CodeKitHubWhat Is This Tool?
A period calculator projects your menstrual cycle forward: starting from your last period's first day, it adds your average cycle length repeatedly to predict when the next periods will begin, and uses your typical period length to mark when each one ends. A cycle is counted from the first day of one period to the first day of the next — 21 to 35 days is generally considered a normal range for adults, and the often-cited 28 days is just the average, not a requirement.
The honest caveat: this is pattern extrapolation, not biology sensing. Menstruation responds to stress, illness, travel across time zones, intense exercise, weight change, breastfeeding, perimenopause and medication — any of which can shift a period by days. A prediction one cycle ahead is usually close; six cycles ahead is a rough sketch. If your real dates repeatedly drift from the forecast, that pattern itself is useful information to bring to a doctor.
Why Use It?
- Predicts your next 6 periods in one table — start date, end date, and days from today.
- Adjustable to your body: cycle length from 21 to 35 days, period length from 1 to 10 days (default 5).
- Helps you plan travel, sport, exams and appointments around your cycle — and notice a late period within days rather than weeks.
- Free, instant and private — the dates you enter are processed in your browser and never uploaded or stored.
- Clear about limits: predictions assume average cycles and are labeled as estimates, not medical advice.
How to Use
- Pick the first day of your last period in the date field.
- Set your average cycle length in days (default 28; adjustable from 21 to 35).
- Set how many days your period usually lasts (default 5).
- Press the calculate button and read the 6-cycle forecast table.
Example
Input
Last period started May 1, 2026, cycle 28 days, period 5 daysOutput
Next period: May 29 – June 2, 2026; the one after: June 26 – 30Each predicted start is exactly one cycle length (28 days) after the previous start; each end date is start + 4 days for a 5-day period. With a 35-day cycle the next period would instead start June 5.
How prediction accuracy degrades
The single best way to improve predictions is to update the calculator with each new period's actual start date instead of forecasting many cycles from one old data point — the forecast is only as good as its starting point.
| Situation | Expected accuracy | Better alternative |
|---|---|---|
| Regular cycles, 1 cycle ahead | Usually within ±2–3 days | This calculator is a good fit |
| Regular cycles, 6 cycles ahead | Small errors accumulate; treat as approximate | Re-enter your latest period each month |
| Cycles varying by up to a week | Predictions can miss by ~1 week | Track actual dates for 3–6 months, use your real average |
| Very irregular or absent periods | Calendar prediction is not meaningful | See a doctor — irregularity itself is the finding |
When to see a doctor about your cycle
- No period for 3 months or more (and you're not pregnant, breastfeeding or on cycle-stopping medication).
- Cycles consistently shorter than 21 days or longer than 35 days.
- Bleeding lasting more than 7 days, unusually heavy bleeding, or bleeding between periods.
- Severe pain that interferes with daily life — this is common but not something you're obliged to just endure.
- This tool is for general reference only and cannot diagnose anything. Predictions are least reliable exactly when cycles are irregular — which is also when a doctor's input matters most.
Where this fits in real workflows
- Trying to conceive? The Ovulation Calculator uses the same inputs to estimate your fertile window in each upcoming cycle.
- Period late and a test came back positive? The Pregnancy Due Date Calculator turns your last period's date into a due date and week-by-week timeline.
- Counting days between arbitrary dates — cycle tracking in a spreadsheet, for example — is what the Date Calculator does.
→ Ovulation Calculator · Pregnancy Due Date Calculator · Date Calculator · BMI Calculator
Frequently Asked Questions
When is my next period?
Add your average cycle length to the first day of your last period. With a last period starting May 1 and a 28-day cycle, the next period is expected around May 29. This calculator repeats that step six times so you can see half a year ahead at a glance.
What counts as a normal cycle?
For most adults, a cycle of 21–35 days and a period of about 2–7 days is considered within the normal range. Cycles also don't need to be identical every month — variation of a few days is common. Teenagers often have longer, more variable cycles for the first years after periods begin.
My period is late compared to the prediction. What now?
A few days of drift is normal — stress, illness, travel or exercise can shift a period. If you're sexually active, a pregnancy test is the sensible first check once a period is about a week late. If periods are repeatedly very late, absent for 3 months, or your cycle suddenly changes pattern, see a doctor.
Can I use these dates to avoid pregnancy?
No. Predicting periods is not the same as predicting safe days — ovulation shifts unpredictably, and calendar-based contraception has a high failure rate. If avoiding pregnancy matters to you, talk to a doctor about reliable contraception instead of relying on any date calculation.
Is my data stored anywhere?
No. The calculation runs in your browser with JavaScript; the dates you enter are never sent to a server, stored or logged.