StampDuty

StampDuty is a Stamp Duty Land Tax (SDLT) calculator for residential property in England and Northern Ireland, using the rates in force from 1 April 2025. It supports first-time buyer relief, the higher rates for additional properties and the non-UK resident surcharge. Scotland (LBTT) and Wales (LTT) are not covered.

Requires Ruby 3.3 or later.

Add this line to your application's Gemfile:

gem 'stamp_duty'

And then execute:

$ bundle

Or install it yourself as:

$ gem install stamp_duty

calc = StampDuty.for(275_000)

# a BigDecimal, rounded down to the whole pound as HMRC does
calc.stamp_duty.to_s("F") # => "3750.0"

Pass options for reliefs and surcharges:

StampDuty.for(400_000, first_time_buyer: true)    # 0% to £300k, 5% to £500k
StampDuty.for(300_000, additional_property: true) # +5% on every band
StampDuty.for(300_000, non_resident: true)        # +2% on every band

• First-time buyer relief does not apply above £500,000; standard rates are used instead.
• Surcharges only apply to purchases of £40,000 or more.
• first_time_buyer and additional_property cannot both be true.

The price must be a non-negative, finite number (Integer, BigDecimal, Rational or Float) and the options must be true or false. Anything else raises ArgumentError.

The tax due in each band is available too:

calc.band_amounts.each do |ba|
  puts ba.description
  puts ba.amount.to_s("F")
  puts ba.percentage_rate
end

Up to £125,000
0.0
0
£125,001 to £250,000
2500.0
2
£250,001 to £925,000
1250.0
5

Results are immutable. calc.to_h returns a plain hash for serialising. calc.calculate is kept for compatibility with 0.1.0 and returns the same result.

Type signatures are included in sig/.

After checking out the repo, run bin/setup to install dependencies. Then, run rake test to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install.

To release a new version, update the version number in version.rb and the CHANGELOG, merge to master, then run the Release workflow from the Actions tab. It publishes to rubygems.org using trusted publishing, so no API key is stored anywhere.

Bug reports and pull requests are welcome on GitHub at https://github.com/j-dexx/stamp_duty.

The gem is available as open source under the terms of the MIT License.