Given a country and either the gross or net income, this library can estimate the personal income tax. These are very rough estimates, mostly ignoring any potential tax deductions.

It has the following goals:

• Zero external dependencies.
• Compatible with the money gem.
• Full test coverage.

require 'income_tax'

# €4k per month before taxes in Estonia, which has a fixed 20% tax rate
tax_info = IncomeTax.new("Estonia", "4k/mo")
tax_info.gross_income # => 48000
tax_info.net_income   # => 38400
tax_info.taxes        # => 9600
tax_info.rate         # => "20%"
tax_info.to_f         # => 0.2
tax_info.to_r         # => (1/5)

# Making €4k after taxes
tax_info = IncomeTax.new("Estonia", "4k/mo", :net)
tax_info.gross_income # => 60000
tax_info.net_income   # => 48000

# Using the money gem (will automatically convert to appropriate currency)
require "money"
tax_info = IncomeTax.new("Estonia", Money.new(100_000, "USD"))
tax_info.gross_income # => 89000 (depending on exchange rate)

Unless handed in as money object, values are assumed to be in the local currency, with the notable exception of Zimbabwe, for which USD is used.

This gem uses a very broad definition of countries, whereby the following are counted as separate entities:

• UN member states.
• Overseas territories, outermost regions and colonies.
• Some autonomous regions enjoying partial sovereignty, esp. if they have full autonomy over tax regime and collection (like Hong Kong).
• States with limited but widespread recognition (like Taiwan, Kosovo, Palestine).

Things not considered separate countries:

• Sovereign entities without any actual territory (The Holy See, Sovereign Military Order of Malta).
• States with very limited recognition (like Northern Cyprus, South Ossetia).
• Scotland, Wales, England and Northern Ireland (they are collectively treated as one country).
• Unclaimed or heavily disputed territories.
• Antarctica.

These decisions and differentiations are made to best model international income taxes and are not meant to convey any political opinion.

Of course, suggestions and patches are always welcome.

There is also a full list of countries.

• Federal only: By default, only federal taxes are calculated.
• State taxes: Passing in the state option with either the full name of the state or the two letter abbreviation will also take state taxes into account.
• Washington, D.C.: Use the state option as if it was a state, which by the way it should be.
• Guam, Puerto Rico, and other territories: Treat them like separate countries.

IncomeTax.new("US", "100k")
IncomeTax.new("US", "100k", state: "California")

IncomeTax.new("Guam", "100k")

• Federal only: By default, only federal taxes are calculated.
• Territorial taxes: Passing in the territory option with either the full name of the territory or the two letter abbreviation will also take territorial taxes into account.

IncomeTax.new("CA", "100k")
IncomeTax.new("CA", "100k", territory: "British Columbia")

You can use the canton option with either the full canton name or the two letter abbreviation. If not passed in, it will default to ZH (Zurich).

IncomeTax.new("CH", "100k", canton: "Bern")

You can set the location option to Mainland (default) or Zanzibar.

IncomeTax.new("TZ", "16m", location: "Zanzibar")

• married: Whether or not the tax payer is married. Respected by the French, German, Irish, Maltese, Nepali, Nicaraguan, Swiss and U.S. American tax model.
• children: Number of children the tax payer has to provide for. Respected by the French, Maltese and Swiss tax model.
• joint_statement: If married, whether or not both spouses file a joint statement. Respected by the U.S. American tax model.
• head_of_household: Whether or not the tax payer is the head of the household. Respected by the U.S. American tax model.
• self_employed: Whether the tax payer is employed or freelancing. Respected by the Costa Rican and Nicaraguan tax model.
• age or birthday: To determine age depended taxes. Respected by the Australian, Bangladeshi and Barbadian tax model.
• gender: male or female, respected by the Bangladeshi tax model.
• disabled: Whether or not the tax payer has a physical or psychological disability, respected by the Bangladeshi tax model.
• wounded_freedom_fighter: Whether or not the tax payer is a war-wounded freedom fighter, respected by the Bangladeshi tax model.
• presumptive: Whether or not presumptive taxes are paid. Respected by the Tanzanian tax model. This value has a smart default.
• tax_year: Year this income tax is due for. Supported by all tax models but tax rates might be incomplete for past or future tax years (will fall back on previous year if data is missing).

General limitations:

• It only calculates personal income tax (ie, the tax the employee has to pay).
• Religious taxes (like church taxes in Germany or Khums in Iran) are currently not taken into account.
• Tax deductions are largely not taken into account.
• Double taxation is not taken into account.
• Rules around multiple tax regimes based on citizenship are not taken into account.
• No differentiation between domestic and foreign income.
• It only has limited support for taxes changing over time. Wherever possible, rates for the current or even upcoming year have been used.

Specific examples of what is not taken into account:

• French citizens (or in some cases, any new citizens) living in overseas territories often have to pay French income tax for the first five years.
• French citizens living or working in Monaco have to pay French income tax instead.
• Employees in the West Bank have a high likelihood of paying Israeli income tax instead of Palestinian income tax.
• For people living in Puerto Rico and Guam, there is a chance that they may have to pay US federal income tax instead.
• Mali has a choose your own tax adventure, and livestock is acceptable currency when paying taxes.

These lists are very much incomplete.

Suggestions and patches on addressing any of these are, of course, more than welcome.