Free · Private · Works for irregular cycles
Period Calculator for Next Period Date & Cycle Calendar
Find out when your next period is due, where you are in your cycle today, and your dates for the next 12 months. This period calculator learns from your past periods for irregular cycles and can add every date to your phone calendar.
- Next 12 periods
- Irregular cycle averaging
- Cycle phase today
- Add to calendar
Your cycle calendar
- Period
- Possible early or late days
- Fertile window
- Ovulation
- PMS days
| # | Period starts | Period ends | Ovulation | Fertile window |
|---|
- 21–35Days, the normal adult cycle range
- 2–7Days, a typical period length
- 12Months of predicted dates
- 0Accounts, uploads or tracking
More than a date
Why use this period calculator
Popular period calculators give one date from a fixed cycle length. Real cycles vary, so this one uses your own history and shows a range.
Built for irregular cycles
Add up to five past start dates. We average your real cycle lengths instead of assuming 28 days.
A likely date range
See the earliest and latest your period is likely to arrive, based on how much your cycles vary.
Where you are today
Know your cycle day and phase today, whether your period is due soon, or how many days late it is.
Color-coded calendar
Browse month by month to see period, fertile, ovulation and PMS days, plus a 12-cycle table.
Add dates to your calendar
Download your next 12 periods as a calendar file for Google Calendar, Apple Calendar or Outlook.
Completely private
No account, no app and no upload. You can choose to remember your cycle on this device only.
Takes 20 seconds
How to use the period calculator
Five simple steps.
Enter your last period date
Pick the first day of proper bleeding, not spotting, of your most recent period.
Set your cycle length
Count from the first day of one period to the first day of the next. Use 28 if you are not sure.
Set your period length
Choose how many days your bleeding usually lasts.
Add past dates if irregular
Open the irregular cycles section and add earlier start dates for a personalised average and date range.
Save or share your dates
Browse the calendar, add the dates to your phone calendar, print them, or copy them.
The science
How your next period is calculated
Your next period is predicted by adding your average cycle length to the first day of your last period. Each later period is one more cycle length ahead, which is how the 12-month schedule is built.
Ovulation usually happens about 14 days before a period starts, because the second half of the cycle is fairly steady. The first half varies more, which is why stress, illness, travel or weight changes can make a period early or late.
Why a range is more honest
Even regular cycles shift by a few days. If your past cycles ranged from 27 to 31 days, your next period is most likely somewhere in that window, not on one exact day.
Fertile window
Pregnancy is most likely in the five days before ovulation and on the day of ovulation. For more detail, use our ovulation calculator.
Period formulas
Next period date:
Next period = last period start + cycle lengthOvulation date:
Ovulation = next period − luteal phase (about 14 days)Fertile window:
Fertile days = ovulation − 5 days to ovulation dayKnow your body
The four phases of the menstrual cycle
Timings below are for a 28-day cycle. Longer or shorter cycles mostly change the length of the follicular phase.
- Days 1 – 5
Menstrual phase
The uterine lining sheds. Energy can be lower and cramps are common.
- Days 6 – 13
Follicular phase
Estrogen rises as an egg matures. Many people feel more energetic.
- Around day 14
Ovulation
An egg is released. This is the most fertile time of the cycle.
- Days 15 – 28
Luteal phase
Progesterone rises. PMS symptoms may appear in the last week.
Health check
When to talk to a doctor
Some variation is normal. See a doctor or gynaecologist if any of these apply to you.
- !
Cycles outside 21 to 35 days
Cycles that are regularly shorter than 21 days or longer than 35 days in adults are worth checking.
- !
Missed periods
No period for three months or more when you are not pregnant can point to conditions such as PCOS or thyroid problems.
- !
Very heavy bleeding
Soaking a pad or tampon every hour for several hours, or passing large clots, needs medical advice.
- !
Periods longer than 7 days
Bleeding that lasts longer than a week, or bleeding between periods or after sex, should be checked.
- !
Severe pain
Pain that stops you from normal activities or does not ease with usual pain relief can be a sign of endometriosis.
- !
A late period
If you are sexually active and your period is more than a week late, take a pregnancy test. Stress and illness can also delay a period.
Learn more about periods on the NHS website.
Answers
Period calculator FAQs
How do I calculate my next period?
Add your average cycle length to the first day of your last period. If your last period started on 1 March and your cycle is 28 days, your next period should start around 29 March.
What is a normal menstrual cycle length?
For adults, a cycle between 21 and 35 days is considered normal, and 28 days is the average. Teenagers can have cycles of up to about 45 days while their cycles settle.
How accurate is a period calculator?
It is most accurate when your cycles are regular. Because ovulation can shift with stress, illness or travel, treat the date as an estimate and allow a few days either side.
Can I use a period calculator with irregular periods?
Yes. Add the start dates of your last few periods. The calculator averages your real cycle lengths and shows a likely date range based on how much they vary.
Why is my period late?
Common causes include stress, illness, travel, big changes in weight or exercise, breastfeeding, and conditions such as PCOS or thyroid problems. If you are sexually active and your period is more than a week late, take a pregnancy test.
When am I most fertile?
You are most fertile in the five days before ovulation and on the day of ovulation. Ovulation usually happens about 14 days before your next period. A period calculator is not a reliable method of contraception.
How long does a period usually last?
Most periods last between 2 and 7 days. Bleeding for more than 7 days, or very heavy bleeding, is worth discussing with a doctor.
When does PMS start?
PMS symptoms such as bloating, breast tenderness, cravings or mood changes usually start in the week or two before a period and ease once bleeding begins.
Is my data safe with this period calculator?
Yes. Everything is calculated in your browser. Nothing is uploaded, and your cycle is saved on your device only if you tick Remember my cycle.
Always know when your period is coming
Use the period calculator to plan trips, events and workouts around your cycle, and add your next 12 dates to your calendar in one tap.
Calculate my next periodThis calculator gives estimates for general information only. It is not a medical device, a diagnosis, or a method of contraception. Talk to a doctor about any concerns.