Ovulation and fertile window
Ovulation ~14 days before next period, not after last. Fertile window 5 days before + ovulation day =6 days due to sperm survival and egg lifespan.
TL;DR
- Next period = LMP + cycle length.
- Formulas: Ovulation = LMP + (Cycle -14) days | Fertile = Ovulation ±5/1
- LMP Jan1 28 — Ovulation Jan15
How it works
Next period = LMP + cycle length. Ovulation = next period -14 days = LMP + (cycle-14). Fertile = ovulation -5 to +1 days. Ovulation comes about 14 days before the next period, not 14 days after the last one — that shift is why a 35-day cycle ovulates on day 21, not 14. Th
The math
Cycle 28 → ovulation LMP+14. Cycle 32 → LMP+18. LMP Jan1 28 → ovulation Jan15, fertile Jan10-16.
Subtract 14 from next period, not add 14 to LMP blindly.
Examples
LMP Jan1 28
- Ovulation Jan15
- Fertile Jan10-16
- Next Jan29
Cycle 32
- Ovulation LMP+18
When to use it
✦ Conception
Time intercourse.
✦ Contraception awareness
Avoid window (not standalone).
⚠ Common mistakes
- Assuming day 14 always
- Ignoring cycle variability
FAQ (6)
Accuracy?
Calendar estimate, confirm with LH tests.
Pregnant outside window?
Very unlikely, sperm 5d, egg 12-24h.
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Can my doctor rely on these numbers?
They're solid estimates using published formulas (WHO for BMI, Mifflin-St Jeor for BMR, CKD-EPI for GFR) but they're not a diagnosis. Think of them as a conversation starter: good for tracking trends, but your GP or nephrologist should interpret GFR, heart age or pregnancy dates with labs and context. We link the sources at the bottom for that reason.
How can I share my result with a trainer or doctor?
Use 'Share Link' in the results panel. It builds a URL with your inputs — for example BMI 70kg/175cm — so they see exactly what you entered. No data is stored on our side, the link just carries the numbers.
Sources & References
⚠ Health Disclaimer
This tool provides estimates for informational purposes only and does not constitute medical, veterinary, or professional advice. Always consult a qualified healthcare professional before making decisions about your health or treatment.
How this calculator is maintained
This calculator is written and maintained by Davide Coronese, independent developer and founder of CalcTracker.
Methodology: every formula is checked against official publications and validated by an automated test suite before release. Health, tax and finance pages cite the exact sources they rely on.
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