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Ovulation & Fertile Window Calculator

Enter your last period start date and average cycle to see your ovulation day, fertile window, and next period on a 3-month calendar.

⚠️ For referenceThis tool is not a medical diagnosis or a contraceptive method; it is a reference estimate based on statistical averages. Actual ovulation varies with stress, sleep, weight, illness, and medication, and this cannot replace contraception. For pregnancy planning, irregular cycles, or infertility, consult an OB-GYN.

📝 Your Info

28d

Usually 21–35 days. Use the average of your recent months for better accuracy.

14d
5d

🔒 Your period dates are never sent to a server — they are stored only in this device (localStorage) and the share link (?s= token). The share link contains no words like 'period' or 'ovulation'.

🌸 Result

Today's phase

Ovulation day

Cycle day 15

Ovulation

Mon, Sep 7

D-DAY

Fertile window

Wed, Sep 2

~

Tue, Sep 8

Next period

Mon, Sep 21

D-14

💗 Peak fertilitySat, Sep 5 ~ Tue, Sep 8
🌱 ImplantationSun, Sep 13 ~ Sat, Sep 19
Intercourse during the fertile window — especially peak fertility (2 days before to the day of ovulation) — gives the highest chance of conception. Confirming ovulation with an LH test or basal body temperature improves accuracy.

🗓️ 3-Month Calendar

Period Fertile Peak Ovulation

September 2026

Su
Mo
Tu
We
Th
Fr
Sa
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30

October 2026

Su
Mo
Tu
We
Th
Fr
Sa
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31

November 2026

Su
Mo
Tu
We
Th
Fr
Sa
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30

How it's calculated

  • 🥚 Ovulation = next expected period − luteal phase (default 14 days). The luteal phase varies little between people, so it anchors the ovulation estimate.
  • 💗 Fertile window = 5 days before to 1 day after ovulation. Sperm survive up to ~5 days; the egg lives ~1 day after ovulation.
  • 🌱 Implantation ≈ 6–12 days after ovulation (avg 8–10).

Based on Wilcox et al. (NEJM 1995) and ACOG fertility-awareness methods. All values are estimates and assume a regular cycle.

Guide · FAQ · bal.pe.kr tools

For the detailed method and FAQ, see Guide · FAQ · About. Last updated 2026-07-11.