if i am a 24 day menstrual cycle, does that mean i ovulate on the 12th day?

no, you could be ovulating at any time in your cycle:

The only way for you to know if you are ovulating is to take your basal body temp each morn, same time and before you get up. You should see a pattern in temps that leads up to and after ovulation. You can get the basal thermometer at Wal-Mart for about 8 dollars. I use a wonderful FREE website called Fertilityfriend.com...it graphs your temps for you, keeps track of your fertility signs, tells you when you ovulate and also tells you if there is an implantation dip in your temps. You can also use Ovulation prediction kits, but they only measure your LH surge and give you a ball park time for ovulation...they do not pin point it. Oh, and it is possible to be ovulating while you do not have your period, just like it is possible to have a period and not ovulate

Ovulation normally occurs when levels of the hormone LH (Lutenizing Hormone causes the ovary to release the egg) spike in the blood stream. If there is irregularity in the mechanisms of FSH (follicle stimulating hormone ripens the egg) or LH then ovulation can occur at any time of the month.

What causes periods is a rise and fall of Estrogen and Progesterone. These hormones have the job of preparing the uterus to receive the fertilized egg and slough off the uterine lining if no pregnancy occurs.

The best way to tell when you are ovulating is to use a BASAL BODY Thermometer. They cost about $15 and the day of ovulation you will see a small rise in body temperature. What you would need to do is to plot your early morning temps on a graph and look for the change upwards. Measure your temp in bed with little moving aroung first thing in the morning for best results. This is about the only "do-ti-yourself" mechanisms out there so that you can truely determine time of ovulation.

The other answerer is correct. There is NO formula or rule that will tell you when you ovulate.

You have to track your fertility to figure it out. Try charting your BBT, as she said. FertilityFriend is great for that, and a basal thermometer costs under $10. Also try testing with OPKs and observing your cervical mucus.

Do those things for three or four cycles, at the very least, and average the ovulation dates together. That CD (cycle day) is the middle of the week when you can expect to ovulate. It can vary within 2 or 3 days in either direction each week, so if you choose to stop keeping track of it, that week average is the best you can do.

24 Day Menstrual Cycle

I thought no matter the length of cycle, ovulation occurs 14 days before your period starts so on day 10 in your case.