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Ovulation Calculator

Estimates the ovulation date, the six-day fertile window and the next period start from the first day of the last period and the average cycle length.

When to use

You want to estimate the most fertile days of a regular menstrual cycle.

Do not use when: Cycles vary by more than a few days, hormonal contraception is used, or you need contraception (calendar estimates are not reliable for preventing pregnancy).

Formula

next_period = LMP + cycle_length; ovulation = next_period − luteal_phase; fertile_window = ovulation − 5 … ovulation

Inputs

Parameter	Type	Unit	Required	Description
`last_period_date`	date		yes	First day of the most recent period (YYYY-MM-DD).
`cycle_length_days`	integer	days	default 28	Average number of days from one period start to the next. Range: ≥ 20, ≤ 45
`luteal_phase_days`	integer	days	default 14	Days from ovulation to the next period; 14 is the population average. Range: ≥ 10, ≤ 18

Outputs

Output	Type	Description
`ovulation_date`	date	Next period start minus the luteal phase.
`fertile_window_start`	date	Five days before ovulation (sperm survival).
`fertile_window_end`	date	Ovulation day.
`next_period_date`	date	LMP + cycle length.
`pregnancy_test_date`	date	Day of the expected next period.

Example

LMP 2026-09-01, 28-day cycle: {"last_period_date":"2026-09-01","cycle_length_days":28} → {"ovulation_date":"2026-09-15","fertile_window_start":"2026-09-10","fertile_window_end":"2026-09-15","next_period_date":"2026-09-29"}

GET https://tttkmbb.com/api/v1/calculate/ovulation?last_period_date=2026-09-01&cycle_length_days=28

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/ovulation (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/ovulation · Markdown: https://tttkmbb.com/health/ovulation.md · JSON definition: https://tttkmbb.com/health/ovulation.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="ovulation"
OpenAPI operationId: calculate_ovulation_date
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

FAQ

How reliable is the estimate?

Ovulation timing varies even in regular cycles; ovulation predictor kits or basal body temperature tracking are more precise than calendar methods.

Related calculators

Pregnancy Due Date Calculator — Once pregnant, estimate the due date from the same LMP.