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Period Calculator

Medical review: Dr. Sarah Mitchell, MD · Board-certified OB-GYN with 15+ years of clinical experience · Updated March 17, 2026

Methodology and sources

Cycle-Length Focused

28 Day Cycle Calculator

Instantly find your ovulation day, fertile window, and next period for a 28-day menstrual cycle - the most common cycle length, calculated precisely for you.

📋 This calculator is pre-configured for a 28-day cycle. Just enter your last period date - everything else is already set.
FreePre-set for 28 daysPrivateNo sign-upInstant results

Medical review: Dr. Sarah Mitchell, MD · Board-certified OB-GYN · Updated March 2026

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How It Works

How It Works

Why 28 Days Is a Useful Reference Cycle

A 28-day cycle is the familiar reference pattern. It places estimated ovulation near Day 14 and the fertile window around Days 9-15 when a 14-day luteal phase is assumed.

A 28-day cycle is the classic reference cycle in many medical examples. Its special advantage is that ovulation day 14 falls exactly halfway through the cycle, which makes the fertile window easier to explain and remember.

A fixed cycle-length page like this works best when your timing is usually close to the same number of days each month. If your cycle shifts around more than that, move to the main period calculator or the irregular period calculator.

Why this cycle length matters

A 28-day cycle search is often looking for the standard calendar example: when ovulation happens, when fertile days start, and when the next period should arrive.

Timing shift

The advantage of a steady 28-day pattern is interpretability. The first half, ovulation day, and luteal phase are easy to map, which makes it useful for learning the formula.

Use it well

Use this page when your recent cycles cluster around 27-29 days. If your real pattern differs by more than a day or two, the adjustable period calculator will fit better.

When not to rely on it alone

Do not treat Day 14 as universal. Illness, stress, postpartum changes, PCOS, thyroid issues, or perimenopause can shift ovulation even if the previous cycle was 28 days.

The 28-Day Cycle Formula

Ovulation Day = Day 14
Fertile Window = Day 9 - Day 15
Peak Fertility = Day 12 - Day 14
Next Period = Day 29 (= Day 1 of next cycle)
Luteal Phase = Day 15 - Day 28

Formula: Ovulation = Cycle Length - Luteal Phase Length. For a 28 day cycle with a 14 day luteal phase, that means 28 - 14 = Day 14.

Signs of Ovulation on Day 14

Useful signs include egg-white cervical mucus, a positive LH test 24-36 hours earlier, a mild one-sided pelvic twinge, and a temperature rise that appears just after ovulation.

If you need more precision than a fixed cycle-length page can offer, compare this result with the ovulation calculator.

Frequently Asked Questions

Frequently Asked Questions

With a 28 day cycle and a typical 14-day luteal phase, ovulation is estimated around Day 14. Count Day 1 as the first day of bleeding.
The fertile window is usually Days 9-15. It includes the 5 days before ovulation, ovulation day, and the short post-ovulation buffer shown by this calculator.
The peak fertile days are Days 12-14. Intercourse before ovulation can be especially useful because sperm may already be present when the egg is released.
Yes. A 28-day cycle can fall within the typical adult range of about 21-35 days when the pattern is reasonably consistent.
The phases are the menstrual phase, follicular phase, ovulation, and luteal phase. In this 28 day cycle page, ovulation is centered on Day 14.
Count from the first day of one period to the first day of the next period for several cycles. If the number repeatedly matches this page, this pre-set calculator is a good fit.
Yes, pregnancy is possible if that day falls inside the fertile window. For a 28 day cycle, sperm survival means days before ovulation can still lead to conception.
This is the estimated ovulation day. The dominant follicle releases an egg, and the egg remains viable for about 12-24 hours.
Most calculators estimate ovulation by subtracting a 14-day luteal phase from the full cycle length. For 28 days, that gives Day 14.
Use this page as a quick reference, but use the main period calculator or ovulation calculator if your cycle length differs by more than a day or two.
Yes. Stress, illness, travel, sleep disruption, and medication changes can delay ovulation even in a usually regular cycle.
Around ovulation, cervical mucus often becomes clear, slippery, stretchy, and similar to egg white. This is one useful body sign to compare with calculator timing.

Related Tools

Related Tools

Ovulation Calculator

Track ovulation with a more adjustable ovulation-first tool.

Open Tool

Fertile Window Calculator

See fertile timing and conception probability in a dedicated view.

Open Tool

Period Tracker

Log cycle history locally and compare your real pattern over time.

Open Tool

Safe Period Calculator

Review lower-fertility and higher-fertility timing on the same calendar logic.

Open Tool

Due Date Calculator

Map the pregnancy timeline if this cycle leads to conception.

Open Tool

Other Cycle Lengths

Other cycle lengths

26-day cycle28-day cycle — you are here30-day cycle32-day cycle35-day cycle

Sources

Medical references and methodology

This page explains cycle timing for planning and education. It does not replace medical advice. Current editorial review remains aligned with the rest of the site: Board-certified OB-GYN with 15+ years of clinical experience. Last reviewed: March 17, 2026.