Quick start
- Open the Period Calculator.
- Enter the first day of the last period.
- Use the first example, "28-day cycle: LMP Apr 1, 2026, cycle 28, period length 5", if you want to see a filled-out calculation before entering your own values.
- Calculate, read the formula line, then copy the result only after the units and assumptions look right.
Best uses
Start here if one of these sounds like your job. The examples below show which inputs matter most.
- Estimate the next upcoming period start date from an older or recent last-period entry.
- Estimate the expected end date from period length.
- List following period dates for planning reminders, travel, or appointments.
- Check start-to-start cycle math without using it as contraception or diagnosis.
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 upcoming period start date from an older or recent last-period entry. Estimate the expected end date from period length.
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: LMP Apr 1, 2026, cycle 28, period length 5. The example result is Next start Apr 29; expected end May 3; following start May 27.
- 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
In plain language: Next period start = last period start plus cycle length. If that predicted start is not in the future, the calculator keeps adding cycle length until it reaches the next upcoming start. Expected end = next period start plus period length minus 1 day. Following periods add cycle length again. Read the result together with the notes on the page, because health and fitness numbers often need personal context.
Read the formula note when you need to understand where the number came from, especially before comparing results over time.
How to read the answer
Read the main estimate first, then read the note beside it. For health, pregnancy, nutrition, kidney, alcohol, or training decisions with real consequences, use 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 results come from a small input mistake or from using a rough estimate for a decision it cannot safely answer.
- 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 health tool can help check the same topic from another angle, but one number should not replace proper care.
- 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.
Worked examples for Period Calculator
Next start Apr 29; expected end May 3; following start May 27
Next start May 5; expected end May 10; following start Jun 4
Next start May 8; expected end May 11; following start Jun 3
The live result keeps adding cycle length until the next predicted start is upcoming
FAQ in plain language
When should I use the Period Calculator?
Use it for simple educational checks, trend tracking, or planning tasks like these: Estimate the next upcoming period start date from an older or recent last-period entry. Estimate the expected end date from period length. It can help you understand a number, but it cannot explain your whole health situation.
What do the main Period Calculator inputs mean?
Enter the first day of the last period, your usual cycle length from one period start to the next, and period length in bleeding days. Use the start date, not the last day of bleeding. If cycles vary, use a recent average and read the answer as a planning estimate for regular-ish cycles.
What is the Period Calculator doing with my inputs?
In plain language: Next period start = last period start plus cycle length. If that predicted start is not in the future, the calculator keeps adding cycle length until it reaches the next upcoming start. Expected end = next period start plus period length minus 1 day. Following periods add cycle length again. Read the result together with the notes on the page, because health and fitness numbers often need personal context.
How should I read the Period Calculator result?
Read the next start, expected end, following period, and third period as calendar estimates. The calculator keeps adding cycle length until it reaches the next predicted start in the future, then uses period length only to estimate the end date. Stress, illness, medication changes, postpartum changes, travel, and normal variation can move real dates.
Why does the Period Calculator skip ahead from an older last period?
The live result is meant to show the next upcoming predicted start. If the first cycle after the last period is already in the past, the calculator keeps adding the cycle length until the predicted start is still ahead of today.
Should I count from period start or period end?
Use period start to period start. Cycle length means the number of days from the first day of one period to the first day of the next. Period length means how many bleeding days to use when estimating the expected end date.
Why does period length change the end date but not the next start?
Cycle length controls the next start date. Period length only estimates the expected end by counting from the predicted start. A longer period does not automatically mean the next cycle starts later.
Related tools
- Ovulation CalculatorEstimate ovulation date and fertile window from cycle details.
- Conception CalculatorEstimate conception date and fertile window from cycle details.
- Due Date CalculatorEstimate pregnancy due date from LMP and cycle length.
Keep exploring
If this guide is close but not exact, these links keep you near the same kind of problem.
- Health & FitnessBrowse the full category for related tools that help with the same job.
- All free toolsSearch the complete Access Free Tools library by task, category, or tool name.
- All calculator and utility guidesFind more plain-language examples, formulas, mistakes, and result explanations.
- Free calculator resourcesStart here when you are not sure which calculator page fits.
Privacy and copying results
Recent answers stay visible only while you work in the current browser tab. They are not sent to a server.
Use Copy answer when you want to save the inputs and result in notes, homework, a message, or a project list. Check the units, labels, and limits before copying.