the heatmap marks as 88 or 100. If your goal is fertility awareness, treat any day with a score above zero as a day where pregnancy is possible.
The science behind it
The six-day window — and why it isn't seven
The classic Wilcox study (NEJM 1995) followed 221 healthy women trying to conceive. Daily urinary hormone measurements identified the day of ovulation; daily logs recorded intercourse. The resulting graph is now textbook: conception probability rises sharply five days before ovulation, peaks on the day before and the day of, then crashes. Pregnancies were extraordinarily rare from intercourse two or more days after ovulation, even though the fertile window is often described loosely as "around ovulation."
The biological rationale is simple. Sperm in optimal (estrogen-stimulated, fertile-quality) cervical mucus survive up to five days. The egg is viable for only 12–24 hours after ovulation. Conception requires both gametes alive at the same time. The math gives you a fertile window of approximately six days.
Cervical mucus is the best non-clinical signal
If you observe one fertility sign, observe cervical mucus. As estrogen rises in the days before ovulation, mucus changes from sticky/cloudy to slippery/clear/stretchy ("egg-white" consistency). Couples who time intercourse to days with peak-quality mucus achieve pregnancy faster on average than couples who time only to a calendar prediction (Stanford et al., 2002).
Cervical mucus observation has a learning curve but is free, requires no equipment, and is often more accurate than calendar prediction for users with irregular cycles.
LH testing: the most precise non-clinical predictor
Home luteinizing hormone (LH) tests detect the LH surge that triggers ovulation, which happens 12–48 hours before the egg releases. A positive LH test is the most reliable consumer-grade signal that ovulation is imminent. Begin testing 4–5 days before your projected ovulation day for the best chance of catching the surge.
Note: PCOS users may have multiple "false" LH peaks per cycle without ovulating. If you have PCOS and rely on LH tests, pair them with basal body temperature charting to confirm that ovulation actually happened (a sustained 0.3–0.5°F rise lasting 14+ days is the BBT signature of a true ovulatory cycle).
Basal body temperature confirms — it does not predict
Progesterone released by the corpus luteum after ovulation raises core body temperature by roughly 0.3–0.5°F. If you measure your temperature every morning before getting out of bed (basal body temperature, BBT), you can see the rise within a day or two of ovulation. BBT confirms that ovulation has happened — but only retrospectively. It is not useful for prospective timing in the current cycle.
BBT is most useful for: confirming whether you ovulate at all (anovulatory cycles show no rise), identifying short luteal phases (rise lasts fewer than 10 days), and estimating ovulation day in retrospect to refine future predictions.
The age effect
Female fertility declines from the early thirties, sharply after 35, and very sharply after 40. At 30, a healthy couple has roughly a 20% chance of conception per cycle of well-timed intercourse; at 40 it is about 5%. A calculator cannot change this; if you are 35 or older and have been trying for six months without success, talk to a reproductive endocrinologist sooner rather than later. (Under 35: try for 12 months before seeking workup.)
References
- Timing of sexual intercourse in relation to ovulation — Wilcox AJ et al., New England Journal of Medicine 1995
- Days of perceived peak fertility and conception — Dunson DB et al., Human Reproduction 2000
- International evidence-based guideline for the assessment and management of polycystic ovary syndrome 2023 — International PCOS Network 2023
- Female fertility and the menstrual cycle — StatPearls 2023
PCOS & irregular cycles: the calendar can't keep up
For PCOS and irregular cycles — the textbook 28-day model is rarely your reality.
If you have PCOS, irregular cycles, or recently came off hormonal contraception, calendar prediction is a starting point, not an answer. We do our best — PCOS mode adjusts the luteal assumption, widens buffers, and warns you when your cycle data suggests anovulation — but you will get more useful information from body signals than from a date on a calendar.
- LH test daily beginning around cycle day 10 and continuing until you get a positive (or until cycle day 30 if you are running long).
- Track cervical mucus daily. PCOS users sometimes show "false" mucus peaks followed by a return to dry days — note the pattern over several cycles.
- Chart BBT for 2–3 cycles to confirm whether you actually ovulate. If there is no clear thermal shift, you are likely having anovulatory cycles and a clinician's input is the next step.
- Re-anchor the calendar after each period. PCOS cycles drift; assumptions that were valid last month may not be this month. This calculator updates every time you enter a new LMP.
How to use this calculator
- Enter your last menstrual period (LMP). The first day of full bleeding from your most recent period. The whole calculation is anchored here.
- Enter your average cycle length. Most adult cycles are 24–35 days. Use 28 if you are unsure, but switch to PCOS mode if your cycles regularly exceed 35 days.
- Read your fertile window. You will see your peak day, your fertile window (6 days), and a heatmap of daily fertility scores so you can prioritize timed intercourse around the peak.
- Optional: combine with body signals. Calendar prediction is a starting point. For couples actively trying to conceive, combine with cervical mucus observation, basal body temperature, and a luteinizing hormone (LH) test kit.
- Track for several months. A single cycle's data is rarely enough — track 3–6 months and revisit if your patterns shift.
Limitations & medical disclaimer
- Calendar prediction has uncertainty of ±1–3 days even in regular cycles. For irregular or PCOS cycles, treat any single ovulation date as an estimate, not a fact.
- This calculator does not detect ovulation in real time. To know that ovulation actually happened, use BBT (retrospective) or LH testing (24–48 hours ahead).
- Daily fertility scores are visual aids approximating relative conception probability. They are not absolute probabilities and do not adjust for individual factors like age, partner sperm quality, or underlying conditions.
- Not a contraceptive method. Calendar-based fertility awareness has a typical-use failure rate of about 24% per year. Pair with barriers if pregnancy would be unwelcome.
- HerCalc is for educational use only and does not replace medical advice. If you have been actively trying to conceive without success, talk to a clinician — 12 months under age 35, 6 months at 35+.
HerCalc tools are educational and do not replace professional medical advice. Always consult a qualified clinician for diagnosis or treatment decisions.
Frequently asked questions
When is the best time to have sex if I am trying to get pregnant? +
The fertile window is the five days before ovulation plus the day of ovulation — a six-day window where conception is possible. Probability peaks on the day before and the day of ovulation. Wilcox et al. (1995) found that timed intercourse anywhere in this window can produce a pregnancy in about a third of cycles for healthy couples under 35.
For maximum chance per cycle, aim for intercourse every 1–2 days through the entire window. "Saving up" sperm is unnecessary; daily ejaculation does not meaningfully reduce sperm quality.
How accurate is an ovulation calculator? +
For regular cycles, calendar-based ovulation prediction is accurate within ±1–2 days for most users. For irregular cycles or PCOS, accuracy degrades sharply. To narrow it further, combine calendar prediction with: cervical mucus observation (egg-white-consistency mucus signals high fertility), basal body temperature (a 0.3–0.5°F rise confirms ovulation has happened — in retrospect), and luteinizing hormone (LH) tests (a positive LH test means ovulation is likely within 12–48 hours).
What does ovulation feel like? +
Some users feel mittelschmerz — a brief, one-sided lower-abdominal twinge mid-cycle that can be sharp or dull. Others feel nothing at all. Common signs in the day or two before ovulation include: increased cervical mucus that stretches like raw egg white, slightly tender breasts, a small libido bump, and a mild basal body temperature dip followed by a sustained rise. None of these signs alone confirms ovulation; together they are reasonably reliable.
Can I ovulate while still bleeding? +
Rarely, but yes. Very short cycles (around 21 days) compressed against a long bleed can produce an early ovulation that overlaps with the tail of your period. This is uncommon and usually associated with very short follicular phases. If you have a 21-day cycle and are trying to conceive (or trying to avoid pregnancy), assume you may be fertile any day after Day 5.
I have PCOS — does this calculator work for me? +
Calendar prediction alone is unreliable for PCOS users. Our PCOS mode uses a 13-day luteal phase, widens the fertile-window buffer, and surfaces anovulation warnings when your cycle data is highly variable — but it cannot tell you whether you ovulated this month. For PCOS users actively trying to conceive, the most reliable approach combines calendar tracking with daily ovulation prediction kits (LH testing) and basal body temperature, and works closely with a reproductive endocrinologist if cycles are very long or absent.
How long does an egg survive? +
12–24 hours after ovulation. After that the egg disintegrates and pregnancy is no longer possible from that ovulation. This is why the fertile window is asymmetric: 5 days <em>before</em> ovulation (sperm survival) plus 1 day after (egg survival).
How early can a pregnancy test detect pregnancy? +
Sensitive home tests can detect human chorionic gonadotropin (hCG) about 10 days after ovulation, but accuracy peaks at 14 days post-ovulation — usually the day your period would be due. Testing earlier than that increases the risk of a false negative. First-morning urine is more concentrated and gives the most reliable result.
Why does the fertility heatmap show numbers like 88 or 70? +
The heatmap is a daily fertility score (0–100) approximating relative conception probability across the fertile window. It is calibrated against the Wilcox et al. NEJM 1995 cohort: the day of ovulation and the day before are the peak (~100, ~88), tapering down through the previous five days, with a small tail (~55) on the day after ovulation thanks to egg viability. The numbers are visual aids, not precise probabilities.