How to use the Period Calculator

Quick start

1. Open the Period Calculator.
2. Enter the first day of the last period.
3. Use the first example, "28-day cycle: Last period Apr 1, 5 days long", if you want to see a filled-out calculation before entering your own values.
4. Calculate, read the formula line, then copy the result only after the units and assumptions look right.

Best uses

Use this guide when one of these tasks matches what you are trying to do.

• Estimate the next period start date.
• Estimate expected period end date.
• List upcoming cycles for planning.
• Use calendar estimates while remembering cycles can change.

What this calculator is for

The Period Calculator estimates the next period start, expected end, and upcoming cycle dates from your usual cycle pattern.

Use it when you want to: Estimate the next period start date. Estimate expected period end date.

What to enter

Good answers start with clean inputs. Before calculating, check the labels, units, and dates so the tool is solving the same problem you actually have.

• Enter the first day of the last period.
• Enter average cycle length from one period start to the next period start.
• Enter period length to estimate the expected end date.

Example walkthrough

Try the calculator example: 28-day cycle: Last period Apr 1, 5 days long. The example result is Next period estimate.

• If the last period started Apr 1 and the cycle is 28 days, the next start is estimated 28 days later.
• If period length is 5 days, the expected end is shown from that start date.

Formula and steps

The calculator adds cycle length to the first day of the last period until it finds the next expected period start date.

The formula line on the calculator page is there so the answer is not a mystery. Read it when you need to understand where the number came from.

How to read the answer

Use the result as an educational estimate. For health, pregnancy, nutrition, kidney, alcohol, or training decisions with real consequences, get qualified professional guidance.

• Use the dates for planning, not diagnosis.
• A cycle that arrives earlier or later than usual can be normal once in a while.

Common mistakes to avoid

Most bad calculator results come from a small input mistake or from using a good estimate for the wrong decision.

• Do not count from the last day of bleeding when the field asks for start date.
• Do not assume predictions stay accurate during irregular cycles, postpartum changes, or medication changes.
• Do not use period prediction as contraception.

What to try next

A related calculator can help check the same topic from another angle instead of relying on one number.

• Use Ovulation Calculator to estimate a fertile window.
• Track actual dates over a few cycles for better averages.

Sources and safety notes

This guide uses public-health, clinical, or peer-reviewed references where the calculator needs a specific formula or interpretation boundary.

Source links are provided for transparency, but they do not turn the calculator into medical advice or a replacement for professional care.

• Johns Hopkins Medicine: Calculating a due date
• Johns Hopkins Medicine: Fertile window basics

Examples from the calculator

Common questions

Related tools

• Ovulation Calculator Estimate ovulation date and fertile window from cycle details.
• Conception Calculator Estimate conception timing from cycle and ovulation assumptions.
• Due Date Calculator Estimate pregnancy due date from LMP and cycle length.

History, privacy, and copying

Recent answers stay visible in the page while you work. The history is kept only in the current browser tab and is not sent to a server.

Copy answer copies the expression and result so you can paste it into notes, homework, a message, or another document.