How is ovulation estimated?

Ovulation is approximated as 14 days before your next predicted period. The fertile window spans about 5 days before ovulation through 1 day after.

What cycle length should I enter?

Use the average length across several recent cycles. Common ranges are 21–35 days, with 28 days being a typical reference.

Can I use this tool as birth control?

No. The calculator provides educational estimates and is not a medical device. For contraception, speak with a clinician about appropriate methods.

What if I have irregular periods?

Use the cycle variation field to model earlier or later dates. Persistent irregularity warrants medical evaluation.

Does the period length affect fertility predictions?

Period length affects period window estimates but not the ovulation placement, which is calculated from the next predicted period start.

Period Calculator

Q: How accurate is this period calculator?

It uses standard calendar-based estimates, which are most accurate when your cycles are regular and you provide a representative average cycle length. Cycle variability, illness, stress, medications, and life changes can shift dates.

Professional period calculator to predict your next period, ovulation day, and fertile window based on your last period and average cycle length. Accurate, accessible, and mobile-friendly.

Period Calculator

This professional-grade period calculator predicts your next period start date, ovulation day, and fertile window using the first day of your last period (LMP), your average cycle length, and your typical period length. It is designed for people who menstruate and want reliable, accessible, mobile-first estimates to plan ahead. Educational use only.

Interactive Calculator

Last period start date (LMP) *

Average cycle length (days) *

Period length (days) *

Cycle variation (± days) [optional]

Results

Ready

Next period (predicted start)

—

Predicted period window

—

Ovulation (estimated)

—

Fertile window

—

Upcoming cycles

Next 6 starts

This tool provides educational estimates only and is not a medical device. If your cycles are consistently irregular, missing, unusually heavy, or painful, consult a qualified clinician.

Data Source and Methodology

Authoritative references:

American College of Obstetricians and Gynecologists (ACOG). “Your Menstrual Cycle.” Reviewed 2022. acog.org/womens-health/faqs/your-menstrual-cycle
Centers for Disease Control and Prevention (CDC). “Fertility Awareness-Based Methods.” Updated 2023. cdc.gov/contraception/about/fertility-awareness.html

Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.

The Formula Explained

1) Next period start: D_next = D_LMP + CL

2) Period window: [D_next, D_next + (PL - 1)]

3) Ovulation (estimate): O = D_next - 14

4) Fertile window: [O - 5, O + 1]

5) With cycle variation ±v: D_next ∈ [D_LMP + (CL - v), D_LMP + (CL + v)]

Where D_LMP is the first day of your last menstrual period, CL is your average cycle length in days, PL is your typical period length, and v is the cycle variation you choose to account for early/late shifts.

Glossary of Variables

LMP (Last Menstrual Period): The first day of your last period.
Average cycle length (CL): Number of days from day 1 of one period to day 1 of the next (typical range: 21–35 days).
Period length (PL): Typical number of days you bleed during a period (often 3–7 days).
Cycle variation (v): Optional ± days to model early or late cycles; increases the output ranges.
Next period start (D_next): The predicted date your next period begins.
Ovulation (O): Estimated as 14 days before the next predicted period start.
Fertile window: The 6-day interval: 5 days before ovulation through 1 day after.

How It Works: A Step-by-Step Example

Suppose your LMP was March 3, 2025, your average cycle length is 30 days, your period lasts 5 days, and you choose a cycle variation of ±2 days.

Next period start: D_next = March 3 + 30 = April 2, 2025. With ±2, range is March 31–April 4.
Period window: April 2–April 6, 2025. With range, March 31–April 4 start, each lasting 5 days.
Ovulation: O = D_next − 14 = March 19, 2025. With ±2, range March 17–March 21.
Fertile window: March 14–March 20, 2025 (range: March 12–March 22).

Note: These are calendar-based estimates that can shift with stress, illness, medication, or life changes.

Frequently Asked Questions (FAQ)

How accurate is this period calculator?

It is most accurate for regular cycles using a representative average cycle length. Variability and health factors can shift dates.

Why is ovulation placed 14 days before my period?

The luteal phase (ovulation to next period) averages around 14 days for many people, which underpins calendar-based estimates.

What if I do not remember my exact LMP?

Use your best estimate and consider a larger cycle variation to widen the result ranges.

Can this replace medical advice or contraception?

No. This tool is for education only. Consult a clinician for diagnosis, treatment, or contraception guidance.

How many future cycles can I view?

The calculator shows the next six predicted starts. You can adjust inputs to see how results change.

Should I track symptoms as well?

Tracking symptoms (e.g., cervical mucus, basal body temperature) may improve personal accuracy but is outside this simple calendar model.

Audit: Complete

Formula (LaTeX) + variables + units

This section shows the formulas used by the calculator engine, plus variable definitions and units.

Formula (extracted LaTeX)

\[','\]

','

Formula (extracted text)

1) Next period start: D_next = D_LMP + CL 2) Period window: [D_next, D_next + (PL - 1)] 3) Ovulation (estimate): O = D_next - 14 4) Fertile window: [O - 5, O + 1] 5) With cycle variation ±v: D_next ∈ [D_LMP + (CL - v), D_LMP + (CL + v)]

Variables and units

No variables provided in audit spec.

Sources (authoritative):

cdc.gov/contraception/about/fertility-awareness.html — cdc.gov · Accessed 2026-01-19
https://www.cdc.gov/contraception/about/fertility-awareness.html
Health and Fitness — calcdomain.com · Accessed 2026-01-19
https://calcdomain.com/health-fitness
acog.org/womens-health/faqs/your-menstrual-cycle — acog.org · Accessed 2026-01-19
https://www.acog.org/womens-health/faqs/your-menstrual-cycle

Changelog

Version: 0.1.0-draft
Last code update: 2026-01-19

0.1.0-draft · 2026-01-19

Initial audit spec draft generated from HTML extraction (review required).
Verify formulas match the calculator engine and convert any text-only formulas to LaTeX.
Confirm sources are authoritative and relevant to the calculator methodology.

Verified by Ugo Candido on 2026-01-19
Profile · LinkedIn

Full original guide (expanded)

Period Calculator

Interactive Calculator

Last period start date (LMP) *

Average cycle length (days) *

Period length (days) *

Cycle variation (± days) [optional]

Results

Ready

Next period (predicted start)

—

Predicted period window

—

Ovulation (estimated)

—

Fertile window

—

Upcoming cycles

Next 6 starts

This tool provides educational estimates only and is not a medical device. If your cycles are consistently irregular, missing, unusually heavy, or painful, consult a qualified clinician.

Data Source and Methodology

Authoritative references:

American College of Obstetricians and Gynecologists (ACOG). “Your Menstrual Cycle.” Reviewed 2022. acog.org/womens-health/faqs/your-menstrual-cycle
Centers for Disease Control and Prevention (CDC). “Fertility Awareness-Based Methods.” Updated 2023. cdc.gov/contraception/about/fertility-awareness.html

Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.

The Formula Explained

1) Next period start: D_next = D_LMP + CL

2) Period window: [D_next, D_next + (PL - 1)]

3) Ovulation (estimate): O = D_next - 14

4) Fertile window: [O - 5, O + 1]

5) With cycle variation ±v: D_next ∈ [D_LMP + (CL - v), D_LMP + (CL + v)]

Glossary of Variables

LMP (Last Menstrual Period): The first day of your last period.
Average cycle length (CL): Number of days from day 1 of one period to day 1 of the next (typical range: 21–35 days).
Period length (PL): Typical number of days you bleed during a period (often 3–7 days).
Cycle variation (v): Optional ± days to model early or late cycles; increases the output ranges.
Next period start (D_next): The predicted date your next period begins.
Ovulation (O): Estimated as 14 days before the next predicted period start.
Fertile window: The 6-day interval: 5 days before ovulation through 1 day after.

How It Works: A Step-by-Step Example

Suppose your LMP was March 3, 2025, your average cycle length is 30 days, your period lasts 5 days, and you choose a cycle variation of ±2 days.

Next period start: D_next = March 3 + 30 = April 2, 2025. With ±2, range is March 31–April 4.
Period window: April 2–April 6, 2025. With range, March 31–April 4 start, each lasting 5 days.
Ovulation: O = D_next − 14 = March 19, 2025. With ±2, range March 17–March 21.
Fertile window: March 14–March 20, 2025 (range: March 12–March 22).

Note: These are calendar-based estimates that can shift with stress, illness, medication, or life changes.

Frequently Asked Questions (FAQ)

How accurate is this period calculator?

It is most accurate for regular cycles using a representative average cycle length. Variability and health factors can shift dates.

Why is ovulation placed 14 days before my period?

The luteal phase (ovulation to next period) averages around 14 days for many people, which underpins calendar-based estimates.

What if I do not remember my exact LMP?

Use your best estimate and consider a larger cycle variation to widen the result ranges.

Can this replace medical advice or contraception?

No. This tool is for education only. Consult a clinician for diagnosis, treatment, or contraception guidance.

How many future cycles can I view?

The calculator shows the next six predicted starts. You can adjust inputs to see how results change.

Should I track symptoms as well?

Tracking symptoms (e.g., cervical mucus, basal body temperature) may improve personal accuracy but is outside this simple calendar model.

Audit: Complete

Formula (LaTeX) + variables + units

This section shows the formulas used by the calculator engine, plus variable definitions and units.

Formula (extracted LaTeX)

\[','\]

','

Formula (extracted text)

1) Next period start: D_next = D_LMP + CL 2) Period window: [D_next, D_next + (PL - 1)] 3) Ovulation (estimate): O = D_next - 14 4) Fertile window: [O - 5, O + 1] 5) With cycle variation ±v: D_next ∈ [D_LMP + (CL - v), D_LMP + (CL + v)]

Variables and units

No variables provided in audit spec.

Sources (authoritative):

cdc.gov/contraception/about/fertility-awareness.html — cdc.gov · Accessed 2026-01-19
https://www.cdc.gov/contraception/about/fertility-awareness.html
Health and Fitness — calcdomain.com · Accessed 2026-01-19
https://calcdomain.com/health-fitness
acog.org/womens-health/faqs/your-menstrual-cycle — acog.org · Accessed 2026-01-19
https://www.acog.org/womens-health/faqs/your-menstrual-cycle

Changelog

Version: 0.1.0-draft
Last code update: 2026-01-19

0.1.0-draft · 2026-01-19

Initial audit spec draft generated from HTML extraction (review required).
Verify formulas match the calculator engine and convert any text-only formulas to LaTeX.
Confirm sources are authoritative and relevant to the calculator methodology.

Verified by Ugo Candido on 2026-01-19
Profile · LinkedIn

Period Calculator

Interactive Calculator

Last period start date (LMP) *

Average cycle length (days) *

Period length (days) *

Cycle variation (± days) [optional]

Results

Ready

Next period (predicted start)

—

Predicted period window

—

Ovulation (estimated)

—

Fertile window

—

Upcoming cycles

Next 6 starts

This tool provides educational estimates only and is not a medical device. If your cycles are consistently irregular, missing, unusually heavy, or painful, consult a qualified clinician.

Data Source and Methodology

Authoritative references:

American College of Obstetricians and Gynecologists (ACOG). “Your Menstrual Cycle.” Reviewed 2022. acog.org/womens-health/faqs/your-menstrual-cycle
Centers for Disease Control and Prevention (CDC). “Fertility Awareness-Based Methods.” Updated 2023. cdc.gov/contraception/about/fertility-awareness.html

Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.

The Formula Explained

1) Next period start: D_next = D_LMP + CL

2) Period window: [D_next, D_next + (PL - 1)]

3) Ovulation (estimate): O = D_next - 14

4) Fertile window: [O - 5, O + 1]

5) With cycle variation ±v: D_next ∈ [D_LMP + (CL - v), D_LMP + (CL + v)]

Glossary of Variables

LMP (Last Menstrual Period): The first day of your last period.
Average cycle length (CL): Number of days from day 1 of one period to day 1 of the next (typical range: 21–35 days).
Period length (PL): Typical number of days you bleed during a period (often 3–7 days).
Cycle variation (v): Optional ± days to model early or late cycles; increases the output ranges.
Next period start (D_next): The predicted date your next period begins.
Ovulation (O): Estimated as 14 days before the next predicted period start.
Fertile window: The 6-day interval: 5 days before ovulation through 1 day after.

How It Works: A Step-by-Step Example

Suppose your LMP was March 3, 2025, your average cycle length is 30 days, your period lasts 5 days, and you choose a cycle variation of ±2 days.

Next period start: D_next = March 3 + 30 = April 2, 2025. With ±2, range is March 31–April 4.
Period window: April 2–April 6, 2025. With range, March 31–April 4 start, each lasting 5 days.
Ovulation: O = D_next − 14 = March 19, 2025. With ±2, range March 17–March 21.
Fertile window: March 14–March 20, 2025 (range: March 12–March 22).

Note: These are calendar-based estimates that can shift with stress, illness, medication, or life changes.

Frequently Asked Questions (FAQ)

How accurate is this period calculator?

It is most accurate for regular cycles using a representative average cycle length. Variability and health factors can shift dates.

Why is ovulation placed 14 days before my period?

The luteal phase (ovulation to next period) averages around 14 days for many people, which underpins calendar-based estimates.

What if I do not remember my exact LMP?

Use your best estimate and consider a larger cycle variation to widen the result ranges.

Can this replace medical advice or contraception?

No. This tool is for education only. Consult a clinician for diagnosis, treatment, or contraception guidance.

How many future cycles can I view?

The calculator shows the next six predicted starts. You can adjust inputs to see how results change.

Should I track symptoms as well?

Tracking symptoms (e.g., cervical mucus, basal body temperature) may improve personal accuracy but is outside this simple calendar model.

Audit: Complete

Formula (LaTeX) + variables + units

This section shows the formulas used by the calculator engine, plus variable definitions and units.

Formula (extracted LaTeX)

\[','\]

','

Formula (extracted text)

1) Next period start: D_next = D_LMP + CL 2) Period window: [D_next, D_next + (PL - 1)] 3) Ovulation (estimate): O = D_next - 14 4) Fertile window: [O - 5, O + 1] 5) With cycle variation ±v: D_next ∈ [D_LMP + (CL - v), D_LMP + (CL + v)]

Variables and units

No variables provided in audit spec.

Sources (authoritative):

cdc.gov/contraception/about/fertility-awareness.html — cdc.gov · Accessed 2026-01-19
https://www.cdc.gov/contraception/about/fertility-awareness.html
Health and Fitness — calcdomain.com · Accessed 2026-01-19
https://calcdomain.com/health-fitness
acog.org/womens-health/faqs/your-menstrual-cycle — acog.org · Accessed 2026-01-19
https://www.acog.org/womens-health/faqs/your-menstrual-cycle

Changelog

Version: 0.1.0-draft
Last code update: 2026-01-19

0.1.0-draft · 2026-01-19

Initial audit spec draft generated from HTML extraction (review required).
Verify formulas match the calculator engine and convert any text-only formulas to LaTeX.
Confirm sources are authoritative and relevant to the calculator methodology.

Verified by Ugo Candido on 2026-01-19
Profile · LinkedIn

Formulas

(Formulas preserved from original page content, if present.)

Version 0.1.0-draft

Citations

Add authoritative sources relevant to this calculator (standards bodies, manuals, official docs).

Changelog

0.1.0-draft — 2026-01-19: Initial draft (review required).