"There is pretty much a standard formula, but even that will vary. With drinking water, for example, you know that people need at least 15 liters a day, but there are situations where that's not possible so there is a period in which people can survive with 6 liters a day for a limited amount of time."
The ICRC sent out a kit of medical material Thursday, which he said was supposed to be enough for 10,000 people for three months to cover basic diseases and medical conditions. "There is clearly an overall calculation template behind that," he said, but added that "it may turn out that there is a huge outbreak of disease and the supplies don't last anywhere near three months."
Agencies use the so-called "Sphere" handbook that gives guidelines for how best to respond to humanitarian emergencies.
"A lot depends on local assessment," Westphal said. "For shelter, what they're going to do now is look at a limited area and see how many houses are destroyed, and extrapolate that for the whole affected area. It's a combination of established standards and local assessment."
He said aid specialists make a continuous effort to learn from every single crisis.
"These operations are so complex," Westphal said. "We have to be realistic. There will be mistakes, and the only way we can get better is by analyzing and learning from previous disasters."
Pablo Medina, operations coordinator of the International Federation of Red Cross and Red Crescent Societies, said, "At this very early hour, with such limited amount of information, what you have to do is base your calls on past experience on previous earthquakes, on media reports and on information on the ground," Medina told the AP.
Initial planning is conservative and is normally revised upward as more information becomes available. This time, the Red Cross decided to send 100 experts to Haiti.
"That's fairly big," Medina said.
Associated Press writers Frank Jordans in Geneva; Meera Selva in London; David McFadden in San Juan, Puerto Rico; Tini Tranh in Beijing; Kwang-tae Kim in Seoul, South Korea; Rod McGuirk in Canberra, Australia; Tran Van Minh in Danang, Vietnam; and Hrvoje Hranjski in Manila contributed to this report.