Calculate Fiscal Horsepower (Tax HP)
Choose a method based on your country or registration system, enter your vehicle details, and get an estimate instantly.
What Is Fiscal Horsepower?
Fiscal horsepower (often called tax horsepower, CV, or fiscal power) is a number used by governments to calculate registration taxes, annual road taxes, or ownership fees. It is not the same as your car’s real engine output. In other words, a vehicle with high mechanical horsepower does not always have high fiscal horsepower—and vice versa.
Countries define fiscal horsepower differently. Some systems rely on CO₂ emissions and power output; older systems used engine dimensions such as cylinder bore and number of cylinders. That’s why calculators must specify the formula they use.
How This Fiscal Horsepower Calculator Works
1) France-style method (kW + CO₂)
This method estimates fiscal horsepower with a formula based on engine power in kilowatts and CO₂ emissions in g/km:
- CV = (CO₂ / 45) + (kW / 40)1.6
- Fiscal HP = rounded CV
It is commonly used as a practical estimate for French registration contexts. Depending on the year, fuel type, and local rules, official paperwork may still differ slightly.
2) RAC taxable horsepower method (historic)
The RAC formula is a classic taxable horsepower method historically used in the UK and referenced in vintage car discussions:
- Tax HP = (D² × n) / 2.5
- D = bore diameter in inches, n = number of cylinders
Since many modern specs are listed in millimeters, this calculator converts mm to inches automatically.
Why Fiscal Horsepower and Engine Horsepower Are Different
Mechanical horsepower measures what an engine can produce. Fiscal horsepower is an administrative index designed for taxation policy. Governments use it to create incentives around emissions, efficiency, and vehicle class.
- Engine horsepower: performance output (bhp, hp, kW).
- Fiscal horsepower: tax category metric for fees and registration.
- CO₂ role: in many modern systems, emissions significantly affect taxes.
Quick Example Calculations
Example A: France-style
Suppose your car has 110 kW and 135 g/km CO₂:
- CO₂ part: 135 / 45 = 3.00
- Power part: (110 / 40)1.6 ≈ 5.05
- CV ≈ 8.05 → Fiscal HP ≈ 8
Example B: RAC method
Suppose bore is 82 mm and cylinders are 4:
- 82 mm = 3.23 inches
- Tax HP = (3.23² × 4) / 2.5 ≈ 16.7
Depending on how your authority rounds values, your payable class may use the nearest whole number or the exact value.
How to Use Fiscal HP to Plan Vehicle Costs
Fiscal horsepower can help you estimate ownership cost before buying a vehicle. A smart pre-purchase check should include:
- Estimated registration fee based on fiscal HP.
- Annual road tax in your region.
- Insurance differences between vehicle classes.
- Fuel economy and expected maintenance costs.
The optional “tax rate per fiscal HP” field in the calculator gives a fast budgeting shortcut. If your local authority charges a fixed amount per fiscal HP unit, you can estimate annual cost in seconds.
Frequently Asked Questions
Is this an official government tool?
No. It is an educational estimator designed to help with planning and comparison.
Can I use brake horsepower (bhp) directly?
Not for these formulas. The France-style method expects kW and CO₂. The RAC formula expects bore and cylinders.
Why does my official document show a different value?
Authorities may apply specific rounding rules, updates by year, regional coefficients, electric/hybrid adjustments, and administrative fees beyond pure fiscal horsepower.
Final Thoughts
A fiscal horsepower calculator is one of the simplest ways to make smarter vehicle decisions. Whether you are importing, registering, comparing trims, or budgeting annual expenses, knowing your tax horsepower can prevent surprises. Use this page as a quick estimate tool, then verify final numbers with your local registration office.