a library for financial modelling in Ruby.


$ sudo gem install finance


Contributions by @thadd and @bramswenson have made the finance library fully compatible with rails, at the cost of the #years and #months convenience methods on Integer, as well as the #to_d method for converting Numerics into DecNums. These methods have been removed, due to conflicts with existing rails methods.

Correspondingly, finance has been bumped up to version 2.0.



>> require 'finance'

Note: As of version 1.0.0, the entire library is contained under the Finance namespace. Existing code will not work unless you add:

>> include Finance

for all of the examples below, we'll assume that you have done this.


You are interested in borrowing $250,000 under a 30 year, fixed-rate loan with a 4.25% APR.

>> rate =, :apr, :duration => (30 * 12))
>> amortization =, rate)

Find the standard monthly payment:

>> amortization.payment
=> DecNum('-1229.91')

Find the total cost of the loan:

>> amortization.payments.sum
=> DecNum('-442766.55')

How much will you pay in interest?

>> amortization.interest.sum
=> DecNum('192766.55')

How much interest in the first six months?

>> amortization.interest[0,6].sum
=> DecNum('5294.62')

If your loan has an adjustable rate, no problem. You can pass an arbitrary number of rates, and they will be used in the amortization. For example, we can look at an amortization of $250000, where the APR starts at 4.25%, and increases by 1% every five years.

>> values = %w{ 0.0425 0.0525 0.0625 0.0725 0.0825 0.0925 }
>> rates = values.collect { |value| value, :apr, :duration => (5  * 12) }
>> arm =, *rates)

Since we are looking at an ARM, there is no longer a single "payment" value.

>> arm.payment
=> nil

But we can look at the different payments over time.

>> arm.payments.uniq
=> [DecNum('-1229.85'), DecNum('-1360.41'), DecNum('-1475.65'), DecNum('-1571.07'), ... snipped ... ]

The other methods previously discussed can be accessed in the same way:

>> arm.interest.sum
=> DecNum('287515.45')
>> arm.payments.sum
=> DecNum('-537515.45')

Last, but not least, you may pass a block when creating an Amortization which returns a modified monthly payment. For example, to increase your payment by $150, do:

>> rate =, :apr, :duration => (30 * 12))
>> extra_payments = 250000.amortize(rate){ |period| period.payment - 150 }

Disregarding the block, we have used the same parameters as the first example. Notice the difference in the results:

>> amortization.payments.sum
=> DecNum('-442745.98')
>> extra_payments.payments.sum
=> DecNum('-400566.24')
>> amortization.interest.sum
=> DecNum('192745.98')
>> extra_payments.interest.sum
=> DecNum('150566.24')

You can also increase your payment to a specific amount:

>> extra_payments_2 = 250000.amortize(rate){ -1500 }


I started developing finance while analyzing mortgages as a personal project. Spreadsheets have convenient formulas for doing this type of work, until you want to do something semi-complex (like ARMs or extra payments), at which point you need to create your own amortization table. I thought I could create a better interface for this type of work in Ruby, and since I couldn't find an existing resource for these tools, I am hoping to save other folks some time by releasing what I have as a gem.

More broadly, I believe there are many calculations that are necessary for the effective management of personal finances, but are difficult (or impossible) to do with spreadsheets or other existing open source tools. My hope is that the finance library will grow to provide a set of open, tested tools to fill this gap.

If you have used finance and find it useful, I would enjoy hearing about it!


Currently implemented features include:

  • Uses the flt library to ensure precision decimal arithmetic in all calculations.
  • Fixed-rate mortgage amortization (30/360).
  • Interest rates
  • Various cash flow computations, such as NPV and IRR.
  • Adjustable rate mortgage amortization.
  • Payment modifications (i.e., how does paying an additional $75 per month affect the amortization?)


This library is released under the terms of the LGPL license.

Copyright (c) 2011, William Kranec. All rights reserved.

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see