How do I calculate my menstrual cycle length?

My period in December started on the 16th and then finished at around 21st/22nd. This month, my period started 2 days ago, on the 10th and will probably finish on Saturday 14th/Sunday 15th. With this information, how long is my menstrual cycle? Is is the length of the last day of your last period to the first... show more My period in December started on the 16th and then finished at around 21st/22nd. This month, my period started 2 days ago, on the 10th and will probably finish on Saturday 14th/Sunday 15th. With this information, how long is my menstrual cycle?

Is is the length of the last day of your last period to the first day of your next period, or?
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