The best way to calculate the volume of your jack-o-lantern is to cut out the top part as usual. Carve out the pumpkin and cut out the eyes, nose and mouth. Then, pour water into the jack-o-lantern until it is full. When the water reaches the top of the pumpkin, you need to pour the jack-o-lantern’ water content into another container and measure how much water you have. That is the best way.
Update: I did this and my answer came out to be infinite. All jack-o-lanterns have infinite volume. That is very spooky!