{"openapi":"3.1.0","info":{"title":"Taxley free UK Corporation Tax tools","version":"1.0.0","description":"Free calculators for UK limited companies: the same results as the tool pages on taxley.co.uk. No key needed; please link to the share_url when you use a result. General guidance from Taxley's free calculator, based on the rules published on GOV.UK. 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Wait a minute and try again."}}},"post":{"operationId":"ukCorporationTaxEstimatePost","summary":"Corporation Tax calculator (JSON body)","description":"Estimates the Corporation Tax a UK limited company pays on its taxable profits for one accounting period of up to 12 months. Applies the small profits rate, the main rate and marginal relief, with associated companies, short periods and periods that straddle 1 April.","x-openai-isConsequential":false,"requestBody":{"required":true,"content":{"application/json":{"schema":{"type":"object","properties":{"period_start":{"type":"string","format":"date","description":"First day of the accounting period, YYYY-MM-DD (for example 2025-04-01)."},"period_end":{"type":"string","format":"date","description":"Last day of the accounting period, YYYY-MM-DD. 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Wait a minute and try again."}}}},"/api/v1/tools/late-penalty":{"summary":"Corporation Tax late filing penalty and interest calculator","description":"Works out HMRC's penalties for filing a UK company's Company Tax Return (CT600) late — the flat £200/£400 penalties, doubled for filing deadlines from 1 April 2026, and the 10% or 20% tax-geared penalty — plus late payment interest on the Corporation Tax at HMRC's published rates.","get":{"operationId":"corporationTaxLatePenaltyAndInterest","summary":"Corporation Tax late filing penalty and interest calculator (query string)","description":"Works out HMRC's penalties for filing a UK company's Company Tax Return (CT600) late — the flat £200/£400 penalties, doubled for filing deadlines from 1 April 2026, and the 10% or 20% tax-geared penalty — plus late payment interest on the Corporation Tax at HMRC's published rates.","x-openai-isConsequential":false,"parameters":[{"name":"period_end","in":"query","required":true,"description":"Last day of the Corporation Tax accounting period the CT600 covers (YYYY-MM-DD), on or after 2015-04-01.","schema":{"type":"string","format":"date"}},{"name":"period_of_account_end","in":"query","required":false,"description":"Only if the company's accounts cover more than 12 months (up to 18): the last day of that period of account, no more than 6 months after period_end. 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Wait a minute and try again."}}},"post":{"operationId":"corporationTaxLatePenaltyAndInterestPost","summary":"Corporation Tax late filing penalty and interest calculator (JSON body)","description":"Works out HMRC's penalties for filing a UK company's Company Tax Return (CT600) late — the flat £200/£400 penalties, doubled for filing deadlines from 1 April 2026, and the 10% or 20% tax-geared penalty — plus late payment interest on the Corporation Tax at HMRC's published rates.","x-openai-isConsequential":false,"requestBody":{"required":true,"content":{"application/json":{"schema":{"type":"object","properties":{"period_end":{"type":"string","format":"date","description":"Last day of the Corporation Tax accounting period the CT600 covers (YYYY-MM-DD), on or after 2015-04-01."},"period_of_account_end":{"type":"string","format":"date","description":"Only if the company's accounts cover more than 12 months (up to 18): the last day of that period of account, no more than 6 months after period_end. 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A third late return in a row costs £1,000 or £2,000 instead of £200 or £400."}},"required":["period_end","filed_on"],"additionalProperties":false},"example":{"period_end":"2025-04-30","filed_on":"2026-08-15","tax_due":10000,"paid_on":"2026-08-01","late_previous_two":false}}}},"responses":{"200":{"description":"The result.","content":{"application/json":{"schema":{"$ref":"#/components/schemas/ToolRun"}}}},"422":{"description":"The input was not valid: one plain-English message per field.","content":{"application/json":{"schema":{"$ref":"#/components/schemas/ValidationError"}}}},"429":{"description":"Too many requests. Wait a minute and try again."}}}},"/api/v1/tools/directors-loan":{"summary":"Director's loan s.455 tax calculator","description":"Works out the section 455 tax a UK close company owes on a director's or shareholder's loan still owed at the end of its accounting period, the relief for any part repaid within 9 months, the net tax and its due date. 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Wait a minute and try again."}}},"post":{"operationId":"directorsLoanS455TaxPost","summary":"Director's loan s.455 tax calculator (JSON body)","description":"Works out the section 455 tax a UK close company owes on a director's or shareholder's loan still owed at the end of its accounting period, the relief for any part repaid within 9 months, the net tax and its due date. 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Required when the 12 months to period_end include a rate change (for example a period ending between 6 April 2026 and 4 April 2027). Otherwise it defaults to the period end."}},"required":["period_end","outstanding"],"additionalProperties":false},"example":{"period_end":"2026-03-31","outstanding":25000,"repaid_within_9_months":10000}}}},"responses":{"200":{"description":"The result.","content":{"application/json":{"schema":{"$ref":"#/components/schemas/ToolRun"}}}},"422":{"description":"The input was not valid: one plain-English message per field.","content":{"application/json":{"schema":{"$ref":"#/components/schemas/ValidationError"}}}},"429":{"description":"Too many requests. Wait a minute and try again."}}}},"/api/v1/tools/capital-allowances":{"summary":"Capital allowances calculator for companies","description":"Works out a UK limited company's capital allowances for one accounting period: the annual investment allowance, full expensing, the 50% and zero-emission car first-year allowances, and writing-down allowances at 14% (18% before 1 April 2026, a hybrid rate for a period spanning that date) and 6%.","get":{"operationId":"ukCompanyCapitalAllowances","summary":"Capital allowances calculator for companies (query string)","description":"Works out a UK limited company's capital allowances for one accounting period: the annual investment allowance, full expensing, the 50% and zero-emission car first-year allowances, and writing-down allowances at 14% (18% before 1 April 2026, a hybrid rate for a period spanning that date) and 6%.","x-openai-isConsequential":false,"parameters":[{"name":"period_start","in":"query","required":true,"description":"First day of the Corporation Tax accounting period, as YYYY-MM-DD. 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Gets the annual investment allowance; any excess goes into the special rate pool.","schema":{"type":"number","minimum":0}},{"name":"zero_emission_cars","in":"query","required":false,"description":"Cost of new and unused cars with 0 g/km CO2 (fully electric). 100% first-year allowance for spending up to 31 March 2027; after that they go into the main pool. Enter a second-hand electric car under other_cars_low_co2.","schema":{"type":"number","minimum":0}},{"name":"other_cars_low_co2","in":"query","required":false,"description":"Cost of cars (other than new zero-emission cars) with CO2 emissions of 50 g/km or less. They go into the main pool. Cars never get the annual investment allowance or full expensing.","schema":{"type":"number","minimum":0}},{"name":"other_cars_high_co2","in":"query","required":false,"description":"Cost of cars with CO2 emissions over 50 g/km. They go into the special rate pool (6%).","schema":{"type":"number","minimum":0}},{"name":"main_pool_brought_forward","in":"query","required":false,"description":"Main pool balance carried forward from the previous accounting period (0 if none).","schema":{"type":"number","minimum":0}},{"name":"special_pool_brought_forward","in":"query","required":false,"description":"Special rate pool balance carried forward from the previous accounting period (0 if none).","schema":{"type":"number","minimum":0}},{"name":"main_pool_disposals","in":"query","required":false,"description":"Sale proceeds of main pool assets sold this period that did NOT have full expensing (for example, assets that had the annual investment allowance or were pooled). 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Enter the amount given to the other companies for this period; this company gets the rest.","schema":{"type":"number","minimum":0}}],"responses":{"200":{"description":"The result.","content":{"application/json":{"schema":{"$ref":"#/components/schemas/ToolRun"}}}},"422":{"description":"The input was not valid: one plain-English message per field.","content":{"application/json":{"schema":{"$ref":"#/components/schemas/ValidationError"}}}},"429":{"description":"Too many requests. Wait a minute and try again."}}},"post":{"operationId":"ukCompanyCapitalAllowancesPost","summary":"Capital allowances calculator for companies (JSON body)","description":"Works out a UK limited company's capital allowances for one accounting period: the annual investment allowance, full expensing, the 50% and zero-emission car first-year allowances, and writing-down allowances at 14% (18% before 1 April 2026, a hybrid rate for a period spanning that date) and 6%.","x-openai-isConsequential":false,"requestBody":{"required":true,"content":{"application/json":{"schema":{"type":"object","properties":{"period_start":{"type":"string","format":"date","title":"Accounting period start date","description":"First day of the Corporation Tax accounting period, as YYYY-MM-DD. Must be on or after 2023-04-01."},"period_end":{"type":"string","format":"date","title":"Accounting period end date","description":"Last day of the accounting period, as YYYY-MM-DD. The period can be up to 12 months long."},"spend_date":{"type":"string","format":"date","title":"Date of the purchases","description":"Optional. When the purchases were made (YYYY-MM-DD, inside the period). Defaults to the period end. It only changes the answer for zero-emission cars: their 100% first-year allowance is for spending up to 2027-03-31."},"new_main_plant":{"type":"number","minimum":0,"title":"New plant, machinery and vans","description":"Cost of new and unused plant, machinery, equipment and vans (not cars). Gets the annual investment allowance, then full expensing (100%) on any amount above it."},"used_main_plant":{"type":"number","minimum":0,"title":"Second-hand plant, machinery and vans","description":"Cost of second-hand main-rate plant, machinery, equipment and vans (not cars). Gets the annual investment allowance; any excess goes into the main pool."},"new_special_rate":{"type":"number","minimum":0,"title":"New special rate items","description":"Cost of new and unused integral features (electrical, lighting, heating, water, lifts, air conditioning), long-life assets, thermal insulation and solar panels. Gets the annual investment allowance, then the 50% first-year allowance on any excess."},"used_special_rate":{"type":"number","minimum":0,"title":"Second-hand special rate items","description":"Cost of second-hand integral features, long-life assets, thermal insulation and solar panels. Gets the annual investment allowance; any excess goes into the special rate pool."},"zero_emission_cars":{"type":"number","minimum":0,"title":"New zero-emission cars","description":"Cost of new and unused cars with 0 g/km CO2 (fully electric). 100% first-year allowance for spending up to 31 March 2027; after that they go into the main pool. Enter a second-hand electric car under other_cars_low_co2."},"other_cars_low_co2":{"type":"number","minimum":0,"title":"Cars with CO2 of 50 g/km or less","description":"Cost of cars (other than new zero-emission cars) with CO2 emissions of 50 g/km or less. They go into the main pool. Cars never get the annual investment allowance or full expensing."},"other_cars_high_co2":{"type":"number","minimum":0,"title":"Cars with CO2 over 50 g/km","description":"Cost of cars with CO2 emissions over 50 g/km. They go into the special rate pool (6%)."},"main_pool_brought_forward":{"type":"number","minimum":0,"title":"Main pool brought forward","description":"Main pool balance carried forward from the previous accounting period (0 if none)."},"special_pool_brought_forward":{"type":"number","minimum":0,"title":"Special rate pool brought forward","description":"Special rate pool balance carried forward from the previous accounting period (0 if none)."},"main_pool_disposals":{"type":"number","minimum":0,"title":"Main pool assets sold","description":"Sale proceeds of main pool assets sold this period that did NOT have full expensing (for example, assets that had the annual investment allowance or were pooled). Enter the lower of the sale price and what the asset cost."},"full_expensed_disposals":{"type":"number","minimum":0,"title":"Full-expensed assets sold","description":"Sale proceeds of assets that had full expensing on their whole cost, sold this period. The whole amount is a balancing charge. Enter the lower of the sale price and what the asset cost."},"aia_shared":{"type":"number","minimum":0,"title":"Annual investment allowance used by related companies","description":"Optional. Companies in a group or under common control share one £1,000,000 annual investment allowance. Enter the amount given to the other companies for this period; this company gets the rest."}},"required":["period_start","period_end"],"additionalProperties":false},"example":{"period_start":"2026-04-01","period_end":"2027-03-31","new_main_plant":40000,"used_main_plant":15000,"main_pool_brought_forward":60000}}}},"responses":{"200":{"description":"The result.","content":{"application/json":{"schema":{"$ref":"#/components/schemas/ToolRun"}}}},"422":{"description":"The input was not valid: one plain-English message per field.","content":{"application/json":{"schema":{"$ref":"#/components/schemas/ValidationError"}}}},"429":{"description":"Too many requests. Wait a minute and try again."}}}},"/api/v1/tools/chargeable-gains":{"summary":"Chargeable gains calculator for companies (with indexation)","description":"Works out a UK limited company's chargeable gain or allowable loss when it sells UK land or buildings, or shares it bought in one purchase, including indexation allowance frozen at December 2017.","get":{"operationId":"companyChargeableGainWithIndexation","summary":"Chargeable gains calculator for companies (with indexation) (query string)","description":"Works out a UK limited company's chargeable gain or allowable loss when it sells UK land or buildings, or shares it bought in one purchase, including indexation allowance frozen at December 2017.","x-openai-isConsequential":false,"parameters":[{"name":"asset_type","in":"query","required":true,"description":"\"land_buildings\": UK land or buildings (the company's whole interest; a freehold or a lease with more than 50 years left). \"shares\": shares in one company that were all bought in a single purchase and all sold together. 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