Learn Russian: Диаметр равняется восьмистам девяноста четырём метрам. - The diameter equals eight hundred ninety-four meters.

The verb равняться (to equal) requires its object to be in the dative case. Because of this, every part of the compound number 894 must decline into the dative: восьмистам девяноста четырём.

How does the word for 800 (восемьсот) decline to become восьмистам?

For hundreds from 500 to 900, both halves of the word decline. The first part declines like the standalone number (восемь becomes the dative восьми), and the second part declines like a plural noun for "hundreds" (сот becomes the dative стам). Put them together, and you get восьмистам.

Why does 90 become девяноста instead of having a complex ending?

The numbers 40 (сорок), 90 (девяносто), and 100 (сто) have a uniquely simple declension. In the nominative and accusative, they keep their dictionary form. In all other cases—including the dative used here—they simply take the ending -а. So, девяносто becomes девяноста.

Normally, numbers ending in 4 take the genitive singular, like четыре метра. Why is it четырём метрам here?