From: brabuhr@...
Date: 2010-06-30T21:48:50+09:00
Subject: Re: Increase significant digits in Float

On Wed, Jun 30, 2010 at 8:06 AM, Neve Le <nevele@gmx.com> wrote:
> Siep Korteling wrote:
>>> Xavier Noria wrote:
>>> That 15 is hard-coded in the definition of Float#to_s.
>>
>> Yes, and exposing more digits is not accurate:
>> The float is bogus  after the 16th digit.
>
> Working with big numbers here, it seems that precision is only
> guaranteed to 15 digits on either side of the decimal. When a whole
> number >15 digits is divided the decimal is approximated, and anything
>>17 digits prints a float of n.0 no matter how it's "%n.nf" formatted.

http://ruby-doc.org/core/classes/Float.html
"Float objects represent real numbers using the native architecture‘s
double-precision floating point representation."

http://en.wikipedia.org/wiki/Double_precision_floating-point_format

You could try BigDecimal.