Quick Answer: The blood soaking scene in both the major theatrical releases of Carrie were done with practical effects. In Brian De Palma's original 1976 film and Kimberly Peirce's 2013 remake, a bucket of fake blood mixture was literally dumped onto the head of the film's star. However, the process that went into creating the blood and figuring out exactly how to perform the dump was approached quite differently between the two, with De Palma simply doing it once and Peirce running numerous tests to figure out the perfect pour.
When Kimberly Peirce, director of the 2013 Carrie remake asked Brian DePalma, director of the original 1976 Carrie, how many takes it took him to get the prom blood soaking scene done right, his response was, “What do you mean? We did one.” Art Director Jack Fisk went on a ladder and dumped a bucket of blood onto Sissy Spacek’s head. End of story.
Either DePalma lucked out or Peirce is a perfectionist; her team spent hundreds of gallons of fake blood in test dumps getting it to look just right.
In today’s cinematic landscape, a lot of blood is done with CGI. It’s easier for a team to CGI a bullet going through someone’s cheek than rig them up with prosthetics and a squib. It saves cleanup time on-set when blood is added in during post-production rather than spilled all over the place in reality. But in both the 1976 and 2013 versions of Carrie, the blood is for real -- at least in the famous prom scene. And in the recent version, to ensure the hit atop Chloe Grace Moretz’ head was perfect, numerous test dumps took place to figure out exactly how the stunt should flow for optimum horror quality.
Peirce’s team reportedly did test dumps from varying heights, from four to six feet above the subject, and with various volumes of bucket spanning three to five gallons each. They also played with the density of the blood, the angle of the dump, the camera setup, and the frame-rate by which the dump was captured. Her team claims to have exhausted hundreds of gallons of fake blood in deciding exactly which looked best. Different heights, volumes and consistencies change the way the blood hits the subject, the way it splashes from the body, and the amount it soaks its surroundings. To Peirce, all of these elements are important. The blood dump is the most important part of Carrie, so everything about it should be perfect.
The end result is actually two different blood dumps which were cut together to create a repetitious effect. The soak perfectly washes from Carrie’s head, sprays the pristine jacket of her prom date, soaks her dress and bouquet, and leaves enough of her face visible that her emotional response is equally terrified and terrifying.
DePalma’s Carrie was soaked in Karo syrup and food coloring. Sissy Spacek recalls that, when the gymnasium fires were lit behind her following the blood dump scene, the mixture got hot and she started to cook like a candy apple. The Carrie remake employed CGI blood for many scenes in the film, but the bucket dump stayed old school, and splattered enough variations of the mixture to make up for the lack of genuine blood mixture elsewhere in the film.
Who's to say which method is right -- doing it once, or doing it ad nauseam to achieve the perfect pour. Either way, both versions look great and elicit the chilling horrors of their design, catapulting their respective Carries into madness.