health calculator

Menstrual Cycle / Period Calculator

Predict your next period, estimated ovulation day, and fertile window from your last period start date and average cycle length.

Next period is predicted as last period start date plus average cycle length. Ovulation is estimated by counting back 14 days from that predicted next period, since the luteal phase is more consistent (~14 days) than the more variable follicular phase. This is a general estimate, not a contraception or conception-planning tool.

Formula Verified by Dr. Sarah Jenkins, PhD

Quick actions: Save inputs, view saved runs, print, share, or export results

Saved Runs

Inputs

Last period start date

Average cycle length (days)

Typical period length (days)

Results

Predicted Next Period

Sep 12, 2026

Period Estimated ThroughAug 19, 2026

Estimated Ovulation DayAug 29, 2026

Estimated Fertile WindowAug 24, 2026 - Aug 29, 2026

Tip

Save this page to check your estimated next period date and cycle day without re-entering your dates each time.

Formula

Cycle prediction method

Next Period = Last Period Start + Cycle Length. Ovulation = Next Period − 14 days. Fertile Window = Ovulation − 5 days through Ovulation.

Ovulation is estimated backward from the predicted next period, since the luteal phase (~14 days) is more consistent than the follicular phase, which varies with total cycle length.

Step by step

How the calculation works

1Enter the date your last period started.
2Enter your typical cycle length (21-45 days) and period length.
3Review the predicted next period, estimated ovulation day, and rough fertile window.
4Treat all estimates as a window, not an exact date, especially if your cycles vary month to month.

Example

Worked example: 30-day cycle

Last period started March 1 with a 30-day average cycle: next period predicted around March 31. Counting back 14 days puts estimated ovulation around March 17, with a fertile window estimate of roughly March 12-17.

Related guides

Learn more about this calculation

Guide

Ovulation Tracking: How to Identify Your Fertile Window

Learn how ovulation tracking works, what the fertile window really means, and how to use calendar, BBT, mucus, and LH methods carefully.

Guide

How to Use Do The Calculation Calculators: A Practical Step-by-Step Guide

Learn the fastest reliable workflow for using Do The Calculation calculators, reading results, checking formulas, and using save, print, share, and export actions correctly.

Menstrual Cycle / Period Calculator

Predict your next period, estimated ovulation day, and fertile window from your last period start date and average cycle length.

Formula Verified by Dr. Sarah Jenkins, PhD

How to understand and use this calculator

What this tool estimates, and what it deliberately doesn't

This calculator predicts your next period's likely start date, an estimated fertile window, and your current day within the cycle, based on the date your last period started and your typical cycle length. It is a factual cycle-tracking estimate, not a fertility planning or contraception tool — predictions here are useful for anticipating a period, not for timing or avoiding conception. If pregnancy planning is what you're actually trying to do, the Ovulation Calculator and Pregnancy Conception Calculator are built specifically for that and go into more relevant detail than this tool does.

Why ovulation is estimated backward from the next period, not forward from the last one

A common oversimplification is assuming ovulation always happens 'around day 14' counted forward from the start of the last period — this only holds for an exact 28-day cycle. The more reliable method counts backward from the predicted start of the NEXT period, because the luteal phase (from ovulation to the next period) is fairly consistent at about 14 days across most cycle lengths, while the follicular phase (from the last period to ovulation) is the part that actually varies cycle to cycle. That means someone with a 35-day cycle likely ovulates around day 21, not day 14 — counting forward from the last period would put the estimate off by a full week. The basal body temperature calculator applies this same backward logic from an actual observed temperature shift rather than an assumed date, if you're charting alongside cycle tracking.

What counts as a typical cycle length

A cycle is measured from the first day of one period to the first day of the next. Cycle lengths of 21 to 35 days are both considered typical, with 28 days being the commonly cited average — but 28 days is a population average, not the number any individual is expected to match exactly. Cycle length can also shift over time due to age, stress, activity level, or hormonal changes, which is part of why this calculator asks for your own average rather than assuming 28.

When predictions become unreliable

Cycle-length-based prediction assumes reasonably regular cycles. It becomes meaningfully less reliable with irregular cycles, conditions like PCOS, recent hormonal birth control changes, postpartum recovery, or perimenopause — all of which can shift timing in ways a simple average can't anticipate. If your cycles vary by more than about a week from month to month, treat any prediction here as a rough window rather than a specific date, and consider discussing persistent irregularity with a doctor.

Worked example

Last period started March 1, with a 30-day average cycle: the next period is predicted around March 31 (March 1 + 30 days). Counting back 14 days from that predicted date puts estimated ovulation around March 17, with a fertile window estimate spanning roughly March 12-17 (the days leading up to and including estimated ovulation, since sperm can survive several days while the egg survives about 24 hours after release).

Related concepts

Cycle length

The number of days from the first day of one period to the first day of the next; 21-35 days is typical.

Luteal phase

The relatively consistent ~14-day span from ovulation to the start of the next period, used to estimate ovulation backward from a predicted period date.

Fertile window

The estimated days around ovulation when conception is possible, accounting for sperm survival time before ovulation and egg viability after.

Frequently Asked Questions

How accurate is this calculator?+

For a regular cycle, next-period predictions are typically accurate within a few days. Ovulation and fertile-window estimates are inherently rougher, since they depend on assumptions about luteal phase length that vary somewhat between individuals.

Can I use this as birth control or to plan a pregnancy?+

No — this tool is for general cycle awareness, not contraception or conception planning. Use the Ovulation Calculator or Pregnancy Conception Calculator for those specific purposes, and talk to a doctor about reliable contraception or fertility planning methods.

Why does the calculator count backward from the next period for ovulation instead of forward from the last one?+

Because the luteal phase (ovulation to next period) is more consistent, around 14 days, across different cycle lengths than the follicular phase (last period to ovulation) is. Counting backward from the predicted next period gives a more reliable ovulation estimate, especially for cycles longer or shorter than 28 days.

What's a normal cycle length?+

21 to 35 days is generally considered typical, with 28 days as a commonly cited average — but average doesn't mean expected for every individual.

What if my cycles are irregular?+

Predictions become less reliable the more your cycle length varies month to month. If your cycles are consistently irregular, this calculator's estimates should be treated as a loose window, and it's worth discussing the irregularity with a doctor.

Does stress or illness affect these predictions?+

Yes. Stress, illness, travel, significant weight changes, and other disruptions can shift ovulation timing and period onset in ways that a simple average-based calculation can't anticipate for that specific cycle.

What's the difference between this and the Ovulation Calculator?+

This tool gives a general overview of your cycle (next period, current cycle day, rough fertile window). The Ovulation Calculator is purpose-built around conception timing specifically and covers that use case in more depth.

Cycle prediction method

Next Period = Last Period Start + Cycle Length. Ovulation = Next Period − 14 days. Fertile Window = Ovulation − 5 days through Ovulation.

Ovulation is estimated backward from the predicted next period, since the luteal phase (~14 days) is more consistent than the follicular phase, which varies with total cycle length.

How to understand and use this calculator

What this tool estimates, and what it deliberately doesn't

Why ovulation is estimated backward from the next period, not forward from the last one

What counts as a typical cycle length

When predictions become unreliable

Worked example

Related concepts

Cycle length

The number of days from the first day of one period to the first day of the next; 21-35 days is typical.

Luteal phase

The relatively consistent ~14-day span from ovulation to the start of the next period, used to estimate ovulation backward from a predicted period date.

Fertile window

The estimated days around ovulation when conception is possible, accounting for sperm survival time before ovulation and egg viability after.

Frequently Asked Questions

How accurate is this calculator?+

Can I use this as birth control or to plan a pregnancy?+

Why does the calculator count backward from the next period for ovulation instead of forward from the last one?+

What's a normal cycle length?+

21 to 35 days is generally considered typical, with 28 days as a commonly cited average — but average doesn't mean expected for every individual.

What if my cycles are irregular?+

Does stress or illness affect these predictions?+

What's the difference between this and the Ovulation Calculator?+

Menstrual Cycle / Period Calculator

Inputs

Results

Cycle prediction method

How the calculation works

Worked example: 30-day cycle

Learn more about this calculation

Ovulation Tracking: How to Identify Your Fertile Window

How to Use Do The Calculation Calculators: A Practical Step-by-Step Guide

Menstrual Cycle / Period Calculator

Inputs

Results

Key output metrics

How to understand and use this calculator

What this tool estimates, and what it deliberately doesn't

Why ovulation is estimated backward from the next period, not forward from the last one

What counts as a typical cycle length

When predictions become unreliable

Worked example

Related concepts

Frequently Asked Questions

Cycle prediction method

How the calculation works

Worked example: 30-day cycle

Learn more about this calculation

Ovulation Tracking: How to Identify Your Fertile Window

How to Use Do The Calculation Calculators: A Practical Step-by-Step Guide

Key output metrics

How to understand and use this calculator

What this tool estimates, and what it deliberately doesn't

Why ovulation is estimated backward from the next period, not forward from the last one

What counts as a typical cycle length

When predictions become unreliable

Worked example

Related concepts

Frequently Asked Questions