Turbo Size Calculator - Capacity to Compressor Size

A turbo size calculator matches a turbocharger to your engine by translating the engine's swept volume and your target horsepower into a single compressor inducer diameter in millimetres. If you are planning a turbo build on a naturally aspirated petrol engine, this turbo size calculator tells you whether a small, medium, or large turbo will support your goal before you spend money on hardware.

• • Project-car planning: Pick the right turbo size for a turbocharged engine build before ordering parts.
• • Budget-friendly builds: Pick the smallest turbo that still meets a horsepower goal, so the car spools up quickly without extra cost.
• • High-power builds: Confirm a target power is realistic for a given engine capacity before buying expensive internals.

Turbo sizing is more art than exact science, but the industry has converged on a simple rule: pair engine displacement with target power, then use the result to pick a compressor wheel size. Both variables drive the same underlying number, which is how much air the turbo must move per minute. The rest of the build still has to follow, including stronger internals, a proper fuel system, and enough exhaust flow.

Once the turbo size is in hand, the boost horsepower calculator estimates the actual power that same hardware will deliver, so the user can confirm the target before buying parts.

The calculator uses a single-turbo sizing heuristic that maps the product of engine capacity and target power to a compressor inducer diameter in millimetres. The same inputs also give specific output in horsepower per litre, which the calculator uses to flag requests that are too low to need a turbo or too high to be feasible for a street build.

• engineCapacity: Swept volume of every cylinder combined, entered in cc or litres and converted to litres internally.
• desiredPower: Target power output after turbocharging, entered in hp or kW and converted to mechanical hp (1 hp = 0.7457 kW).
• turboSize: Compressor inducer diameter in millimetres, the standard way to label a turbocharger.
• specificOutput: Target power per litre of displacement, used to sanity-check the request.

The formula is intentionally simple because turbo sizing is dominated by the volume of air the compressor has to move, and that volume scales with the product of displacement and target power. Specific output is shown alongside the size so the user can tell whether the request is plausible: a street-built petrol engine rarely sustains more than 200 to 250 hp/L reliably, while purpose-built race engines can reach 350 hp/L with the right parts.

According to Garrett Motion Boost Adviser, matching a turbo to an engine is done by entering crank horsepower and engine displacement into a manufacturer-published selector that sorts the catalogue against choke flow. Sizing a 2.494 L engine for 500 hp, a 2.7 L for 400 hp, and a 5.9 L Cummins for 800 hp on a single compressor lands at 47 to 57 mm, which the calculator's 15.5 mm-per-decade heuristic reproduces.

When the factory spec sheet lists torque in lb-ft and rpm, the horsepower calculator converts those numbers into the horsepower target the sizing formula needs.

Four ideas drive every turbo sizing decision. Understanding them makes the calculator's number easier to interpret and the rest of the build easier to plan.

Most of the confusion around turbo sizing comes from vendors listing turbos by part number, A/R ratio, or turbine wheel size instead of compressor size. The calculator normalises everything to compressor inducer diameter, so the answer is comparable across brands.

Squeezing a target power out of a given displacement usually means lowering the compression ratio, and the compression ratio calculator helps the user pick a piston set that keeps the engine safe on pump fuel.

Two inputs, two unit selectors, and a single millimetre answer. The tool is designed to give a usable result in under a minute.

1. 1 Enter engine capacity: Type the engine's swept volume in cc or L, and pick the matching unit. Most engine specs are listed in cc, so cc is the default.
2. 2 Enter the desired power: Type the power you want the engine to make after turbocharging, then pick hp or kW. Marketing brochures usually quote hp, so hp is the default.
3. 3 Read the recommended turbo size: The result is the compressor inducer diameter in millimetres. Match it to a turbo whose published compressor wheel diameter sits at or just above that number.
4. 4 Check the status flag: A status of 'ok' means the request is realistic. 'not_necessary' means a turbo is not worth the cost, and 'not_feasible' means the target power is too high for a street-built engine.
5. 5 Plan the rest of the build: Use the specific output reading to decide whether the engine needs stronger internals, an upgraded fuel pump, or a larger intercooler before the turbo is bolted on.

For a 2.0 L engine aimed at 300 hp, enter 2000 in cc and 300 in hp, then read 42 mm and 150 hp/L. A 42 mm turbo from a major manufacturer is a sensible street choice, and 150 hp/L is well within the range of a stock bottom end on premium fuel.

After the size is decided, the BSFC calculator shows how much fuel the engine will burn at the new power level, which feeds the fuel pump and injector choice.

A sizing estimate before buying parts saves money, prevents wasted weekend builds, and keeps the project moving.

• • Saves time on parts research: Instead of comparing every turbo in a brand's catalogue, the user starts with a single target diameter and narrows the list to models that match.
• • Prevents expensive over- or under-buying: An undersized turbo chokes a build at high rpm, while an oversized turbo kills low-end response. The result keeps the user in the middle ground.
• • Reveals unrealistic target power: The 'not_feasible' status flags power targets that would demand a fully built engine, so the user can adjust expectations before paying for parts.
• • Works for naturally aspirated petrol engines: The calculator is tuned for the most common swap project, an NA petrol engine being upgraded with a single turbocharger, and skips diesel-specific complications.
• • Supports both cc/L and hp/kW inputs: Unit selectors mean the user can paste in factory spec sheets directly without pre-converting to imperial.
• • Returns a single, comparable number: Compressor inducer diameter in millimetres is the way the industry labels turbos, so the result lines up with manufacturer spec sheets in seconds.

Pairing the size with the specific output reading also helps the user decide whether the rest of the driveline can handle the new power. A turbo that flows 500 hp on paper is only useful if the clutch, tyres, and cooling system can deliver it.

A larger turbo can demand more fuel than the stock pump can deliver, so the fuel pump calculator checks whether the existing fuel system can keep up with the new target power.

Several real-world factors shift the ideal turbo size up or down, and a few approximations are worth calling out before ordering parts.

• • The calculator assumes a naturally aspirated petrol engine being upgraded with a single turbocharger. Diesel engines, factory-turbocharged engines, and twin-turbo setups need a different curve.
• • The result is the compressor inducer diameter, not the exducer or the turbine wheel. Real turbo part numbers depend on both compressor and turbine wheel dimensions plus the A/R ratio of the housing.
• • The model is a heuristic, not a CFD simulation. A real turbo build should still include compressor maps from the manufacturer and a check against the engine's BSFC at the target power.

These factors rarely change the answer by more than 5 to 10 mm, which is why a single-number sizing estimate is still useful as a starting point. The 'not_feasible' status is itself a factor: if the request would demand more than 350 hp/L, the build needs more than just a bigger turbo, so the calculator steers the user toward a different design choice instead of an ever-larger wheel.

According to Wikipedia: Turbocharger, a turbocharger's performance is closely tied to the relative sizes of the turbine and compressor wheels, and the compressor must move enough air at peak demand to support the target flywheel power. That airflow-to-power relationship is the engineering basis for matching compressor inducer diameter to displacement and target horsepower.

Turbo size is only half the question, and the turbo boost hp gain calculator estimates the power gain a given boost pressure will deliver on top of the engine's baseline output.