The University of California, Irvine Libraries host Jim Martindale's impressive compilation of over 16,000 specialized online calculators from over 3,500 sources. The calculators come from a variety of sources and are broken down into four categories:

• Alphabetical
  • A-E
  • F-H
  • I-N
  • O-Z
• Mathematics
  • Statistics
• Science A-Z
  • Chemistry
  • Physics & Astrophysics
• Engineering A-Z
  • Electrical & Computer

Whether you need to convert ounces to miligrams or barrels of oil to US and Imperial gallons, you're bound to find the source for your solution here.