When should I use double instead of decimal?

Question

I can name three advantages to using double (or float) instead of decimal:

1. Uses less memory.
2. Faster because flo

Answer 1

Note: this post is based on information of the decimal type's capabilities from http://csharpindepth.com/Articles/General/Decimal.aspx and my own interpretation of what that means. I will assume Double is normal IEEE double precision.

Note2: smallest and largest in this post reffer to the magnitude of the number.

Pros of "decimal".

• "decimal" can represent exactly numbers that can be written as (sufficiently short) decimal fractions, double cannot. This is important in financial ledgers and similar where it is important that the results exactly match what a human doing the calculations would give.
• "decimal" has a much larger mantissa than "double". That means that for values within it's normalised range "decimal" will have a much higher precision than double.

Cons of decimal

• It will be Much slower (I don't have benchmarks but I would guess at least an order of magnitude maybe more), decimal will not benefit from any hardware acceleration and arithmetic on it will require relatively expensive multiplication/division by powers of 10 (which is far more expensive than multiplication and dividion by powers of 2) to match the exponent before addition/subtraction and to bring the exponent back into range after multiplication/division.
• decimal will overflow earlier tha double will. decimal can only represent numbers up to ±2⁹⁶-1 . By comparision double can represent numbers up to nearly ±2¹⁰²⁴
• decimal will underflow earlier. The smallest numbers representable in decimal are ±10^-28 . By comparision double can represent values down to 2^-149 (approx 10^-45) if subnromal numbers are supported and 2^-126 (approx 10^-38) if they are not.
• decimal takes up twice as much memory as double.

My opinion is that you should default to using "decimal" for money work and other cases where matching human calculation exactly is important and that you should use use double as your default choice the rest of the time.