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Predict your next period start from your last period (LMP) and average cycle length. Also estimates ovulation (~14 days before next bleed) and a fertile window (5 days before ovulation through ovulation + 1 day). Pair with our ovulation window calculator. Educational—not for contraception.
Last updated: June 5, 2026
Calendar period prediction fails often as birth control. See a clinician for heavy bleeding, severe pain, fertility concerns, or contraception selection.
Days from period day 1 to day before next period. Default 28.
Full-flow bleeding days. Default 5.
Next period starts
Sun, March 29
Bleeding through Apr 2 (5 days)
Ovulation
Mar 15 (day 15)
Fertile window
Mar 10 – Mar 16
Next 3 cycles
Cycle 1
Mar 29 – Apr 2
Fertile Mar 10 – Mar 16
Cycle 2
Apr 26 – Apr 30
Fertile Apr 7 – Apr 13
Cycle 3
May 24 – May 28
Fertile May 5 – May 11
Model assumptions
Ovulation = next period − 14 days. Fertile = 5 days before ovulation through ovulation + 1 day. Not for contraception.
Default example: LMP March 1, 2026, 28-day cycle, 5-day period.
28-day cycle (default)
2026-03-29
Ovulation Mar 15 · Fertile Mar 10–Mar 16
30-day cycle
2026-03-31
Cycle day 17 ovulation
35-day cycle
2026-04-05
Next period ~5 weeks after LMP
21-day cycle
2026-03-22
Early ovulation—day 8
Calendar-day model with fixed 14-day luteal phase—not a hormonal simulation.
| Step | Formula | 28-day example |
|---|---|---|
| Next period start | Current cycle day 1 (LMP) + cycle length | LMP Mar 1 + 28 days → next period Mar 29 |
| Period end (bleeding) | Period start + (period duration − 1) | 5-day period starting Mar 29 → bleeding through Apr 2 |
| Ovulation (before next period) | Next period start − 14 days | Mar 29 next period → ovulation Mar 15 (cycle day 15) |
| Fertile window | Ovulation − 5 days through ovulation + 1 day | Mar 15 ovulation → fertile Mar 10–Mar 16 (7 calendar days) |
| Range | Label | Ovulation (model) |
|---|---|---|
| 21–24 days | Short | ~Day 8–11 |
| 25–30 days | Average | ~Day 12–17 |
| 31–35 days | Longer | ~Day 18–22 |
| >35 days or <21 | Review | Variable |
| Bleeding | Meaning | Note |
|---|---|---|
| 2–3 days | Short bleed | Can be normal; note if sudden change |
| 4–6 days | Typical | Most teaching references use ~5 days |
| 7+ days | Prolonged | Discuss with clinician if heavy or recurring |
| Spotting only | Not cycle day 1 | Count full-flow bleeding as day 1 of period |
Low estrogen/progesterone
Uterine lining sheds; day 1 = first full bleeding
Rising estrogen, FSH
Highly variable length—drives cycle-length differences
LH surge
Egg release; ~12–24 h fertilization window for the oocyte
Progesterone peak
Relatively stable 12–16 days; this tool assumes 14
LMP September 1, 2026, 30-day cycle, 5-day period—computed step by step:
Next period
Thu, October 1
Ovulation
Thu, September 17
Fertile window
Sep 12 – Sep 18
| Pattern | Action |
|---|---|
| Cycle <21 or >35 days | Track 3–6 months; thyroid, PCOS, perimenopause workup if persistent |
| Variation >7–9 days month to month | Calendar prediction unreliable—use LH/BBT or clinician review |
| Missed periods (amenorrhea) | Rule out pregnancy; evaluate if ≥3 missed or 90+ days without bleeding |
| Sudden heavy bleeding or clots | Urgent evaluation—do not rely on app forecasts alone |
| Sign | Urgency |
|---|---|
| Bleeding between periods or after sex | Schedule visit |
| Severe pain, fever, or foul discharge | Same-day / urgent care |
| Trying to conceive >12 months (<35) or >6 months (≥35) | Fertility evaluation |
| No period after stopping hormonal contraception >3 months | Clinician review |
This period calculator uses calendar arithmetic and a fixed 14-day luteal assumption. It does not measure hormones, predict pregnancy, or replace clinical care. Fertility awareness methods have high typical-use failure rates when used alone for contraception. For personalized guidance, consult ACOG resources or your healthcare provider.
Suggested hashtags: #WomensHealth #MenstrualCycle #Ovulation #TTC