Ovulation Calculator
Predicting Forward or Checking Backward
There are two natural ways to ask when ovulation happened: forward from the last period, using how long your cycles typically run, or backward from when the next period is expected, using how long your luteal phase typically runs. Both methods locate the same event through different anchors, and this calculator supports either one depending on which date you have more confidence in.
The Formulas
From next expected period: Ovulation Day = Next Expected Period Date − Luteal Phase Length
Both formulas isolate the same quantity — the follicular phase length — from opposite ends. The first assumes a standard 14-day luteal phase; the second lets you specify your actual luteal phase length if you know it, which is generally the more stable number cycle to cycle.
Two Routes to the Same Estimate
| Mode | Required input | Assumes |
|---|---|---|
| From last period | Last period start date, cycle length | 14-day luteal phase |
| From next expected period | Next expected period date, luteal phase length | Your specified luteal phase, not 14 days |
If your luteal phase isn't close to 14 days, the second mode is the more accurate of the two — it doesn't force that assumption.
When Each Mode Fits
- From last period — the simpler option when you're not tracking luteal phase length and just want a forward estimate from your last period.
- From next expected period — better if you've been charting basal body temperature or cervical mucus over several cycles and know your actual luteal phase length, since it removes the standard 14-day assumption entirely.
How to Use This Calculator
- Select a mode: From Last Period Date or From Next Expected Period Date.
- For the first mode, enter the Last Period Start Date and Average Cycle Length (days) (default 28, range 20–45).
- For the second mode, enter the Next Expected Period Date and Luteal Phase Length (days) (default 14, range 8–20).
- Select Calculate for the estimated ovulation day and fertile window.
Related Calculations
For the single-mode version of this calculation with a matching cycle-length breakdown table, see the Ovulation Calculator. Once you have a likely conception date, run it through the Conception Calculator.