# How many DPO am I today?

Counts the days past ovulation (DPO) on a chosen day, from the ovulation day or the last period and cycle length, and says what usually happens at that DPO: implantation, the period due date, and when a test is reliable.

- Page: https://www.acalculator.org/health/dpo-calculator
- JSON spec: https://www.acalculator.org/health/dpo-calculator.json
- Version: 047d24c3586b

## Default answer

Example with the default inputs (What do you know? Last period, First day of your last period September 14, 2026, Cycle length (days) 28, DPO on October 5, 2026) on the example date Monday, October 5, 2026: On October 5, 2026 you are 7 days past ovulation (DPO).

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| from | What do you know? | The first day of your last period, or the day you ovulated. |
| lmp | First day of your last period | The first day of bleeding of your most recent period. |
| cycle | Cycle length (days) | Days from the first day of one period to the first day of the next; 28 on average. |
| ovulation | Ovulation day | The day you ovulated, for example from an ovulation test or a temperature chart. |
| on | DPO on | The day to count to; today by default. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| dpo | Days past ovulation | Days from ovulation to the chosen day. |
| stage | What usually happens | The usual events at this DPO. |
| ovulation | Ovulation | The ovulation day: entered, or the last period + cycle length − 14 days. |
| period | Next period due | Ovulation + 14 days. |
| toPeriod | Days until the period is due | 14 − DPO; 0 or less means the period is due or late. |
| implantFrom | Implantation window from | Ovulation + 6 days. |
| implantTo | Implantation window to | Ovulation + 12 days. |

## Method

DPO = chosen day − ovulation day; ovulation = last period + cycle length − 14 days (or the day entered); period due = ovulation + 14 days.

## Assumptions

- Ovulation comes 14 days before the next period (ACOG average); real cycles vary.
- Implantation happened 6 to 12 days after ovulation, most often on day 8, 9 or 10 (Wilcox and others, 1999).
- Most home pregnancy tests are reliable from the first day of a missed period (womenshealth.gov).

## Worked examples

1. from = period, lmp = 2026-09-01, cycle = 28, on = 2026-09-24 gives ovulation = 2026-09-15, dpo = 9, stage = In the most likely implantation days (8 to 10 DPO), period = 2026-09-29, toPeriod = 5, implantFrom = 2026-09-21, implantTo = 2026-09-27. Source: ACOG Committee Opinion 700, Methods for Estimating the Due Date (ovulation about 14 days before the next period), https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date; Wilcox AJ, Baird DD, Weinberg CR, Time of implantation of the conceptus and loss of pregnancy, N Engl J Med 1999;340:1796 (implantation 6 to 12 days after ovulation, 84% on day 8, 9 or 10), https://doi.org/10.1056/NEJM199906103402304.
2. from = ovulation, ovulation = 2026-12-25, on = 2027-01-08 gives dpo = 14, period = 2027-01-08, toPeriod = 0, stage = Period due today: a home pregnancy test is reliable from a missed period. Source: ACOG Committee Opinion 700, Methods for Estimating the Due Date (ovulation about 14 days before the next period), https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date.
3. from = period, lmp = 2028-02-10, cycle = 32, on = 2028-03-03 gives ovulation = 2028-02-28, dpo = 4, period = 2028-03-13. Source: ACOG Committee Opinion 700, Methods for Estimating the Due Date (ovulation about 14 days before the next period), https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date.

## FAQ

### What does DPO mean?

DPO stands for days past ovulation. The ovulation day is 0 DPO, the next day 1 DPO, and so on. People trying to conceive use it to time a pregnancy test during the two-week wait before the next period.

### How do I work out my DPO?

Count the days from your ovulation day to today. If you do not know the ovulation day, estimate it as the first day of your last period plus your cycle length minus 14 days. With a 28-day cycle that started on September 1, ovulation is about September 15, so September 24 is 9 DPO.

### When does implantation happen?

In a 1999 study that tracked hormones daily, implantation happened 6 to 12 days after ovulation, and on day 8, 9 or 10 in 84% of the pregnancies. So 6 to 12 DPO is the usual window.

### How many DPO can I take a pregnancy test?

The hCG hormone appears only after implantation and then rises. Most home tests are reliable from the day of a missed period, which is about 14 DPO in a regular cycle. Testing earlier can give a negative result even when you are pregnant.

### Is my period late at 15 DPO?

If ovulation was on the day you think, the period is due about 14 days after it, so at 15 DPO it is a day late. Ovulation dates are often off by a few days, especially when they are estimated from the last period.

### Why is the luteal phase 14 days?

The time from ovulation to the next period is the luteal phase. It is about 14 days on average and varies less than the first part of the cycle, which is why the cycle length minus 14 gives the ovulation day.

## Sources

- Wilcox AJ, Baird DD, Weinberg CR. Time of implantation of the conceptus and loss of pregnancy. N Engl J Med. 1999;340(23):1796-1799. https://doi.org/10.1056/NEJM199906103402304
- American College of Obstetricians and Gynecologists. Committee Opinion No. 700: Methods for Estimating the Due Date (ovulation about 14 days before the next period). Obstet Gynecol. 2017;129(5):e150-e154. https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date
- Office on Women’s Health (HHS). Pregnancy tests: most home tests are accurate after a missed period, retrieved 2026-10-05. https://womenshealth.gov/a-z-topics/pregnancy-tests