Your Cycle
Results
Period Calculator Guide
This predicts your next several periods from your last one and your average cycle length, so you can plan around them.
Next period = First day of last period + cycle length
Repeat for each cycle ahead. A 28-day cycle starting 1 January means periods on 29 January, 26 February, and so on.
What Counts as Day 1
Day 1 is the first day of proper bleeding — not spotting. Getting this wrong by a day or two is the most common reason predictions drift, and it's worth being strict about, because every calculation here counts from it.
What's Actually Normal
A typical cycle runs 21–35 days, and variation of a few days month to month is completely normal. "Regular" doesn't mean identical.
Cycles are usually longest and most erratic in the first years after periods begin and again approaching menopause. In between, most people settle into a pattern with occasional surprises from stress, illness, travel, hard training or weight change.
Why Predictions Drift
This calculator uses one average. Your body doesn't. The follicular phase — before ovulation — is the variable part, and anything that delays ovulation delays your period by exactly as much. A stressful fortnight can push a period back a week without anything being wrong.
If your cycles genuinely vary a lot, use the prediction as a rough zone rather than a date, and track a few months to find your real average.
When to Get It Checked
Worth a conversation with a doctor if cycles are consistently shorter than 21 days or longer than 35, if they suddenly change pattern, if bleeding is very heavy — soaking through a pad or tampon hourly for several hours — lasts more than seven days, or if you bleed between periods. Also if periods stop for three months and you're not pregnant.
None of that means something is wrong. It means it's worth asking.
Planning Around It
Six cycles ahead is usually enough for holidays, events and appointments. Remember the prediction gets less reliable the further out it goes — small errors in your average compound with each cycle.
To find your fertile days instead, use the ovulation calculator.
Do not use this to avoid pregnancy. Cycle-based prediction is unreliable — sperm survive up to five days, ovulation shifts with stress and illness, and calendar methods fail for roughly 1 in 4 users over a year. If you are trying not to get pregnant, talk to a healthcare provider about contraception that actually works.
This is an estimate for general information, not medical advice. Every pregnancy and every cycle is different, and only your doctor or midwife can give you dates and guidance for your situation. Nothing here replaces professional care.
Need a Calculator We Don't Have Yet?
Can't find the calculator you need? We'll build it. Submit your request and we'll evaluate it for our growing collection.
Request Calculator