Fertile window predictions that sharpen over time
Crimson is a free fertility tracker app for iOS and Android that predicts your fertile window from your own logged cycles, not population averages. The fertile window is the roughly six days each cycle when pregnancy is possible: the five days before ovulation plus ovulation day itself. Sperm can survive up to five days; the egg lasts about 12 to 24 hours. Timing really is everything.
Most apps average your cycle lengths and call it a day. Crimson uses weighted moving averages, so your most recent cycles count more than what happened eight months ago. Bodies change. Your predictions should keep up. After one or two logged cycles the estimates are rough; by three or four they personalize to your pattern; past six they're well calibrated. The math behind it is covered in our cycle tracking guide.
Ovulation tracking without the homework
Cycle dates alone can predict a lot. Add physical signals and the picture gets sharper. Crimson supports more than 70 symptoms and signals, and quick logging takes seconds, not a journaling session.
Cervical mucus
Changes in consistency track your approach to ovulation. Egg-white, stretchy mucus usually marks peak fertility. Log it daily in trying-to-conceive mode and watch the pattern line up with your predicted window.
Basal body temperature
A small temperature rise after ovulation confirms it happened. Log your morning readings and Crimson keeps them alongside everything else, so you can see the shift across cycles.
Ovulation signs
One-sided twinges, breast tenderness, libido shifts. These feel random until you log them for a few cycles and Crimson's symptom correlations show they arrive like clockwork.
Cycle-phase insights
Menstrual, follicular, ovulation, luteal. Crimson detects your current phase and tells you what to expect next, so you're never decoding a calendar on your own.
Just want a quick estimate?
An AI that speaks fluent cycle
Crimson's AI assistant works out of the box, no API key required. Because it knows your current phase, your logged symptoms, and that you're trying to conceive, its answers are about your cycle rather than the average one. Free accounts get a daily AI allowance. Crimson Pro makes it unlimited, and new accounts start with a 14-day Pro trial. See how that compares to the other big apps in our tracker comparison.
Honest limits: Ovulation predictions are estimates. Stress, travel, illness, and conditions like PCOS can shift ovulation in ways no algorithm sees coming, and irregular cycles are harder to predict, period. These predictions are not contraception and should never be used to prevent pregnancy. If you've been trying for 12 months (or 6 months if you're 35 or older), it's worth talking to a healthcare provider. Questions about the app itself? We're here.
Private by design
Fertility data reveals more about your life than almost anything else on your phone. Crimson stores data locally first, then syncs across your devices over an encrypted connection (TLS 1.2+), with encryption at rest on the server. A free account is required so your history survives a lost phone. We never sell health data, and there are no ads. If the two pink lines show up, your data moves straight into pregnancy mode with you.
Frequently asked questions
How accurate are Crimson's ovulation predictions?
They're educated estimates, and they get better with data. After one or two logged cycles, predictions are rough. By three or four cycles they personalize to your pattern, and past six cycles they're well calibrated. Irregular cycles will always be harder to predict, and no app can pinpoint ovulation to the hour — ovulation predictor kits or basal body temperature can add confirmation.
Can I use Crimson as birth control?
No. Crimson's fertile-window predictions are estimates designed for people trying to conceive or simply understand their cycle. They are not contraception and were never validated for preventing pregnancy. If you need contraception, talk to a healthcare provider about methods designed and tested for that purpose.
Is the fertility tracker free?
Yes. Ovulation and fertile-window predictions, symptom tracking, cycle-phase insights, and a daily AI allowance are all free, forever. Crimson Pro is an optional subscription that adds unlimited AI, and every new account starts with a 14-day Pro trial.
What should I log when trying to conceive?
At minimum, log your period start and end dates each cycle — that's what powers the predictions. For a sharper picture of your fertile window, also log cervical mucus changes, basal body temperature if you take it, ovulation pain, and sexual activity. Crimson supports more than 70 symptoms and signals, and quick logging takes seconds.
Start tracking your fertile window
Free ovulation and fertile-window predictions that improve with every cycle. Private by default. Pro unlocks more.