Last period started 1 January, 28-day cycle, 5-day period
Next periods begin about 29 January, 26 February and 26 March, each lasting until day 5.
Period calculator
Every future period is projected with the same cycle length. Real cycles drift by a few days, so treat later dates as less certain. Ovulation is taken as 14 days before the next period.
days
days
Next period starts
19 Jan 2026
In 18 days. It should last until about 23 Jan.
The next six periods
- Period 119 Jan to 23 Jan · ovulation 5 Jan
- Period 216 Feb to 20 Feb · ovulation 2 Feb
- Period 316 Mar to 20 Mar · ovulation 2 Mar
- Period 413 Apr to 17 Apr · ovulation 30 Mar
- Period 511 May to 15 May · ovulation 27 Apr
- Period 68 Jun to 12 Jun · ovulation 25 May
Predicting your next periods
If your cycles are regular, a calendar is a simple way to plan. Note the first day of your last period, add your usual cycle length, and you have the next date. Add it again for the one after.
With a 28-day cycle and a 1 January start, the next periods fall around 29 January, 26 February and 26 March. Each lasts about five days, and ovulation falls about two weeks before each one.
The further ahead the date, the less certain it is. Cycles drift by a few days even for people with regular ones, and the error adds up. So treat the first prediction as fairly firm and the sixth as a rough plan for travel or events.
A cycle that keeps changing by more than a week, or periods that are unusually heavy or painful, is a reason to talk to a doctor. A simple note of dates, kept for a few months, helps that conversation.
Sources
What this calculator does not account for
- Cycle changes from stress, illness or medicine.
- Irregular cycles.
- Any medical condition.
How this is calculated
Formula: Repeating the cycle
next period =last period start + cycle length × kWhat the symbols mean
- cycle lengthDays from the first day of one period to the first day of the next, often 21 to 35.
- k1 for the next period, 2 for the one after, and so on.
Good to know
- Each period's end is its start plus the period length minus 1 day.
- Ovulation is placed 14 days before each period.
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Worked examples
Each example is worked out by the same code as the calculator above.
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<iframe src="/embed/period-calculator" title="Period calculator" width="100%" height="720" style="border:0;max-width:42rem" loading="lazy"></iframe>Questions
How do I count my cycle length?
From day one of a period to the day before the next period starts. Average several months.
What is a normal cycle?
Cycles from 21 to 35 days are generally considered normal in adults, and the length can vary by a few days.
Why is my period late?
Stress, illness, travel, weight change, PCOS and pregnancy can all delay it. If you are late and sexually active, a pregnancy test is the first step.
When should I see a doctor?
If periods are very painful, last over 7 days, are very heavy, or stop for three months, speak to a doctor.