Enter your engine’s cylinder bore and number of cylinders (or total displacement) to calculate the taxable horsepower using the RAC/SAE fiscal rating formula.
Related Calculators
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- Net Horsepower To Gross Horsepower Calculator
- Corrected Horsepower Calculator
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Formula
The RAC/SAE taxable horsepower formula:
HP = (D2 × N) ÷ 2.5
where HP = taxable horsepower, D = cylinder bore diameter in inches, N = number of cylinders.
When only displacement is known, bore is estimated assuming a square engine (stroke = bore):
D = ∛(4V / π)
where V = per-cylinder displacement in cubic inches.
Interpretation
Taxable horsepower is a nominal rating used historically for vehicle taxation and registration in the UK, France, and other countries. It does not represent real engine output (bhp) — a modern 2.0 L four-cylinder rated at around 160 bhp has a taxable HP of roughly 16. The value scales with bore area and cylinder count only; stroke, compression, tuning, and RPM are ignored. Use the following rough bands for context:
- Under 10 HP: very small engines, mopeds, microcars.
- 10–20 HP: light cars and motorcycles.
- 20–30 HP: typical mid-size vintage car.
- 30–50 HP: large or luxury vintage car.
- Over 50 HP: very large multi-cylinder engines.
Example Values
| Bore (in) | Cylinders | Taxable HP |
|---|---|---|
| 2.50 | 4 | 10.0 |
| 3.00 | 4 | 14.4 |
| 3.25 | 6 | 25.4 |
| 3.50 | 8 | 39.2 |
| 4.00 | 8 | 51.2 |
FAQ
Is taxable horsepower the same as brake horsepower?
No. Taxable HP is a fiscal figure based only on bore and cylinder count. Brake horsepower (bhp) measures actual power output at the crankshaft and is typically several times higher.
Why isn’t the stroke included in the formula?
The RAC formula was set in 1910 when it was assumed stroke was fixed by convention. Manufacturers quickly responded by building long-stroke, small-bore engines to minimize tax, which is why many vintage British engines are undersquare.
What units should I use for bore?
The formula requires inches. The calculator converts mm to inches automatically if you select that unit.
How accurate is the “from displacement” mode?
It assumes a square engine (stroke equals bore) to back-calculate bore. Real engines vary, so the result is an estimate — use the bore-and-cylinder mode when you know the actual bore.
