Academic preparation Caltech says you should have [1]:
4 years of math (including calculus)
1 year of physics
1 year of chemistry
3 years of English (4 years recommended)
1 year of U.S. history/government (waived for international students)
They really want you to have calculus. From the admissions FAQ [3]:
What if my high school does not offer Advanced
Placement (AP) or International Baccalaureate (IB)
courses?
Many schools don't offer the AP or IB curricula.
Regardless of the curriculum your school offers, we
expect that you will have challenged yourself with
demanding courses. The Caltech curriculum will require
that you have completed rigorous courses in calculus and
in physics. If these courses are not available in your
high school, we strongly encourage you to take them at a
local college or online.
Here is Caltech's required first year math course [2]:
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Ma 1 abc. Calculus of One and Several Variables and Linear Algebra. 9 units (4-0-5); first, second, third terms. Prerequisites: high-school algebra, trigonometry, and calculus. Special section of Ma 1 a, 12 units (5-0-7). Review of calculus. Complex numbers, Taylor polynomials, infinite series. Comprehensive presentation of linear algebra. Derivatives of vector functions, multiple integrals, line and path integrals, theorems of Green and Stokes. Ma 1 b, c is divided into two tracks: analytic and practical. Students will be given information helping them to choose a track at the end of the fall term. There will be a special section or sections of Ma 1 a for those students who, because of their background, require more calculus than is provided in the regular Ma 1 a sequence. These students will not learn series in Ma 1 a and will be required to take Ma 1 d. Instructors: Marx, Katz, Mantovan, Aschbacher, Ni, Kechris.
Ma 1 d. Series. 5 units (2-0-3); second term only. Prerequisite: special section of Ma 1 a. This is a course intended for those students in the special calculus-intensive sections of Ma 1 a who did not have complex numbers, Taylor polynomials, and infinite series during Ma 1 a. It may not be taken by students who have passed the regular Ma 1 a. Instructor: Staff.
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Note: don't freak out over the unit numbers. Caltech's unit scale is different from that of most other schools. 1 unit corresponds to 1 hour of work per week, so a 9 unit course is one that should take 9 hours a week. The numbers in parentheses, such as (4-0-5) break it down by number of hours of lecture, number of hours of lab, and number of hours of outside preparation and homework.
Ma1 does start with a "review of calculus", so you could probably do it without having had calculus in high school if you were really good, but it would be rough.
[1] http://www.admissions.caltech.edu/applying/freshman
[2] http://catalog.caltech.edu/courses/listing/ma.html
[3] http://admissions.caltech.edu/uploads/File/general/FAQapplyi...