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About this tool
Work out luteal phase length from your ovulation date and the next period start date, with follicular and cycle length too.
The Luteal Phase Length Calculator measures the second half of your cycle — the days from ovulation to the start of your next period — from two dates you already have on your chart. It uses the standard fertility-charting rule that the luteal phase runs from the day after ovulation up to and including the day before the period, so a textbook 28-day cycle with ovulation on day 14 returns 14 days. Add the previous period start date and it also returns follicular phase length and total cycle length, which always add up to the cycle.
Open Luteal Phase Length Calculator on AltFTool — it loads instantly in your browser.
Enter the values you already know.
Fine-tune the options to match your scenario.
Read the result and use it in your planning or reporting.
Both conventions are shown, so your figure matches whichever your charting app uses.
Follicular and luteal lengths are reported separately and always sum to the cycle length.
Nothing is assumed about ovulation timing — it comes from the date you actually confirmed.
About 12 to 14 days is most common, and roughly 11 to 17 days is generally described as normal. A luteal phase under 10 days is called short; whether that alone reduces the chance of conception is still debated, so it is worth discussing with a clinician rather than acting on one cycle.
Count from the day after ovulation up to and including the day before your period starts. With ovulation on cycle day 14 and the next period on cycle day 29, the luteal phase covers days 15 to 28, which is 14 days. Some apps count ovulation day itself as luteal day 1 and report one day more.
It is the more stable half of the cycle — most of the variation in total cycle length comes from the follicular phase, because ovulation can happen earlier or later. That is why an unusually long or short cycle is usually explained by when you ovulated, not by the luteal phase.
If your period has not arrived by about 18 days past ovulation, a home test is the usual next step, because a luteal phase that long without bleeding is uncommon. This tool is informational and is not a substitute for advice from a doctor or fertility specialist.
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