Period Calculator
Your cycle details
Not sure of your average? Most people use 28 days cycle and 5 days period as a starting point - you can fine-tune it any time.
Your calendar
Enter your last period start date on the left. Your next three cycles, fertile window and ovulation day are drawn on this sheet.
What is a Period Calculator?
A period calculator (also called a menstrual cycle calculator or period tracker) predicts your next period, ovulation day, and fertile window using the date your last period started, your average cycle length, and how many days your period usually lasts. It's an estimate based on averages - your actual dates can shift due to stress, travel, illness, or hormonal changes.
How is Ovulation Calculated?
Ovulation is calculated by counting backward 14 days from your next expected period, not forward from your last one. This is because the luteal phase - the time between ovulation and your next period - is medically consistent at around 14 days, while the follicular phase (period to ovulation) can vary a lot between people and even between cycles. So a longer cycle usually means ovulation happens later, not earlier.
Understanding Your Cycle Phases
🩸 Menstrual Phase
🌱 Follicular Phase
🥚 Ovulation Phase
🌙 Luteal Phase
Irregular Periods and PCOS in India
Estimates depend on diagnostic criteria, but a nationwide, multicenter ICMR-PCOS study of Indian women found PCOS (Polycystic Ovary Syndrome) in up to 1 in 5 (19.6% by Rotterdam criteria; 7.2% by stricter NIH criteria) - one of the most common causes of irregular periods. Cycles shorter than 21 days or longer than 35 days, or periods that are frequently unpredictable, are worth discussing with a gynaecologist - especially if paired with other symptoms like acne, weight changes, or excess hair growth. This calculator flags irregular cycles, but it cannot diagnose PCOS or any other condition.
Safe Days and Family Planning
"Safe days" (the rhythm or calendar method) refers to the days outside your fertile window when pregnancy is less likely. It is one of the least reliable forms of contraception - NHS guidance puts typical-use effectiveness at 76% (about a 24% failure rate) versus 91-99% with strict, correctly taught use, because ovulation timing shifts cycle to cycle, especially for people with irregular periods. Use this information for awareness, not as your primary method of contraception; speak with a doctor about condoms, hormonal options, or IUDs for reliable protection.
Frequently Asked Questions
How do I know my next period date?
Add your average cycle length (commonly 28 days) to the first day of your last period. This calculator does that automatically and also shows your next 3 predicted periods.
My period is late - should I worry?
A few days' delay is common and can be caused by stress, travel, illness, or weight changes. If you're more than a week late, consider a pregnancy test if relevant, and see a doctor if it keeps happening.
How accurate is a period calculator?
It's an estimate based on your average cycle, not a guarantee. Accuracy is highest for regular cycles (21-35 days) and drops for irregular cycles, recent childbirth, breastfeeding, or conditions like PCOS.
Can I use this to avoid or plan pregnancy?
You can use it for general awareness, but the fertile window and safe days shown here are estimates. For reliable contraception or fertility planning, talk to a doctor about proven methods.
Is my data private?
Yes. All calculations happen in your browser. Your dates are stored only in your browser's local storage if you choose to keep them - nothing is sent to Toolsda's servers.