# 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.

- Calculator id: `ovulation` · Category: Health (`health`) · Tool name: `calculate_ovulation_date`
- Canonical page: https://tttkmbb.com/health/ovulation · This document: https://tttkmbb.com/health/ovulation.md · JSON definition: https://tttkmbb.com/health/ovulation.json

## Purpose

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.

**Use when:** 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).

## Input

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

## Output

| Field | Type | Unit | 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. |

## Formula

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

## Data Sources

- Wilcox AJ, Weinberg CR, Baird DD (1995) Timing of sexual intercourse in relation to ovulation, N Engl J Med 333:1517-1521 — https://pubmed.ncbi.nlm.nih.gov/7477165/ (peer_reviewed, retrieved 2026-09-23)
- NHS – Trying to get pregnant: when am I most fertile? — https://www.nhs.uk/pregnancy/trying-for-a-baby/trying-to-get-pregnant/ (reference, retrieved 2026-09-23)

Data freshness: `static`. Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output.

## API

- `GET https://tttkmbb.com/api/v1/calculate/ovulation?last_period_date=…`
- `POST https://tttkmbb.com/api/v1/calculate/ovulation` with JSON body `{"inputs": {…}}`
- Response: unified envelope (`success`, `request`, `result.values`, `result.units`, `sources`, `freshness`, `timestamp`, `next_actions`, `links`); see https://tttkmbb.com/docs/response-format.md
- Schema: https://tttkmbb.com/api/v1/calculators/ovulation · OpenAPI operationId `calculate_ovulation_date` in https://tttkmbb.com/openapi.json
- Authentication: none. Rate limit: fair use, see https://tttkmbb.com/docs/rate-limits.md.

## MCP

- Server: `https://tttkmbb.com/mcp` (Streamable HTTP, JSON-RPC 2.0, no auth)
- Tool:  `run_calculator` with `{"calculator_id": "ovulation", "inputs": {…}}`

## Example

- LMP 2026-09-01, 28-day cycle: inputs `{"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
```

## Limitations

Cycles vary by more than a few days, hormonal contraception is used, or you need contraception (calendar estimates are not reliable for preventing pregnancy). All values are computed from the formula above; no measurement or live data is involved.

## 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

- [Pregnancy Due Date Calculator](https://tttkmbb.com/health/pregnancy-due-date.md) — Once pregnant, estimate the due date from the same LMP.