Two numbers, one machine, neither sufficient
Every blower on a shelf carries a cfm figure and an mph figure. Taken alone each is meaningless as a ranking: a machine can move a lot of air slowly or a little air quickly, and both can be true of machines that behave very differently. What actually pushes a leaf is momentum — the mass of air arriving each second multiplied by how fast it is going — and that needs both numbers.
With the defaults, 700 cfm at 200 mph in air at 0.0765 lb per cubic foot works out at 53.6 pounds of air a minute leaving at 293 feet a second, which is 8.14 pounds of force, or 36.2 newtons. That last unit is the one makers print when they print anything sensible at all.
The check that costs one division
Divide the airflow by the speed and you get the area the air must be passing through for both figures to describe the same place. 700 cfm at 200 mph gives 5.73 square inches, which is a round hole 2.70 inches across. If the tube on the machine measures 2.5 inches inside, the two figures are consistent within about fifteen percent and multiplying them means something.
Where it goes wrong is when the airflow was measured at the fan housing and the speed at the end of a narrowed nozzle. Both readings are honest; the product is not, because that much air never leaves at that speed. The implied area then comes out far larger than the nozzle, which is the tell. It does not make the comparison between two machines useless — if both are quoted the same way the ranking survives — but it does mean the absolute force number is inflated.
Speed is expensive, and force does not say so
Force rises in a straight line with speed at fixed airflow. The kinetic power in the stream rises with the square of it. Doubling the air speed of the default machine from 200 to 400 mph takes the force from 8.14 to 16.27 pounds and the air power from 2.17 to 8.68 horsepower — twice the force for four times the power. That is the whole reason blower design is a compromise rather than a race, and why big backpack machines chase airflow while handhelds chase speed.
The air power figure is also a reality check on the machine rating. Two point one seven horsepower of kinetic energy in the air stream is what the fan actually delivered; the engine rating is much larger, and the difference is fan efficiency, duct losses and everything else between combustion and moving air. A large gap there is normal and not a fault.
Density, the input nobody changes
The force is proportional to air density, so the same machine at the same cfm and mph makes less force in thin air. Altitude and heat both thin it. The field defaults to the textbook sea level figure at 59 F and it is there to be replaced, not because a blower is fussy but because the arithmetic is honest about what it assumed.
What the number will not settle
Whether a machine suits your work. A wide slow stream and a narrow fast one can print the same newtons and do quite different things — one sweeps a broad band of dry leaves off a lawn, the other lifts a wet mat off tarmac. Nozzle shape, whether the stream stays coherent a few feet out, weight on your back and how long you can stand the noise are all real and none of them are in this arithmetic. The force figure is one axis, correctly computed, and that is all it claims to be.
Questions people ask
Is CFM or MPH more important on a leaf blower?
Neither on its own. What moves a leaf is momentum, which is airflow multiplied by speed, so a machine can lead on cfm and trail on force. 700 cfm at 200 mph gives 8.14 pounds of force; 450 cfm at 250 mph gives 6.54, so the higher mph machine is the weaker one on this measure despite the bigger headline speed.
How do you calculate leaf blower force in newtons?
Multiply air density by airflow to get the mass leaving each minute, convert to mass per second, and multiply by the air speed in feet per second. 0.0765 lb per cubic foot times 700 cfm is 53.6 lb a minute, which at 293 feet a second gives 8.14 pounds of force, or 36.2 newtons once converted.
Why do the CFM and MPH numbers not seem to match the nozzle?
Because they are often measured at different places. Airflow at the fan housing and speed at a narrowed nozzle are both honest readings of one machine, but the product of them overstates the force, since that much air never leaves at that speed. Dividing airflow by speed gives the area the pair implies, and comparing it against the tube diameter shows the gap in one line.
How much power is actually in a blower air stream?
Less than the engine rating, by a long way. The default machine puts 2.17 horsepower of kinetic energy into the air, and whatever the engine is rated at, the difference is fan efficiency and duct losses. A large gap is normal. The air power also rises with the square of the speed while force rises only in proportion, which is why chasing mph gets expensive quickly.
Does altitude change how well a blower works?
The force is proportional to air density, so thinner air at altitude or on a hot day carries less mass at the same volume flow and makes proportionally less force. How much thinner is a separate calculation from pressure, temperature and humidity, and the page links to it. What the machine does with that thinner air is a question for the machine.