I've seen variants of this question, but nothing explicitly like the one below:
Three feet equals/equal a yard.
Which is correct? Is there a definitive explanation?
Please indicate BrE vs AmE if you have a view.
Equals is more idiomatic in American English, but either is acceptable.
To determine whether "three feet" is treated as singular or plural, try using it in other contexts. In American English, and I believe in UK English as well, you would say:
Three feet is pretty long for a garter snake
Three feet are too many for a human being to have.
When expressing a distance, rather than describing the things your socks go on, "three feet" is treated as singular. Think of it as shorthand for "a distance of three feet" rather than "three of the items known as feet."
The trick with your phrase is that it can be interpreted in either way. Because it is a distance, you could think of it as:
A distance of three feet is equivalent to a distance of one yard.
in which case you use "equals." Or you could think of it as:
Three of the items known as feet are equivalent to one of the items known as yard.
in which case you use "equal."
Because either reading is possible, it's grammatically ambiguous. In my experience, though, "equals" is more common, following the way "three feet" would be treated in other sentences.
1 equals 1.
1 is equal to 1.
My level of patience equals 0.
My level of patience is equal to 0.
Three feet equals one yard.
Three feet is equal to one yard.
Equals is generally used unless using a verb "is" and the phrase "equal to". While reading 3 ft = 1 yd you would say "three feet equals a yard," or "three feet is equal to a yard".
Equals is used as a verb. To use equal in mathematics (generally an adjective) you need an accompanying verb.
I have heard people use it as a verb, but it is not a common practice around the midwest/southern United States. I've lived in three regions here and do not hear it used.
Edit: From what I am reading, BrE often uses equal as more of a verb, but generally requires an object.
Examples taken from Dictionary.com:
So far the rate of production doesn't equal the demand. If A equals B and B equals C, then A equals C.
No matter how he tries, he can't equal his brother's achievements.
If you looks at these, they are accompanies by "does not" and "can not" in order to use them.
Equals is okay in some circumstances.
For example, if you were to say the following out loud:
2 + 2 = 4
You would say:
two plus two equals four
So it would make perfect sense to use the word equals in speech and if you did it would be understood pretty much instantly. When people are speaking of a formula or equation they may use the word equals where an equals sign would be were they to write out their sentence. I don't see any issue with using the word equals so long as it is a translation of how the equation would have been written.
Using the word "is", on the other hand, makes sense for comparing items that are not measured.
A banana is a fruit
Bob is a ninja
So the answer is both are fine to use. However, usage should depend on how technical what you are saying is.