tooloctopus.

Period calculator

Project upcoming period dates from your last start date and usual cycle length.

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Period calculator processes your input in this browser. Inputs and results are excluded from analytics. Recent tools save tool names, visit counts and last-visit times. Saved tools store only their names. Draft saving is optional and stays on this device; delete saved text with the draft controls. Loading text from a URL is optional and contacts that server; its URL and your IP address are visible to the server. An optional “Use in” action keeps a handoff in this tab. The next tool removes it on arrival and ignores it if more than a minute has passed.

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More inputs and assumptions

Answer

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Project upcoming period dates from your last start date and usual cycle length.

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Precision and calculation record
Rounding changes displayed numbers, not the calculation. Text, fractions, matrices and CSV schedules keep their own formatting. Automatic keeps up to 12 significant digits, with money shown to cents where applicable.
Saved scenario
Save the current answer, then change an input to compare. This clears when you leave.
Formula and assumptions
The cycle is counted from the first day of one period to the first day of the next. Repeating your chosen length creates calendar estimates, not predictions of ovulation or fertility. Cycles can vary; do not use this as contraception or to confirm or rule out pregnancy. A missed period or a change in bleeding needs appropriate follow-up.
Automatic precision shows numbers to 12 significant digits. Thousands separators such as 1,000 are accepted; use a dot for decimals.
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Updated 
Using Period calculator
Project upcoming period dates from your last start date and usual cycle length.
The cycle is counted from the first day of one period to the first day of the next. Repeating your chosen length creates calendar estimates, not predictions of ovulation or fertility. Cycles can vary; do not use this as contraception or to confirm or rule out pregnancy. A missed period or a change in bleeding needs appropriate follow-up.
Next: Pregnancy calculator
A worked example
For first day of last period (yyyy-mm-dd) = 2026-01-01, usual cycle length (days) = 28, usual bleeding duration (days) = 5, next estimated period start = 2026-01-29.
Before you use the result
The cycle is counted from the first day of one period to the first day of the next. Repeating your chosen length creates calendar estimates, not predictions of ovulation or fertility. Cycles can vary; do not use this as contraception or to confirm or rule out pregnancy. A missed period or a change in bleeding needs appropriate follow-up.
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ToolOctopus. “Period calculator.” Updated 2026-10-03. https://tooloctopus.com/period-calculator.
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