The short answer: add your usual cycle length to the first day of your last period. If your last period started on 30 March and your cycle is usually 29 days, your next period is due around 28 April. Track three or more cycles to find your real average, because very few people have an exact 28-day cycle.
First, what counts as a cycle
Your menstrual cycle runs from the first day of one period (the first day of real bleeding, not spotting) to the day before your next period starts. That first day of bleeding is called day 1.
A cycle anywhere from 21 to 35 days is normal, and it's common for your cycle to vary by a few days from month to month. A period itself usually lasts 2 to 7 days, often about 5.
Step by step: find your next period date
- Write down the start date of your last three to six periods. Use a calendar, notes app or period tracker.
- Count the days between each start date. Count from day 1 of one period up to, but not including, day 1 of the next.
- Work out your average. Add the cycle lengths together and divide by how many cycles you counted. Round to the nearest whole day.
- Add your average to the start date of your latest period. That's your expected next period date.
Worked example: a regular cycle
| Period started | Days until next period |
|---|---|
| 3 January | 28 |
| 31 January | 29 |
| 1 March | 29 |
| 30 March | This is the latest period |
Average cycle: (28 + 29 + 29) ÷ 3 = 28.7, which rounds to 29 days. Next period: 30 March + 29 days = 28 April.
Worked example: an irregular cycle
If your cycles vary more, an average can mislead. Use your shortest and longest cycles to get a window instead.
Say your recent cycles were 26, 33, 29 and 31 days, and your last period started on 2 June. Your next period is likely between 2 June + 26 days and 2 June + 33 days: between 28 June and 5 July. Keep protection handy across that whole window.
Bonus: estimating when you ovulate
Ovulation usually happens around 10 to 16 days before your next period, not 14 days after your last one. So in the regular example above, with a period due on 28 April, ovulation is most likely between about 12 and 18 April.
Please don't rely on this as contraception. Ovulation timing can shift from cycle to cycle, and sperm can survive for up to 7 days, so pregnancy is possible on more days than a calculation suggests.
What can throw your dates off
- Stress, illness or travel
- Big changes in weight or exercise
- Starting, stopping or changing hormonal contraception, or taking the emergency pill
- Breastfeeding
- PCOS, thyroid problems and other health conditions
- The first few years after periods start, and the years before menopause
If your period comes much earlier than you calculated, read why your period might come early. If it's late, see our guide to late periods.
Make tracking easy
- Note day 1 every month. It takes five seconds and gives you, and your doctor, a clear picture.
- Note anything unusual too: very heavy days, clots, pain, or bleeding between periods.
- Be ready a few days early. Wearing a period panty in the days around your expected date means an early start isn't a problem. A panty liner works for lighter pre-period spotting.
Some people also adjust what they eat across the cycle. See our nutrition guide for each phase of your cycle.
When to see a doctor
- Your cycles are regularly shorter than 21 days or longer than 35 days
- Your cycle length changes a lot from month to month
- You've missed three periods in a row and aren't pregnant
- Your periods haven't started by age 15
Frequently asked questions
Do I count from the first or last day of my period? Always the first day of proper bleeding.
Is a 28-day cycle the only normal one? No. Anything from 21 to 35 days is normal, and few people have exactly 28 days every month.
How many cycles should I track? At least three, ideally six, to get a reliable average.
Are period tracker apps accurate? They use the same maths as this guide. They get better with more data, but they can't predict a change caused by stress or illness.
Related reading
- Period Late but Not Pregnant? 10 Common Reasons
- Period Came Early? 9 Reasons and When to Worry
- Discharge Before Your Period or Early Pregnancy? How to Tell the Difference
This article is for general information and doesn't replace advice from your doctor.
Sources: NHS: Periods, NHS: Periods and fertility in the menstrual cycle, NHS: Missed or late periods