VB.Net Why does Math.Round round 5 to the nearest even number, and what can I do about it?

Question

Why is Math.Round(0.125, 2) rounding to 0.12?

Dim foo As Decimal
foo = Math.Round(0.125, 2)

foo is now 0.12 but it should b 0.13

I heard it's because some standard in .Net rounds to the nearest even number, but that's just bad math. 12.5 will round down to 12, but 13.5 will round up to 14. Is there a way to fix this?

It is banker's rounding. Use the Round() overload that takes a MidpointRounding argument to change it. — Hans Passant, Dec 16 '13 at 17:33
http://stackoverflow.com/questions/311696/why-does-net-use-bankers-rounding-as-default — , Dec 16 '13 at 17:33

score 12 · Accepted Answer · edited Jun 20 '20 at 09:12

From the documentation on the Math.Round(decimal) method:

If the fractional component of d is halfway between two integers, one of which is even and the other odd, the even number is returned.

The same logic applies to the Math.Round(decimal, int) overload. Notice:

Math.Round(0.125, 2) // 0.12
Math.Round(0.135, 2) // 0.14
Math.Round(0.145, 2) // 0.14

It's not 'bad math'; it's a common rounding strategy known as 'round-to-even'. From Wikipedia:

This variant of the round-to-nearest method is also called unbiased rounding, convergent rounding, statistician's rounding, Dutch rounding, Gaussian rounding, odd-even rounding, bankers' rounding or broken rounding, and is widely used in bookkeeping.

This is the default rounding mode used in IEEE 754 computing functions and operators.

If you want finer control over how it rounds, you can specify a MidpointRounding parameter

Math.Round(0.125, 2, MidpointRounding.AwayFromZero) // 0.13

Ah, MidpointRounding.AwayFromZero does the trick. I learn something new every day. Thanks! — BClaydon, Dec 16 '13 at 17:39

VB.Net Why does Math.Round round 5 to the nearest even number, and what can I do about it?

1 Answers1