How is ovulation date estimated?

Using the standard luteal-phase model: ovulation typically occurs 14 days before the next expected period. The calculator subtracts 14 from the predicted next-period date (LMP + cycle length).

How accurate is the prediction?

The calculator uses the calendar method, which assumes a regular cycle. For most people with consistent cycles it is accurate to within 1–3 days. Predictions are less reliable for irregular cycles, after stopping hormonal contraception, while breastfeeding, or around perimenopause.

Does it work if my cycle is not 28 days?

Yes. Enter your average cycle length (commonly 21–35 days). The calculator adapts the prediction so that ovulation lands 14 days before the next period regardless of cycle length.

Can this calculator be used for contraception?

No. The calendar method is not a reliable contraceptive — natural cycle variation, illness, stress and lifestyle changes can shift ovulation. For contraception, use a clinically approved method and consult your doctor.

🌸 Ægløsningsberegner

Predict your fertile window, ovulation day and next period from your last menstrual period and average cycle length.

First day of last period (LMP)

Gennemsnitlig cykluslængde (dage)

⚠️ Not medical advice. Cycle prediction is based on the calendar method and assumes a regular cycle. Speak to your GP or healthcare provider for personalised fertility guidance — and do not rely on this tool for contraception.

Om ægløsnings-lommeregneren

Ovulation Calculator bruger standard kalendermetoden til at forudsige dagene i din cyklus, hvornår befrugtning er mest sandsynlig. Indtast den første dag i din seneste menstruation (LMP) og din gennemsnitlige cykluslængde, og lommeregneren returnerer dit fertile vindue, ægløsningsdagen, dine mest fertile dage, datoen for din næste menstruation og en seks-måneders prognose.

The science in brief

An average menstrual cycle lasts 21–35 days. Ægløsning — the release of the egg — typically occurs 14 days before the start of the next period, regardless of cycle length. This is the luteal-phase model used by clinicians worldwide.

Fertile window: sæd overlever ~5 dage i reproduktionssystemet; ægget lever ~24 timer. Så de 5 dage inden ægløsningen plus ægløsningsdagen selv danner et seks-dages vindue, hvori befrugtning er mulig.
Most fertile days: the day of ovulation and the two days before it, when conception probability is highest.
Next period: LMP plus the average cycle length.
Due date if conception happens this cycle: Naegele's rule — LMP + 280 days.

How to use this calculator

Enter the first day of your last period (LMP). This is day 1 of the cycle.
Indtast din average cycle length. Hvis du ikke kender den, beholde standardindstillingen på 28 dage. Typisk interval er 21–35 dage.
Press Calculate Fertile Window. The fertile window, ovulation day, next-period date and six-month forecast appear instantly.

Common uses

Familieplanlægning — timing af samleje for den højeste chance for befrugtning
Sporing af uregelmæssige cyklusser for at opdage mønstre
Health monitoring — late or missed periods can flag underlying conditions
Pairing with our Pregnancy Due Date Calculator once a positive test arrives

Ofte stillede spørgsmål

What is the fertile window? Det seks-dages spænd, der slutter på ægløsningsdagen. Befrugtning er mulig på alle disse dage.

How accurate is this prediction? For people with regular cycles, accuracy is typically within 1–3 days. Stress, illness, travel, hormonal changes, breastfeeding and perimenopause can all shift ovulation, so the calendar method should be combined with cycle-tracking tools (basal body temperature, cervical mucus, ovulation predictor kits) for higher accuracy.

Can this be used for contraception? Nej. Cyklusvariabilitet gør kalendermetoden uegnet som eneste prævention. Brug en klinisk godkendt metode og konsulter din læge.

Why does the prediction assume ovulation 14 days before the next period? The luteal phase (post-ovulation) is the most consistent part of the cycle, lasting 12–16 days for most people. The follicular phase varies more, which is why ovulation timing depends on cycle length rather than a fixed day-14 rule.

What if my cycle is irregular? Brug din gennemsnitlige cykluslængde over de seneste 3–6 måneder som bedste skøn. Ved meget uregelmæssige cyklusser anbefales yderligere metoder til fertilitetssporing.

Is my data shared anywhere? Nej. Alle beregninger kører udelukkende i din browser — intet uploades.

⚠️

Vigtig bemærkning: This tool provides estimates based on the calendar method and is not a substitute for medical advice. Always consult a qualified healthcare provider for personalised fertility, pregnancy or contraception guidance.

Relaterede lommeregnere

Period Calculator — Predict your next 6 periods, fertile window and ovulation day from your cycle data.
Pregnancy Due Date Calculator — Estimeret termindato, nuværende uge og trimester fra seneste menstruation.
Sleep Calculator — Bedste sengetidspunkt og opvågningstider baseret på søvncyklusser.