Period Calculator

What this calculator does

See when your next period is likely to start and end, for the next six cycles. A menstrual cycle runs from the first day of one period to the first day of the next. If you know roughly how long yours run, the math is a calendar count. Take the first day of your last period and add one cycle length. The calculator repeats that six times and shows how many days each period should last.

Not sure how long your cycle is? Enter the start dates of up to six recent periods instead. The calculator counts the days between each pair and averages them. It also shows your shortest and longest cycle, so you can see how much they vary. These dates are estimates to help you plan trips, events and supplies. They aren't a form of contraception and can't confirm or rule out pregnancy. Cycle length naturally changes from month to month. If yours changes a lot, it's worth mentioning to a clinician.

Where it's used

Why this math matters outside the classroom:

• Planning trips. Someone with a 28-day cycle whose last period began June 3 expects the next around July 1 and packs accordingly.
• Averaging data. Gaps of 27, 29 and 31 days between four start dates average to 87 ÷ 3 = 29 days.
• Clinic visits. Patients bring their recent start dates and average cycle length to appointments so the clinician sees the pattern.
• Sports schedules. Athletes note expected period dates against a season calendar to plan supplies for away games.
• Calendar counting. Students count forward 30 days from March 20 across the month end to land on April 19.

How to use it

1. Choose whether you already know your average cycle length, or want it worked out from past start dates.
2. Enter the first day of your last period. That's the first day of real bleeding, not spotting.
3. Enter your average cycle length, or the start dates of up to five earlier periods (at least one). The dates can be in any order.
4. Enter how many days your period usually lasts. Then read your next expected period and the six-cycle table.

What the inputs mean

Average cycle length

Days from the first day of one period to the first day of the next. Lengths from 21 to 45 days are accepted. If you're unsure, 28 is a common starting point, but your own average is better.

Start of the period before that

The first day of the period before your last one. Each gap between back-to-back start dates must be 15 to 90 days. That way a skipped month shows up as an error, not as a misleading average.

Period length

The number of days you usually bleed. It sets the end date of each expected period. It doesn't change the start dates.

Formula and method

Average cycle = (latest start date − earliest start date) ÷ number of cycles between them
Next period starts = last period start + average cycle   (rounded to whole days)
Period n starts = last period start + round(n × average cycle)
Period ends = start date + (period length − 1) days

Here, n is which future period you mean: 1 for the next one, 2 for the one after, and so on. "Round" means to the nearest whole day.

Averaging past start dates is the same as adding up the gaps between them and dividing by how many gaps there are. With start dates of Jan 1, Jan 30, Mar 1 and Mar 30, the gaps are 29, 30 and 29 days. That's an average of 29.33 days.

Each future period is placed a whole number of average cycles after your last one, and only then rounded. The calculator doesn't round the cycle first. So a 29.33-day average gives gaps of 29, 30 and 29 days in turn, and the dates don't drift.

The first day of bleeding counts as day 1 of the period. So a 5-day period that starts on the 29th is expected to end on the 2nd of the next month.

Worked example

Your last period started on Thursday, January 1, 2026. Your cycles average 28 days and periods last 5 days.

1. Next period: Jan 1 + 28 days = Thursday, Jan 29, 2026.
2. It is expected to last until Jan 29 + 4 days = Feb 2, 2026.
3. The one after that: Jan 1 + 56 days = Thursday, Feb 26, 2026.
4. Third cycle: Jan 1 + 3 × 28 = 84 days = Mar 26, 2026.

Result: Your next period is expected around Thursday, Jan 29, 2026, lasting until about Feb 2, then again from Feb 26.

Understanding the result

Think of each date as the middle of a window, not an exact day. A period that comes a few days early or late is common, even for people whose cycles are usually regular.

Your own shortest and longest cycles are a good guide to how wide that window is. If they differ by more than about a week, the page tells you, and the estimate becomes less precise.

To see your likely fertile days in the same cycle, try the ovulation calculator. It uses the same dates and counts back from your next period.

Assumptions and limitations

• Future cycles match your average length.
• The dates you enter are the first day of each period.
• Cycles can shift with stress, illness, travel, changes in weight or exercise, and hormonal contraception. Pregnancy, breastfeeding and perimenopause (the years leading up to menopause) change them too. A calendar can't account for any of these.
• The calculator doesn't diagnose anything and can't tell you whether you're pregnant. A late period can have many causes. A pregnancy test or a clinician can help.
• It isn't contraception and shouldn't be used to avoid pregnancy.

Frequently asked questions

How this calculator is checked

Method: Calendar method: next period = last period start + average cycle length; average cycle = mean gap between consecutive period start dates; later periods placed at whole multiples of the average and rounded to the nearest day. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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