Methodology

Every HerCalc calculator is built on a published formula. This page documents the math, the assumptions, the limitations, and the studies behind each tool. If a method is unreliable for a particular user, we say so on the tool itself — but here you can read the full reasoning.

Period & cycle prediction

The Period Calculator predicts your next six cycles, ovulation date, fertile window, and luteal phase. The default assumption is a 28-day cycle with ovulation 14 days before the next period (the luteal-phase rule). When you enable Irregular or PCOS mode, the calculator switches to an evidence-based variant:

• Standard mode (regular cycles 24–35 days): Cycle length you provide; ovulation = next period − 14 days; fertile window = ovulation − 5 days through ovulation + 1 day.
• Irregular mode: Calculator uses the median (not mean) of your last 3–6 cycle lengths and reports a confidence range based on standard deviation. Fertile window is widened by ±2 days.
• PCOS mode: Luteal phase extended to 16 days (some PCOS users have a longer luteal phase). Anovulatory-cycle warning when stdDev exceeds 7 days. Fertile window is approximated, not predicted.

References: ACOG Practice Bulletin 651 (Menstruation in Girls and Adolescents); Bull JR et al. NPJ Digital Medicine 2019 — "Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles."

Ovulation & fertile window

The Ovulation Calculator uses the same luteal-phase logic as the Period Calculator and produces a 14-day fertility heatmap. Daily fertility scores follow a bell-shaped distribution centered on ovulation:

• Ovulation day: 100 (peak)
• Day −1: 88; Day −2: 88; Day −3: 70; Day −4: 50; Day −5: 30 (sperm survival edge)
• Day +1: 55 (egg viability tail)

These scores are derived from the classic Wilcox et al. (1995, NEJM) study showing that conception occurs almost exclusively in the six-day window ending on the day of ovulation, with probability peaking on the day before and the day of ovulation.

Safe-period (rhythm method)

The Safe-Period calculator applies the Knaus–Ogino rhythm method, widened by your reported cycle variation. We display two reliability bands:

• Pre-ovulation safe (low reliability): from end of period to one day before the earliest possible fertile day.
• Post-ovulation safe (moderate reliability): from one day after the latest possible fertile day to the day before next period.

The calendar method has a typical-use failure rate of approximately 24% per year (Trussell J. Contraception 2011). It is significantly less reliable than condoms, IUDs, or hormonal birth control. We display this prominently and recommend the symptothermal method (BBT + cervical mucus + LH testing) for users who want fertility-awareness contraception.

Due date

The Due Date Calculator supports five dating methods. In order of accuracy:

1. IVF transfer date (most precise): EDD = transfer date + (266 − embryo age in days). For a Day-5 blastocyst transfer, that's transfer + 261 days. For Day-3, transfer + 263. For Day-6, transfer + 260. Implemented internally as LMP = transfer − 17/19/20 days, then EDD = LMP + 280.
2. First-trimester ultrasound (CRL): Robinson–Fleming formula — gestational age in days = 8.052 × √CRL(mm) + 23.73. Most accurate at 7–12 weeks (±5 days). After 22 weeks, ultrasound dating carries ±10–14 day uncertainty.
3. Conception date: EDD = conception + 266 days. Reliable only when ovulation was confirmed (LH test, BBT chart, or known intercourse window).
4. LMP (Naegele's rule): EDD = LMP + 280 days. Adjusted for non-28-day cycles by adding (cycle − 28) days. Assumes regular ovulation on day 14.
5. Reverse from known EDD: LMP = EDD − 280 days; conception = EDD − 266.

References: Robinson HP, Fleming JE. BJOG 1975; ACOG Committee Opinion 700 (Methods for Estimating the Due Date); ASRM Practice Committee 2017 — IVF dating.

Pregnancy week-by-week

The Pregnancy Week tool computes weeks + days from your reference (LMP, EDD, or conception) and surfaces a fetal snapshot at that gestational age. Length and weight estimates are derived from Hadlock fetal growth curves (Hadlock FP et al. Radiology 1991), with crown-rump length below week 14 and crown-heel length thereafter. Fruit-likeness mappings are commonly cited approximate sizes, included for intuition only.

BMI for women

BMI = weight (kg) / [height (m)]². We use seven WHO categories:

• <16 — Underweight (severe)
• 16.0–16.9 — Underweight (moderate)
• 17.0–18.4 — Underweight (mild)
• 18.5–24.9 — Healthy weight
• 25.0–29.9 — Overweight
• 30.0–34.9 — Obesity class I
• 35.0–39.9 — Obesity class II
• ≥40 — Obesity class III

BMI alone is not enough for women. When a waist measurement is provided, we also compute waist-to-hip ratio (WHR) and flag elevated cardiometabolic risk per WHO 2008 thresholds (WHR ≥ 0.85 for women). When pregnant, postpartum, or on hormone replacement, we suppress BMI categorization and show life-stage-specific guidance instead.

Body shape (FFIT-inspired)

The Body Shape Calculator uses bust, waist, and hip measurements (and optionally high-hip) to classify into seven shapes: Hourglass, Pear, Apple, Rectangle, Inverted triangle, Oval, Diamond. Classification rules follow the Female Figure Identification Technique (Lee J-Y et al., International Journal of Clothing Science 2007) with thresholds adapted from Simmons KP et al. Journal of Textile and Apparel 2004:

• Defined waist requires (bust − waist) ≥ 22 cm AND (hip − waist) ≥ 22 cm.
• Bust/hip balanced if |bust − hip| ≤ 5 cm.
• Apple distinguished from rectangle by waist/hip ≥ 0.95.

Data, accuracy, and updates

All calculations happen client-side. We store nothing. When we update a formula, threshold, or study reference, we add a "Last reviewed" date to the tool page and an entry to its corresponding blog post if one exists.

Medical disclaimer

HerCalc is for educational use. It does not replace a clinician, a dating ultrasound, or a personalized treatment plan. If you are pregnant or trying to conceive, your prenatal-care provider's dating decision is authoritative.