Cycle Day Calculator
Calculate cycle day from the last period start date and a target/current date using date-only arithmetic.
Enter or adjust the values above, then calculate.
Cycle Day: what Day of Your Cycle Is It?
Calculate cycle day from the last period start date and a target/current date using date-only arithmetic.
Inputs this tool uses
- Last period start
- Date to check
What you get
Cycle day counted from the entered period start.
How to use the Cycle Day Calculator
- Enter last period start, date to check.
- Select any method, activity level or option shown on this page, then choose Calculate.
- Read the main result together with the supporting values, method, assumptions and limitations below before using the estimate.
How this calculation works
Cycle tools use date arithmetic from observed period starts and user-entered cycle assumptions. Estimates are never presented as contraception.
Example calculation
The sample values loaded into the calculator produce Day 1. This example exists to show how the output is structured; replace the sample values with your own inputs for your calculation.
Assumptions and limitations
- Cycle and ovulation dates are estimates and can vary from cycle to cycle.
- This tool is not contraception and does not diagnose fertility or pregnancy.
Sources and review status
Source set last verified 2026-09-25. No clinician review is claimed unless a real named reviewer is displayed.
Connected tools
Continue with the closest related calculation without re-entering sensitive values automatically.
Estimate the next period date from recent cycle lengths or the last start date, showing uncertainty rather than a single guaranteed date.
Open tool →◌Period TrackerStore observed period dates locally, derive cycle statistics and forecasts on-device, and keep estimates visually separate from observations.
Open tool →◌Cycle Length CalculatorCalculate individual cycle lengths, average, median and variability from at least two period-start dates.
Open tool →