How is my next period predicted?

Your next period start date is calculated by adding your average cycle length to the start date of your last period. If your last period started on the 1st and your cycle is 28 days, your next period will start on the 29th. The calculator projects six future cycles.

What is a normal cycle length?

A typical menstrual cycle is 21–35 days, measured from the first day of one period to the first day of the next. 28 days is the population average but anything in this range is considered normal. Cycle length can vary by a few days month to month.

How is the fertile window calculated?

Ovulation typically occurs 14 days before the next period starts. The fertile window is the five days leading up to ovulation plus ovulation day itself — six days total — because sperm can survive in the reproductive tract for up to five days.

Is this calculator a reliable form of contraception?

No. Cycle-based predictions are estimates, not contraception. Cycle length varies and stress, illness, travel, hormonal changes and breastfeeding can shift ovulation. Use a recognised contraceptive method if you want to avoid pregnancy.

Why does my cycle vary each month?

Stress, sleep changes, weight changes, intense exercise, illness, travel across time zones, hormonal birth control, perimenopause and certain medications can all shift cycle length by several days. Variations of ±5 days are usually nothing to worry about — but speak to a GP if your cycle becomes very irregular.

🌺 Period Lommeregner

Predict your next six periods, ovulation day and fertile window from the start date of your last period and your average cycle length.

First day of last period

Gennemsnitlig cykluslængde (dage)

Gennemsnitlig menstruationsperiodes varighed (dage)

⚠️ Kun til information. Predictions are estimates based on cycle averages and are not a contraceptive method. Speak to a GP about irregular cycles or fertility planning.

Understanding Your Menstrual Cycle

Menstruationscyklussen er den månedlige hormonelle cyklus, der forbereder kroppen til graviditet. Den måles fra day 1 — den første blødningsdag — til dag 1 i den næste menstruation. Den gennemsnitlige cyklus er 28 dage, men alt fra 21–35 dage er medicinsk normalt. Denne lommeregner bruger din seneste menstruationsdato og gennemsnitlige cykluslængde til at forudsige de næste seks cyklusser, herunder ægløsning og det fertile vindue.

Cyklussens fire faser

Menstrual phase (days 1–5): the uterine lining is shed if no pregnancy occurred.
Follicular phase (days 1–13): the body prepares an egg for release. Oestrogen rises.
Ovulation (around day 14 in a 28-day cycle): a mature egg is released and is viable for ~24 hours.
Luteal phase (days 15–28): if the egg is not fertilised, hormone levels drop and the cycle starts again.

Fertilt vindue forklaret

Det fertile vindue dækker de fem dage inden ægløsningen plus ægløsningsdagen selv, fordi sæd kan overleve op til fem dage inde i reproduktionssystemet. Højeste befrugtningschance er de to til tre dage umiddelbart inden ægløsningen.

When to Speak to a GP

Cyklusser konsekvent kortere end 21 eller længere end 35 dage
Periods that suddenly become much heavier or longer
Bleeding between periods or after sex
Svære menstruationssmerter, der forstyrrer dagligdagen
Forsøger at blive gravid i over 12 måneder uden succes (eller 6 måneder hvis over 35)

⚠️

Vigtig bemærkning: Dette værktøj giver kun cylusestimater og er ikke en erstatning for medicinsk rådgivning eller prævention. For fertilitetsplanlægning, prævention eller uregelmæssige cyklusser, bedes du konsultere en kvalificeret sundhedsfagperson.

Relaterede lommeregnere

Ovulation Calculator — Forudsig dit fertile vindue, ægløsningsdag og næste menstruation ud fra din seneste menstruation og gennemsnitlige cykluslængde.
Pregnancy Due Date Calculator — Estimeret termindato, nuværende uge og trimester fra seneste menstruation.
Sleep Calculator — Bedste sengetidspunkt og opvågningstider baseret på søvncyklusser.