# Rails Tip: Precision and scale for decimals

For when you need that little bit of extra accuracy, specifying precision and scale for a decimal column in your Ruby on Rails migration is pretty simple. The precision represents the total number of digits in the number, whereas scale represents the number of digits following the decimal point. To specify the precision and scale, simply pass those as options to your column definition.

For example:

```
class AddLatLngToAddresses < ActiveRecord::Migration
def self.up
add_column :addresses, :lat, :decimal, :precision => 15, :scale => 10
add_column :addresses, :lng, :decimal, :precision => 15, :scale => 10
end
def self.down
remove_column :addresses, :lat
remove_column :addresses, :lng
end
end
```

This will allow you to have 10 digits after the decimal point and 15 digits max.

One thing to note, however is that Rails will use BigDecimal as the type for the column. BigDecimal provides support for very large or very accurate floating point numbers. Remember those pesky floating point imprecision errors?

```
>> 1.2 - 1.0 == 0.2
=> false
```

Yep, BigDecimal handles that…

```
>> BigDecimal.new('1.2') - BigDecimal.new('1.0') == BigDecimal.new('0.2')
=> true
```

So now, go forth and be accurate.

Also see

Relatively new to Rails and never took the time to understand this simple idea. I just keep Googling it until today, when I realized it would be much better if I just took a few seconds to understand what 'precision' and 'scale' actually did.

Thanks for the clear explanation, quick and easy…

February 1st, 2010 at 10:23 pmThanks!

January 8th, 2012 at 3:21 pm