State expected business miles in the tax year, broken into the first 10,000 (claimed at 45p) and any miles above (claimed at 25p). The tool calculates the simplified expense total and the equivalent income-tax saving at your marginal rate.

Enter total miles driven in the year (business + private). The tool computes business use percentage which is essential for the actual-cost calculation. Most self-employed users sit between 30 and 70% business use.

Enter annual fuel or charging cost, insurance, road tax, MOT, repairs and any capital allowance for the vehicle (typically 18% writing-down allowance per year for cars over 50g/km CO2, 100% first-year for new EVs).

Result panel shows simplified-expense total vs actual-cost total (after business use percentage), the difference, and the better method. It also shows the actual income-tax and Class 4 NI saving at your marginal rate, which is what really lands in your pocket.

If you are planning to replace your vehicle, the tool runs the maths for the new vehicle on both methods. Picking simplified for a brand-new £30,000 EV often costs you thousands compared with actual cost plus first-year allowance.

The tool generates a sample mileage log row with the fields HMRC expects to see, plus a year-end checklist of supporting documents (fuel receipts, insurance schedule, MOT, V5C). If you adopt simplified, you only need miles per journey; if you adopt actual, you need every receipt.

Simplified expenses (45p / 25p) typically win when:

• Vehicle is older or paid off (low fixed costs)
• Business mileage is high (10,000+ miles a year)
• You want low admin overhead

Actual cost typically wins when:

• Vehicle is new and expensive (capital allowance is sizeable)
• Business use percentage is high (60%+)
• Fuel/charging cost per mile is unusually high or you drive an EV
• You have major repair years

For most sole traders the difference is £200 to £1,500 per year in tax — worth doing the calculation.

HMRC business mileage:

• Travel between client sites and back
• Travel to a temporary workplace (under 24 months at one site)
• Travel from home to a one-off client visit if home is your business base
• Travel to suppliers, training and trade shows

Not business mileage:

• Home to a regular fixed workplace
• Detours for personal errands during a business trip
• Most school-run combined trips

Capital allowances on cars depend on CO2 emissions:

• 0g/km (electric, new): 100% first-year allowance
• 1 to 50g/km: main pool 18% writing-down allowance
• 51g/km+: special rate pool 6% writing-down allowance

The allowance is multiplied by business use percentage. For a £35,000 EV used 70% for business, year-1 allowance is £35,000 × 100% × 70% = £24,500 — a substantial tax deduction.

VAT-registered self-employed people can reclaim VAT on business fuel. The simplest route is to apply the HMRC fuel scale charge to private use, which lets you reclaim full input VAT but pays a fixed VAT charge based on CO2 emissions.

For low-mileage self-employed who use the car mostly for private journeys, not reclaiming VAT at all is often simpler and cheaper than the scale charge.

An HMRC enquiry into vehicle expenses typically asks for:

• Mileage log per journey: date, start, end, miles, reason
• Annual total mileage from MOT records
• Fuel receipts (for actual cost) or sample receipts (for simplified)
• Insurance and tax documents

Reconstructing a year of journeys after the fact is hard. A weekly habit of logging takes minutes; doing it during enquiry takes days.