Are doubles faster than floats in C#?

The short answer is, "use whichever precision is required for acceptable results."

Your one guarantee is that operations performed on floating point data are done in at least the highest precision member of the expression. So multiplying two float's is done with at least the precision of float, and multiplying a float and a double would be done with at least double precision. The standard states that "[floating-point] operations may be performed with higher precision than the result type of the operation."

Given that the JIT for .NET attempts to leave your floating point operations in the precision requested, we can take a look at documentation from Intel for speeding up our operations. On the Intel platform your floating point operations may be done in an intermediate precision of 80 bits, and converted down to the precision requested.

From Intel's guide to C++ Floating-point Operations¹ (sorry only have dead tree), they mention:

• Use a single precision type (for example, float) unless the extra precision obtained through double or long double is required. Greater precision types increase memory size and bandwidth requirements. ...
• Avoid mixed data type arithmetic expressions

That last point is important as you can slow yourself down with unnecessary casts to/from float and double, which result in JIT'd code which requests the x87 to cast away from its 80-bit intermediate format in between operations!

1. Yes, it says C++, but the C# standard plus knowledge of the CLR lets us know the information for C++ should be applicable in this instance.

I just read the "Microsoft .NET Framework-Application Development Foundation 2nd" for the MCTS exam 70-536 and there is a note on page 4 (chapter 1):

NOTE Optimizing performance with built-in types
The runtime optimizes the performance of 32-bit integer types (Int32 and UInt32), so use those types for counters and other frequently accessed integral variables. For floating-point operations, Double is the most efficient type because those operations are optimized by hardware.

It's written by Tony Northrup. I don't know if he's an authority or not, but I would expect that the official book for the .NET exam should carry some weight. It is of course not a gaurantee. I just thought I'd add it to this discussion.

I profiled a similar question a few weeks ago. The bottom line is that for x86 hardware, there is no significant difference in the performance of floats versus doubles unless you become memory bound, or you start running into cache issue. In that case floats will generally have the advantage because they are smaller.

Current Intel CPUs perform all floating point operations in 80 bit wide registers so the actual speed of the computation shouldn't vary between floats and doubles.