The Throttle Body Size Horsepower Calculator converts the power target into a bore diameter in mm, the airflow that the bore has to pass, and the nearest common throttle body size.
Work Out Throttle Body Bore Diameter From a Horsepower Target
Enter a power target and this throttle body size calculator returns a bore diameter in millimetres, the airflow that bore has to pass, and the nearest common 5 mm size. It is built for people choosing a throttle body for an engine swap, a boost build, or an intake upgrade.
Throttle Body Size for Common Power Targets
These are the figures the tool returns for a naturally aspirated engine with a single throttle body. They come from the calculator’s own rule, not from a manufacturer’s flow bench.
| Target power | Airflow | Exact bore | Nearest 5 mm step |
|---|---|---|---|
| 300 HP | 450 CFM | 56.7 mm | 60 mm |
| 400 HP | 600 CFM | 65.5 mm | 70 mm |
| 500 HP | 750 CFM | 73.2 mm | 75 mm |
| 600 HP | 900 CFM | 80.2 mm | 85 mm |
| 700 HP | 1,050 CFM | 86.6 mm | 90 mm |
| 800 HP | 1,200 CFM | 92.6 mm | 95 mm |
| 1,000 HP | 1,500 CFM | 103.5 mm | 105 mm |
The step column always rounds up, never down. A 73.2 mm result becomes 75 mm, not 70 mm.
The Two Constants Behind the Bore Number
The math runs in three moves. Power becomes airflow. Airflow becomes area. Area becomes a diameter.
$$CFM = HP \times 1.5 \qquad A = \frac{CFM}{115} \qquad D = 2\sqrt{\frac{A}{\pi}}$$
The first constant is the induction sizing convention of roughly 1.5 CFM per horsepower, quoted widely in carburetor and throttle body sizing guidance. It is a rule of thumb, not a standard.
The second constant, 115 CFM per square inch, could not be traced to any published throttle body specification. Treat it as this calculator’s own rule. It does hold up against physics, though. Flow of 115 CFM through one square inch works out to about 276 feet per second of air speed. That sits just under the 300 ft/s ceiling Corky Bell recommends for throttle sizing in Maximum Boost. The forced induction setting uses 130 CFM per square inch, which is about 312 ft/s.
Power in kW is converted to horsepower first, at 1.341 HP per kW. Every bore figure is reported in millimetres regardless of which unit you pick. Mass flow uses 0.075 lb per cubic foot, the standard air density figure at sea level.
Forced Induction Returns a Smaller Bore, Not a Bigger One
This surprises most people, so it is worth stating plainly. Switch a 500 HP build from naturally aspirated to forced induction and the bore drops from 73.19 mm to 72.20 mm.
The reason is in the two constants. Airflow per horsepower rises from 1.5 to 1.65, which is about 10 percent. But the area rule rises from 115 to 130, which is about 13 percent. The bigger jump wins, so the bore comes out slightly smaller.
There is a physical argument for it. Boosted air is denser, so the same power needs less volume through the throttle. Most builders still size up for boost rather than down. If you are running boost, take the forced induction figure as a floor and check it against the throttle body your intake manifold flange actually accepts.
What This Calculator Does Not Look At
There is no displacement or RPM input. Full sizing math uses CID times RPM times volumetric efficiency, divided by 3,456. This tool skips that and works from power alone. Two engines making the same power at very different RPM get the same answer here.
Throttle shaft and blade area are ignored too. A real bore loses area to the shaft running through it, so the usable opening is smaller than the circle the math assumes. On smaller bores that loss matters more.
Power has a floor of 10 HP or 10 kW and no ceiling. Enter 100,000 and it will hand you a bore over a metre wide without complaint.
Where to Measure a Throttle Body
Manufacturers spec a throttle body by its bore, the inside diameter of the housing where the blade sits. That is the number the calculator returns and the number to compare when shopping.
Why Bigger Is Not Automatically Better
The oversize card adds 10 mm to the calculated bore and shows what happens. On the 500 HP default, area climbs 29 percent and air speed at the same flow drops nearly 23 percent.
Slower air at part throttle costs you low-RPM response and mixture quality. It also compresses the useful part of the pedal travel. Most of the airflow change now happens in the first few degrees of blade movement, which is what people mean when they say a car drives like a light switch.
Common Throttle Body Sizing Questions
Why is the calculated size bigger than my stock throttle body?
Because this is a theoretical minimum for a round opening with no shaft. Real throttle bodies are tapered and radiused, and they flow better than the bare geometry suggests. Factory sizes are also chosen for driveability, not peak power.
How do I split the size across dual or quad throttle bodies?
The calculator divides total area, not diameter. Pick Dual on a 500 HP build and you get 51.75 mm per body, because two of those add up to the same area as one 73.19 mm bore. Halving the diameter instead would leave you a quarter of the area.
Will a bigger throttle body add horsepower?
Only if the current one is actually restricting airflow at peak power. If your existing bore is at or above the calculated figure, a larger one adds nothing at wide open throttle and can cost you part-throttle manners.
Should I use flywheel or wheel horsepower?
Flywheel. Airflow demand is set by what the engine makes, not what reaches the tires. Entering wheel horsepower will undersize the bore by roughly the size of your drivetrain loss.
Does the nearest 5 mm step matter more than the exact number?
Yes, in practice. Throttle bodies are sold in fixed sizes and have to match your manifold flange. Use the exact figure to know which side of a size you fall on, then buy the real part that fits.