An ovulation calendar predicts your fertile days, periods and ovulation days based on your basic cycle information, usually simply your average cycle length.
Usage of simple ovulation calendars
For the most part, ovulation calendars that rely solely on basic cycle parameters can be used to roughly predict your period start date and possible fertile phases. These calendars simply count the days of your cycles and use average cycle statistics. Some calendars let you specify your luteal phase length if you know it (number of days from ovulation to your period) which may improve the accuracy of the predictions.
To accurately know your luteal phase length, ovulation date and fertile time, you need to chart your fertility signals. This requires more advanced tools to analyze and predict your cycles. Simple ovulation calendars are useful to plan ahead if the risk of being inaccurate is not really an issue.
Simple ovulation calendars have the advantage to be extremely easy to use and not require much personal involvement. FertilityFriend.com provides two such calendars one web based and one in the form an iPhone App. You can also use FertilityFriend.com's mobile access or full web based calendar for this purpose with the possibility then to use more advanced and accurate methods later on.
Limitations of simple ovulation calendars
Ideally these simple calendars only work well for women with very regular cycles (same cycle length each month). As soon as your menstrual cycle shows some variation, the accuracy of these calendars decreases rapidly. Even if your cycles are perfectly regular you may not ovulate each cycle, even if you get your period as expected. The only way to really know more about it is to observe your fertility signals.
Smart Ovulation Calendar
FertilityFriend.com's Ovulation Calendar is able to use your own fertility signals to predict your next ovulation/period and also analyze your actual cycle data to find out what your body is actually doing. FertilityFriend.com's Smart Ovulation Calendar lets you record your fertility signals and analyze them to produce an accurate analysis of your cycles. This improves the usefulness of the calendar which can then be used to time intercourse perfectly and increase your conception chances dramatically. With this type of tool you can predict/plan your cycle phases with more accuracy than any of the simple ovulation calendars can provide even if your cycles are perfectly regular.