This is from "How to Design and Build Centrigugal Fans for the Home Shop" by Dave Gingery
HP= horsepower required
C= volume of air in cubic feet per second
h= pressure in inches water column
E= efficiency (.40-.60 average)

HP= ( C * h * 5.2 ) / ( 550 * E )

If we assume .50 for efficiency it can be further simplified

HP=( C * h * 5.2 ) / 275

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To get volume of air, C, in the above equation.

First you need velocity,
Assume a 10 inch diameter wheel 3 inches wide spinning at 1725 rpm

10 * 3.1416=31.416" wheel circumference
31.416 / 12 = 2.618 feet per revolution
2.618 * 1725 rpm = 4516.05 feet per minute

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Next we need velocity pressure in inches of water column (h)

velocity is in ft per second here
4516ft/min / 60 seconds/minute = 75.27 feet per second

h= (velocity / 66) * (velocity / 66)

h= (75.27 / 66) * (75.27 / 66)
h= 1.14 * 1.14
h= 1.3 inches water column pressure

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Output

A=blast area in sq inches
D= wheel diameter in inches
W= wheel width in inches

A=(D * W)/3
A=(10 * 3)/3
A=(30)/3
A=10 square inches blast area......

10 square in/ 144 sq inches per sq foot= .069 square feet blast area

4516 ft per minute * .069 sq ft blast area * .80 efficiency = 249.28 cubic feet per minute

250 cfm / 60 seconds per minute = 4.17 cu ft / sec
....................................................
Back to where we started, you know HP required

HP= ( C * h * 5.2 ) / 275
HP= ( 4.17cfs * 1.3" WC * 5.2 ) / 275
HP= 28.19 / 275
HP= .103 ....1/8 horse would pull this blower
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