Free training tool

Menstrual Cycle Calculator

Your cycle is a rich source of information about your body. Map your phases, fertile window and next period, then start to learn how you tend to feel across the month. The aim isn't to train by a chart, it's to understand yourself and adapt sensibly around good, progressive training.

For information only. This is not medical advice and is not a method of contraception. Predictions assume a fairly regular cycle and are estimates, not certainties. All calculations happen in your browser and your data is never sent anywhere.

Your cycle

Map your cycle.

One period's start to the next. Average is 28.

Advanced: luteal phase length

Days from ovulation to your next period. Sets ovulation timing.

You are in your
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Use it as a lens, not a rulebook.

The old advice was to train strictly by your phase. The better, more honest view, led by researchers like Dr Stacy Sims, is that individual variation is huge, and hormones shift with stress, sleep, travel and life stage, so a generic phase chart can't govern your training. Keep training progressive and periodised, keep strength work in all year round, and fuel consistently. Then let how you actually feel day to day, your energy, sleep and motivation, guide the small adjustments. Track a few cycles and your own patterns will teach you far more than any calendar. This tool is here to help you learn about yourself, not to box you in.

Your cycle calendar

Menstrual Follicular Ovulation Fertile window Luteal

The one period you entered is real; everything after it is a prediction. Predicted period days show as a lighter hatch.

What you might notice through the cycle

Common patterns, not rules. Two women can feel completely different on the same day, so treat these as prompts to check in with yourself rather than instructions.

MenstrualEnergy varies a lot. Recover if you need to, but many train and race well here. Keep iron and fuelling up.
FollicularOften feel strong and recover fast, a natural time to push if your plan and readiness agree.
OvulationFrequently a high-energy window. Warm up well, as joints can be slightly more lax.
LutealEffort and body heat can feel higher. Fuel well, hydrate and protect recovery rather than automatically backing off.

How to actually use this

Mapping your cycle is the easy part. Using it without letting it box you in is the bit that takes a little thought, so here is the short version of how we approach it with athletes.

Why a generic phase chart cannot run your training

You are individualTwo women can feel completely different on the same day of the same length cycle. An average does not describe you.
Cycles moveStress, travel, heavy training and illness all shift them. Under high load some cycles are anovulatory, meaning a bleed without ovulation.
Contraception changes itHormonal contraception alters the picture entirely, and a large proportion of athletes use it.

What to do instead

  1. Keep the plan progressive. Build training around your race and your life. Do not move key sessions because of a date on this calendar.
  2. Track three or four cycles. Note energy, sleep, motivation and session quality against the phases above. Your own patterns will teach you more than any chart.
  3. Adjust by feel, day to day. A flat session is information. Some of those days will land in a predictable place for you, and some will not.
  4. Keep strength work in all month. There is no good reason to drop it, and plenty of reasons to keep it.
  5. Fuel consistently. Under-fuelling disrupts cycles and blunts adaptation. It matters more than session timing.
  6. Take symptoms seriously. Heavy bleeding, severe pain or a cycle that has stopped are worth a conversation with your GP, not something to train through quietly.

Two practical things worth knowing. Heavier periods raise the risk of low iron, which affects endurance directly, so test rather than guess. And project the calendar above forward to race day, so you can plan for it rather than be surprised by it.

Further readingTraining With Your Cycle, Not Ruled By It →

Common questions

Which phase am I in today?

Enter your last period date and cycle length and the calculator works out your day of cycle, then maps it to the menstrual, follicular, ovulation or luteal phase, with a note on what women commonly notice at that stage. Treat it as a prompt to check in with yourself, not a prescription.

How accurate is this?

It's a calendar-based estimate that assumes a fairly regular cycle, with ovulation placed by your luteal-phase length. Real cycles vary, and under stress or heavy training some cycles are anovulatory (a bleed without ovulation), so treat every future date as a prediction and refine it with your own tracking.

How should I plan training around my cycle?

Loosely, and as an individual. Rather than training by a fixed phase chart, keep your training progressive and periodised, and keep strength work in all month. Then use how you feel day to day, your energy, sleep and motivation, to fine-tune intensity, and adapt fuelling and recovery around your symptoms. Track a few cycles to learn your own patterns. The current thinking, from coaches and researchers like Dr Stacy Sims, is to let your training lead with cycle awareness layered on top, not the calendar governing the plan.

Will my race land on my period?

Project your cycle out to race day and the calendar shows which phase the date falls in, so you can plan fuelling, warm-up and expectations rather than being caught out.

Is it safe to train on my period?

For most people, yes. Light to moderate activity can actually ease symptoms. Scale the intensity to how you feel on the day and prioritise recovery.

Don't go it alone.

Cycle awareness is one useful layer in a much bigger picture. The real edge is a coaching team in your corner, mapping and running the whole journey with you: training, strength and conditioning, nutrition and race-day strategy, built around your body, your data and how you actually respond. Let us get you to the start line ready, and across the finish line in the best possible shape.

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