The principles behind every tool
Three rules govern how Cyluna calculators are built. First, established formulas only: every calculation uses a method published by a recognised health authority or in standard clinical use. We never invent a formula, and where a method has a known name (Naegele's rule, the luteal-phase-constant method), we say so. Second, honest uncertainty: where a prediction is inherently imprecise, we show a range or a confidence framing rather than a single confident date. Third, privacy by architecture: every calculation runs entirely in your browser. No input you type is transmitted, stored or seen by anyone.
Period Calculator
Formula: next period = first day of last period + your average cycle length. Ovulation is estimated as cycle length minus 14 days (the luteal-phase-constant method), and the fertile window is placed at ovulation minus 4 to plus 1 days. The 3-month forecast repeats your average cycle length forward.
Why minus 14: the second half of the cycle (luteal phase) is far more consistent than the first, typically 12 to 16 days with 14 the most common. Predicting backward from the next period is therefore more reliable than guessing forward. This method is standard in fertility-awareness practice.
Accuracy and limitations: for regular cycles, predictions land within 1 to 3 days. Accuracy collapses for irregular cycles, after recent contraception changes, and in perimenopause, which is why the tool links to a range-based alternative. The forecast assumes your average cycle length repeats exactly; real bodies do not.
Irregular Period Calculator
Formula: instead of one average, you enter your shortest and longest recent cycle lengths. The tool produces a prediction window between those bounds and weights likelihood toward the middle of the range, reflecting the observed behaviour of variable cycles.
Why a range: for genuinely irregular cycles, any single-date prediction is false precision. A window that honestly spans your variation is more useful clinically and practically. This range-based approach mirrors how clinicians discuss unpredictable cycles.
Accuracy and limitations: the window is only as good as the recent history you enter. Two or three cycles of data produce a wide window; six produce a meaningful one. PCOS-related anovulation can break the pattern entirely in some cycles.
Ovulation Calculator and Ovulation Calendar
Formula: ovulation date = cycle length minus 14 days, applied from the last period start date; the fertile window spans the five days before ovulation plus ovulation day, reflecting documented sperm survival (up to 5 days) and egg viability (12 to 24 hours). The calendar extends the same calculation across future cycles.
Accuracy and limitations: the estimate is only as good as the cycle-length average, and it assumes a consistent luteal phase. If you track OPKs or cervical mucus, those real-time signs will always beat a calendar estimate, which is why our tools cross-reference them.
Cervical Mucus Calculator
Method: this is a classification tool, not a date prediction. You describe today's observations (colour, texture, stretch) and the tool maps them to the standard four-stage mucus model (sticky, creamy, watery, egg-white) used in fertility-awareness methods, with a fertility level per stage. A safety pathway routes descriptions matching common infection patterns to medical guidance instead of a fertility reading.
Accuracy and limitations: the classification reflects the descriptions you choose; observations after sex, arousal or lubricant are easily confused and the tool says so. It cannot detect pregnancy, ovulation, or infection: it organises what you observed against established staging guidance.
Implantation Calculator
Formula: implantation window = ovulation date + 6 days to ovulation date + 12 days, with days 8 to 10 noted as most likely, consistent with the documented biological window. The IVF mode adjusts for embryo age: 5-day blastocysts implant roughly 1 to 3 days after transfer; 3-day embryos roughly 3 to 7 days after. Test-timing suggestions follow documented hCG doubling behaviour (roughly every 48 hours) after implantation.
Accuracy and limitations: the window is a biological range, not a personal measurement: most pregnancies fall inside it, but an individual embryo's timing cannot be known without clinical testing. IVF guidance from your clinic always takes priority.
Pregnancy Calculator
Formula: Naegele's rule: estimated due date = first day of last menstrual period minus 3 months plus 1 year and 7 days, equivalently LMP + 280 days. Current week and trimester are derived from the same date. This is the standard obstetric dating method used worldwide.
Accuracy and limitations: Naegele's rule assumes a 28-day cycle with ovulation on day 14; only about 4 to 5 percent of babies arrive exactly on the estimated date, and the honest full-term range is 37 to 42 weeks. Dating by early ultrasound is more accurate where available. The tool states this clearly in its results.
Late Period Calculator and Pregnancy Test Calculator
Method: these tools convert dates into decision timing. The late calculator measures days past your expected period and frames what is common at each stage of delay. The test calculator computes your current days past ovulation and maps them to documented hCG detectability, from undetectable before implantation through the reliable 14-DPO threshold.
Accuracy and limitations: detectability percentages are population-level approximations from published early-pregnancy testing data, not personal probabilities. A negative test before 14 DPO is genuinely inconclusive; the tools say so rather than implying certainty.
The Am I Late quiz
Method: a decision-tree guidance tool. Your five answers select one of several guidance branches (positive test, red-flag symptoms, extended delay, common late cycle), each mapping to advice consistent with NHS and ACOG patient guidance. It provides decision structure, not diagnosis: the output always names when professional assessment is the right next step.
Where our guidance comes from
Calculator methods and article content are checked against current patient guidance from these sources, linked inline on every article:
- NHS: Periods and menstrual health guidance
- ACOG: patient FAQ collections
- Cleveland Clinic: health library
- NICHD: Menstruation and Menstrual Problems
- Planned Parenthood: Learn
Review and update process
Every tool and article carries a visible last-updated or last-reviewed date. When guidance from the sources above changes, or when a reader reports an error, the affected page is updated and its date moves. Substantive article updates are marked in the page schema as well. See our editorial policy for how content is written, sourced and corrected, and our author page for who is responsible.
What our tools are not
No Cyluna tool diagnoses anything, replaces contraception guidance, or replaces a clinician. Results are estimates for education and planning. If a tool's output and your doctor disagree, your doctor wins. Our medical disclaimer covers this in full.