Next period
Expected start = first day of last period + cycle length. Everything is calculated in whole calendar days, so clock changes and time zones can’t shift a result.
The range uses your variation, widened by the square root of the number of cycles ahead. With variation of ±3 days, the next cycle shows ±3 days and the fourth shows ±6 days.
Ovulation and fertile days
Estimated ovulation = next period start − luteal phase, using 14 days unless you change it. The NHS gives 10 to 16 days as the usual range. Likely fertile days are the 5 days before ovulation plus the day itself, widened by your variation. If a cycle is too short for this to make sense, we show no estimate rather than a meaningless date.
This is not contraception. Ovulation can shift in any cycle, and no calendar can detect that.
Your own pattern in the tracker
- Periods come from your logged flow. Spotting isn’t counted as a period start, and one blank day doesn’t split a period in two.
- Typical cycle is the median of your last 12 cycles, so one unusual month doesn’t skew it.
- Variation is the standard deviation of those cycles, once you have 3 or more. Until then, we assume ±3 days.
- Gaps over 90 days are left out of predictions and flagged separately.
- Regularity is based on the gap between your shortest and longest cycle. Up to 7 days reads as fairly regular, 8 to 9 as some variation, and 10 or more as quite variable. Published limits are 7 to 9 days depending on age.
Late period checker
Up to 3 days either side of your estimate counts as on time. Later than that counts as late, and 90 days or more since your last period is flagged for a doctor.
We don’t give a percentage for any cause. No dataset could honestly turn a handful of checkboxes into a probability, so the tool shows which common causes match your answers, and always leads with urgent symptoms and pregnancy.
Implantation vs period
Your answers are compared with the published pattern for implantation bleeding: 10 to 14 days after ovulation, pink or brown, liner-level flow, up to about 2 days, no clots, and little cramping. The tool reports a closer fit, or that it can’t tell. It never says you are or aren’t pregnant.
Cramp planner and discharge decoder
These apply NHS self-care steps and warning signs directly. They never give medicine doses and never diagnose. They name what a symptom can indicate, then point you to the right kind of care.
Photo colour reading
Your browser averages a small square of pixels around where you tap, converts them to the CIELAB colour space (which is closer to how people see colour), and finds the nearest of seven reference colours. Lightness counts for less than hue, because cameras and room lighting affect it most. Very pale, low-colour areas are treated as background. A match that’s close between two colours is labelled as uncertain, and you can always correct it.
The reference colours are our own approximations, and this method hasn’t been clinically validated. Treat the result as a logging aid, not a measurement.
Sharing
Sharing text and spreadsheets are built in your browser, from the same numbers you see on screen. Tracker summaries list up to your last 12 periods, and optional details are left out unless you tick them. Long summaries are shortened for email, with a note saying so, because some email apps limit the length of a link. Copy text always gives you the full version. Spreadsheet cells are protected, so a note can’t run as a formula when opened.
Translations
This site is written in English. Versions in other languages are translations of the English pages, and the same rules and sources apply to every language.
What these tools can’t do
- Diagnose any condition, or confirm or rule out pregnancy.
- Account for hormonal contraception, which changes your cycle.
- Replace a healthcare professional. If you’re worried, please see one.