UK PAYE Tax Calculator

A Windows desktop application for estimating UK income tax liability across multiple tax years.

IMPORTANT DISCLAIMER

This application is provided for estimation purposes only.

• There is no warranty of any kind, express or implied
• There is no support provided
• Calculations may be wrong, incomplete, or out of date
• Tax rules are complex and change frequently — this tool does not guarantee accuracy
• Do not rely on this application for filing tax returns or making financial decisions
• Always consult HMRC or a qualified tax professional for definitive guidance
• The authors accept no liability for any errors, omissions, or consequences arising from use of this software

Use entirely at your own risk.

Support

If you find this app useful, please consider buying me a coffee!

Features

Income & Employment

• Multiple tax years — tabs for 2023/24, 2024/25, 2025/26 with year-specific rules
• Multiple employments & pensions — add as many income sources as needed per year
• Pension/annuity flag — marks non-employment income (hides NI/expenses fields)
• Savings interest — with ISA tax-free flag
• Scottish taxpayer support — uses Scottish income tax bands when selected

Allowances & Reliefs

• Personal Allowance — with automatic taper above £100,000
• Marriage Allowance — transfer or receive allowance from spouse
• Blind Person's Allowance
• Gift Aid donations — extends basic rate band

Allowable Expenses

• Working from home allowance — HMRC flat rate (£6/week)
• Business mileage — AMAP rates (45p first 10,000 miles, 25p thereafter)
• Professional subscriptions — HMRC-approved bodies only
• Uniform/work clothing allowance — flat rate or actual cost
• Other allowable expenses — with description field

Pension Tax Credits

• Workplace pension contributions — deducted from taxable income (Net Pay/AVC schemes)
• Personal pension contributions (Relief at Source) — calculates additional tax relief claimable:
  • Automatically grosses up net contributions
  • Identifies higher/additional rate taxpayers eligible for extra relief
  • Shows claimable amount with detailed breakdown
  • Provides guidance on how to claim (Self Assessment, tax code adjustment, or writing to HMRC)
  • Warns if contributions exceed the Annual Allowance (£60,000)

Tax Calculation

• Income tax bands — basic (20%), higher (40%), additional (45%) rates
• Scottish bands — starter (19%), basic (20%), intermediate (21%), higher (42%), advanced (45%), top (48%)
• Savings income — starting rate for savings, personal savings allowance
• NI verification — compares paid vs expected employee Class 1 NIC

Payroll Estimator

• Period-by-period breakdown — monthly or weekly PAYE, NI and pension estimates for a full tax year
• Pension modelling — employee and employer pension as percentage of gross or fixed amount per period (salary sacrifice)
• Tax year data import — when selecting a tax year that has a matching tab with salary data, offers to copy gross salary, tax code and taxpayer type into the estimator; auto-populates silently on first load if the default year has data

User Experience

• Formatted numbers — thousand separators and 2 decimal places for currency
• Carry forward — new tax years inherit non-ended employments and savings with zero totals
• Window position/size — remembered between sessions
• Auto-save — data persisted to local app data as JSON
• Export options — Excel (.xlsx), PDF, Word (.docx)

Pension Contributions Guide

The app handles two different types of pension contributions:

Type	Where to Enter	How Tax Relief Works
Workplace pension (deducted from salary)	Employment section → "Pension Contributions"	Full relief automatic via PAYE
Personal pension/SIPP (paid from bank account)	Other Details → "Personal Pension Contributions"	20% added by provider; higher rates claim back

Example: Personal Pension (Relief at Source)

1. You pay £800 from your bank account to a SIPP
2. Pension provider adds £200 (20% basic rate relief) → £1,000 gross
3. If you're a 40% taxpayer, you can claim back an additional £200 from HMRC
4. The app calculates this automatically and shows how to claim

Building

Requires .NET 8 SDK with Windows App SDK workload.

dotnet build PAYETAXCalc.csproj -p:Platform=x64

Running Tests

dotnet test PAYETAXCalc.Tests\PAYETAXCalc.Tests.csproj

MSI Installer

WiX Toolset v5 installer projects are included for x64 and x86 builds.

Build an MSI locally:

dotnet build PAYETAXCalc.Installer.x64.wixproj -c Release

Output: bin\installer\PAYETAXCalc-Setup-x64.msi

The installer project automatically publishes the app self-contained, harvests all output files using HeatWave, and packages them into a single MSI with Start Menu and Desktop shortcuts.

CI / CD

GitHub Actions (.github/workflows/ci.yml) runs on every push and pull request to master:

Job	Trigger	What it does
Build & Test	All pushes / PRs	Restores, builds, and runs all xUnit tests; uploads TRX results as an artifact
Build & Release MSI	Push to master only	Builds the x64 MSI and publishes a GitHub Release tagged with the version from Package.appxmanifest

To release a new version:

1. Update Version in Package.appxmanifest (e.g. 1.2.0.0)
2. Push to master
3. GitHub Actions builds the MSI and creates a release tagged v1.2.0.0 automatically

Tech Stack

• WinUI 3 / Windows App SDK
• .NET 8
• Mica backdrop
• WiX Toolset v5 (MSI installer)
• xUnit for testing
• GitHub Actions for CI/CD

License

See LICENSE file for details.