A carburettor CFM calculation answers one question: how much air, in cubic feet per minute, does this engine draw at the speed you intend to use it? Everything else about carburettor selection follows from that number, and getting it wrong in either direction costs driveability, fuel economy or power. The calculator above works it from swept volume, engine speed, volumetric efficiency and the cycle type.
Arb Digital builds free engineering calculators that size a component honestly and stop there. This page sizes an airflow requirement. It does not approve an engine build, choose jets, or set an ignition curve, because those are measured decisions taken on a dynamometer by a qualified tuner with the engine in front of them.
What This Carburetor CFM Calculator Does
The tool converts your displacement into a per-cycle volume, multiplies by how many cycles happen per minute, scales by volumetric efficiency, and converts the result into cubic feet per minute. It handles four-stroke and two-stroke cycles separately, accepts displacement in cubic inches, litres or cubic centimetres, and lets you apply a manifold pressure ratio if the carburettor is being fed from a pressurised box.
Alongside the headline number, the grid reports the airflow the engine would draw at a hypothetical 100 per cent volumetric efficiency, the same flow in cubic metres per hour, the corresponding air mass flow at standard density, and the equivalent two-barrel catalogue rating. That last figure exists because carburettor flow ratings are measured at a stated test depression, and the two common conventions are not interchangeable.
What the page does not do is pick a part number. The right carburettor for an engine depends on venturi count and size, booster style, throttle bore, fuel curve and the character of the induction system, and none of those reduce to a single flow figure. The number here is where selection starts.
How to Use It
- Enter the total displacement. Every cylinder combined. Entering the volume of a single cylinder is the most common arithmetic slip on this page.
- Use a realistic peak speed. The airflow demand scales linearly with engine speed, so an optimistic redline produces an optimistically large carburettor.
- Be honest about volumetric efficiency. Most street engines never see the numbers people assume. Cylinder head flow, camshaft timing and inlet length set it, and it varies across the rev range rather than sitting at one value.
- Set the cycle correctly. A two-stroke fires every revolution, so for the same displacement and speed it asks for double the air.
- Check the rating convention. If you are comparing against a two-barrel catalogue figure, read the equivalent two-barrel number in the grid rather than the headline four-barrel one.
The Formula: How Carburettor CFM Is Calculated
For a four-stroke engine in imperial units the standard relation is CFM = (CID × RPM × VE) ÷ 3,456. The constant is not arbitrary: 1,728 converts cubic inches to cubic feet, and the second factor of two accounts for the fact that a four-stroke engine completes its induction cycle once every two crankshaft revolutions. NASA Glenn Research Center's page on engine mechanical operation sets out that four-stroke sequence and why the crankshaft turns twice per firing event.
For a two-stroke the divisor becomes 1,728, because induction happens once per revolution. In SI terms the same relation is volumetric flow = displacement × speed ÷ (60 × strokes-per-cycle) × VE, with displacement in cubic metres and speed in revolutions per minute; the tool works internally in SI and converts at the end, which is why the litre and cubic-centimetre inputs give exactly consistent answers. NASA Glenn's page on the internal combustion engine Otto cycle shows the constant-pressure intake stroke that this volume bookkeeping is built on.
Work the defaults. A 350-cubic-inch four-stroke at 6,000 rpm with 85 per cent volumetric efficiency gives 350 × 6,000 = 2,100,000; multiplied by 0.85 gives 1,785,000; divided by 3,456 gives 516.5 CFM. At a hypothetical 100 per cent volumetric efficiency the same engine would ask for 607.6 CFM, which is the grid figure and a useful ceiling to keep in mind.
Air mass flow follows from density. At standard sea-level conditions dry air is close to 1.225 kg/m³, so 516.5 CFM — about 877 m³/h — carries roughly 1,075 kg of air per hour. That mass, not the volume, is what the fuel system has to match, and it is why the air density calculator matters as much as this one on a hot day or at altitude.
Why the Rating Depression Changes the Number
A carburettor's CFM rating is a bench measurement, not a property of the part in isolation. Air is pulled through the assembly at a fixed pressure drop and the flow is recorded. The convention for four-barrel carburettors is a 1.5 inHg depression; for two-barrel carburettors it is 3.0 inHg. Because flow through a restriction rises with the square root of the pressure drop, the same hardware measured at twice the depression reads about 1.41 times higher.
That factor is the single most common source of confusion in carburettor selection. A 500 CFM two-barrel and a 500 CFM four-barrel are not the same airflow. Converted to a common basis, the two-barrel is flowing roughly 354 CFM at the four-barrel's 1.5 inHg test point. The grid figure on this page performs that conversion so the comparison is like for like.
The deeper point is that the rating tells you what the carburettor does at one pressure drop, while the engine imposes whatever pressure drop it likes. On a small engine at part throttle the depression across the venturi may be a fraction of the rating condition, which is exactly where an oversized unit stops metering cleanly.
Volumetric Efficiency Is the Hard Input
Displacement is a fact and engine speed is a decision, but volumetric efficiency is an outcome. It is the ratio of the mass of air actually trapped in the cylinder to the mass that would fill the swept volume at inlet conditions, and it is set by everything in the breathing path: valve area and lift, port shape, camshaft timing and overlap, inlet runner length and diameter, and exhaust scavenging.
It is also not a single number. Volumetric efficiency peaks somewhere in the rev range, usually near the torque peak, and falls away either side. Entering the peak value with the peak engine speed therefore overstates demand slightly, because an engine rarely achieves its best filling at its highest speed. A typical naturally aspirated road engine peaks around 80 to 90 per cent; a well-developed race engine with a tuned inlet can exceed 100 per cent because inertial ram effects push in more charge than static pressure alone would.
If you are working from measured data rather than an estimate, mass airflow is the cleaner route: measure the air mass the engine consumes, convert it back to a volume at inlet conditions, and divide by the swept volume per unit time. The engine displacement calculator and the engine RPM calculator give the two denominators for that sum.
Why Bigger Is Not Better
The instinct to fit the largest carburettor that will bolt on is understandable and usually wrong. A carburettor meters fuel using the pressure signal generated by air accelerating through the venturi. Make the venturi too large for the engine and that signal collapses at low and mid airflow, so the transition from idle circuit to main circuit becomes vague, throttle response softens and the engine loads up.
The opposite error is real but rarer and more visible. An undersized carburettor becomes a restriction at high speed, the manifold depression climbs, and the engine simply stops making power near the top of the range. The symptom is unmistakable on a dyno trace: torque falls away faster than the cam timing would explain.
In practice, sizing near the calculated demand and choosing a unit with the right venturi and booster arrangement beats sizing above it. That said, none of these choices are settled by arithmetic. Jetting, air-bleed sizing, accelerator pump calibration, ignition timing and knock margin are dynamometer decisions for a qualified engine tuner, made with instrumentation on the specific engine. This page sizes a component; it does not approve a build.
Where This Sits Among the Other Engine Tools
This page owns the induction airflow demand. For the swept volume that feeds it, use the engine displacement calculator, and for the drivetrain relationship behind your chosen peak speed, the engine RPM calculator. Compression is a separate question handled by the engine compression ratio calculator. On the fuel side, the air fuel ratio calculator turns this air mass into a fuel mass, and the fuel pump size calculator turns that into a delivery requirement. For pneumatic rather than engine airflow, the SCFM calculator handles standard-condition volume flow, and the flow rate converter moves any of these figures between unit systems.
Arb Digital builds free tools like this one because useful pages earn attention. If you want tools, calculators or content built for your own audience, we can help.
Browse All Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Entering one cylinder's volume — the formula wants total displacement. A 350 CID V8 has a 43.75 CID cylinder, and using the smaller figure gives an eighth of the real demand.
- Comparing ratings across conventions — two-barrel and four-barrel figures are measured at different depressions and differ by about 41 per cent for identical hardware.
- Assuming 100 per cent volumetric efficiency — almost no naturally aspirated engine achieves it across the range, and using it inflates the requirement by around a fifth.
- Using the tachometer redline as peak speed — airflow scales linearly with speed, so a thousand optimistic revolutions per minute is a meaningful oversize.
- Treating the answer as a tuning decision — airflow demand sizes a part. Jetting, timing and knock margin are measured on a dynamometer, not derived from a formula.
Related Free Tools From Arb Digital
Start from the engine displacement calculator, sanity-check your peak speed with the engine RPM calculator, and handle the compression side with the engine compression ratio calculator. Convert this airflow into a fuelling requirement with the air fuel ratio calculator and the fuel pump size calculator. Correct for hot or high-altitude air with the air density calculator, and move between flow units with the SCFM calculator and the flow rate converter. Everything Arb Digital publishes is listed on the free online tools hub.
Frequently Asked Questions
For a four-stroke engine it is CFM equals cubic inches multiplied by rpm multiplied by volumetric efficiency, divided by 3,456. The constant combines the 1,728 cubic inches in a cubic foot with the factor of two for the two crankshaft revolutions a four-stroke needs per induction cycle.
Because they are measured at different bench depressions: 3.0 inches of mercury for two-barrel units and 1.5 for four-barrel units. Flow through a restriction rises with the square root of pressure drop, so the same hardware reads about 41 per cent higher on the two-barrel convention.
Use a figure you have measured or that matches the specification of your cylinder heads and camshaft. A standard road engine is typically 75 to 85 per cent, a well-developed naturally aspirated performance engine 90 to 100 per cent, and a strongly tuned race engine can exceed 100 per cent through inlet ram effects.
No. Metering depends on the pressure signal generated in the venturi, and an oversized venturi produces too weak a signal at low and mid airflow. The result is vague transition, soft throttle response and poor economy, without any gain at the top end that the engine can actually use.
For the same displacement and engine speed, yes, roughly double. A two-stroke completes an induction event every crankshaft revolution rather than every second revolution, so the divisor in the formula is 1,728 rather than 3,456.
The volumetric demand in cubic feet per minute barely changes, but the mass of air in that volume falls with density. Since fuel has to be matched to air mass rather than air volume, a carburettor calibrated at sea level runs rich at altitude unless it is rejetted.
No, and no formula can. Jet selection depends on the specific carburettor circuit design, fuel, air temperature, ignition timing and the mixture the engine actually measures under load. That work is done on a dynamometer with instrumentation by a qualified tuner.
This tool is provided for educational and preliminary sizing use only. It estimates an airflow requirement and does not approve, validate or specify an engine build. Fuelling, ignition timing and knock margin are measured decisions for a qualified engine tuner on a dynamometer, and modifying an engine may affect emissions compliance, insurance and road legality in your jurisdiction.
