Luteal Phase Calculator — Ovulation & Cycle Timing
Enter the first day of your last period, your total cycle length, and your typical luteal phase length to see your estimated ovulation date, luteal phase dates, approximate fertile window, and next period start date.
First day of last period
days
days
Thu, Jan 15, 2026
How does this calculator work?
Ovulation date = last period start + (cycle length − luteal phase length). The luteal phase typically lasts 12–14 days. Next period = last period start + cycle length. Estimates vary with individual cycle irregularity — these dates are guidance only.
Formula
How this is calculated
The menstrual cycle has two phases: the follicular phase (from the period start to ovulation) and the luteal phase (from ovulation to the next period). Unlike the follicular phase, which varies considerably in length, the luteal phase is relatively constant — typically 12–14 days — because it is governed by the fixed lifespan of the corpus luteum (the temporary gland that forms after ovulation and produces progesterone).
This calculator uses that principle to work backwards: given cycle length and luteal length, the follicular phase length = cycle − luteal. Adding that to the last period start date gives the estimated ovulation date. The luteal phase then runs from ovulation to the first day of the next period, which is simply the period start plus the full cycle length.
The fertile window is shown as approximately 4 days before ovulation through ovulation day itself (based on sperm survival of up to 5 days and egg viability of about 24 hours). These are estimates only: actual ovulation timing can vary from cycle to cycle due to stress, illness, travel, and other factors. This calculator is for awareness and family planning education, not medical advice or contraception.
Frequently asked questions
The luteal phase is the second half of the menstrual cycle, from ovulation until the next period begins. It is driven by progesterone from the corpus luteum. A normal luteal phase lasts 12–14 days; a phase shorter than 10 days is sometimes called a "luteal phase defect" and may affect fertility — speak to a doctor if you suspect this.
You can estimate it by tracking basal body temperature (BBT) — the temperature rise after ovulation marks the start of the luteal phase. You can also use LH ovulation predictor kits and count the days to your next period. Most people have a luteal phase of 12–14 days.
The estimates here are educational only. Cycle-based methods (fertility awareness methods) require careful daily tracking and training to use effectively, and are not foolproof. Consult a healthcare provider or certified fertility educator before relying on cycle tracking for contraception or conception planning.
Also known as
TG we-Calculate Editorial Team. (2026). Luteal Phase Calculator — Ovulation & Cycle Timing [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/luteal-phase-calculator
TG we-Calculate Editorial Team. "Luteal Phase Calculator — Ovulation & Cycle Timing." TG we-Calculate. 2026. https://we-calculate.com/calculator/luteal-phase-calculator.
TG we-Calculate Editorial Team, "Luteal Phase Calculator — Ovulation & Cycle Timing," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/luteal-phase-calculator
@misc{wecalculate_luteal_phase_calculator, title = {Luteal Phase Calculator — Ovulation & Cycle Timing}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/luteal-phase-calculator}}, year = {2026}, note = {TG we-Calculate} }
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