Ovulation calculator: fertile window and next cycle
Ovulation math counts from day one of bleeding: with a 28-day cycle starting 1 January 2026, ovulation lands on 15 January, the fertile window runs 10-16 January and the next cycle starts 29 January. A longer cycle only shifts ovulation later, the luteal tail stays near 14 days. This calculator is built for fast, private, offline use in your browser. Every formula runs locally and is verified against official sources. You enter the key numbers, see the result instantly and check the steps.
TL;DR
- Ovulation = last period + cycle − 14; fertile window = ovulation −5 to +1 days; next cycle = last period + cycle length.
- Formulas: ovulation = LMP + cycle − 14 | fertile = ovulation −5..+1
- Example 1: LMP 1 Jan 2026, 28-day cycle — Ovulation: 1 Jan + (28−14) = 15 Jan
How it works
Ovulation = last period + cycle − 14; fertile window = ovulation −5 to +1 days; next cycle = last period + cycle length. Start with clean data, keep the same units and follow the order in the formula. The tool does the arithmetic, you choose rate, period or factor. A second example with different numbers shows how the result scales.
The math
LMP 1 Jan 2026, 28 days: ovulation 1 Jan + (28−14) = 15 Jan, fertile 10-16 Jan, next 29 Jan. Same LMP, 35 days: ovulation 1 Jan + (35−14) = 22 Jan, fertile 17-23 Jan.
The 14-day luteal phase is the fixed part; cycle length moves ovulation.
Examples
Example 1: LMP 1 Jan 2026, 28-day cycle
- Ovulation: 1 Jan + (28−14) = 15 Jan
- Fertile: 10-16 Jan
- Next cycle: 1 Jan + 28 = 29 Jan
Example 2: same LMP, 35-day cycle
- Ovulation: 1 Jan + (35−14) = 22 Jan
- Fertile: 17-23 Jan
- Longer cycles push ovulation later
When to use it
✦ Trying to conceive
Time intercourse: daily in the 10-16 Jan window for a 28-day cycle.
✦ Avoiding fertile days
Know the 17-23 Jan window with a 35-day cycle, always confirmed with a second method.
⚠ Common mistakes
- Counting from period end: day one is the first bleeding day, not the last.
- 14-day rule for all: only the luteal tail is about 14 days, ovulation moves with cycle length.
- One cycle as proof: irregular cycles need 3+ months of history.
FAQ (6)
28-day cycle from 1 Jan?
Ovulation 15 Jan, fertile 10-16 Jan, next cycle 29 Jan.
Irregular cycles?
Track 3+ months; the calculator uses your average length.
What happens to my weight, height or other health data?
It never leaves your phone. BMI, BMR, body-fat — all computed on-device, no upload, no account. That's intentional for health data: you can check a BMI of 22.8 or a TDEE of 2,100 kcal without worrying about a server log. Closing the tab clears the form from memory.
Does it work offline at the gym or on a plane?
Yep. Once the page has loaded, the calculators are fully offline-first. I've used the pace and BMI tools in airplane mode between flights — no spinner, no retry, just instant numbers. Same for sleep cycles if you're planning a shift.
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.
Found an error? Contact us — verified corrections ship within 24 hours.