Calculate alternator pulley ratio or find the missing crankshaft or alternator pulley diameter from two pulley sizes in inches, cm, mm, or m.
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Alternator Pulley Ratio Formula
The alternator pulley ratio compares the crankshaft pulley diameter to the alternator pulley diameter. Use the same diameter unit for both pulleys before applying the formula.
R = D_c / D_a
- R = alternator pulley ratio, unitless
- D_c = crankshaft pulley diameter
- D_a = alternator pulley diameter
If you know the ratio and alternator pulley diameter, calculate the crankshaft pulley diameter with:
D_c = R * D_a
If you know the ratio and crankshaft pulley diameter, calculate the alternator pulley diameter with:
D_a = D_c / R
The calculator has three functions:
- Calculate alternator pulley ratio: enter the crankshaft pulley diameter and alternator pulley diameter.
- Calculate crankshaft pulley diameter: enter the alternator pulley diameter and the ratio.
- Calculate alternator pulley diameter: enter the crankshaft pulley diameter and the ratio.
The calculator converts diameters to a common base unit before calculating, so you can enter the two pulley diameters in inches, centimeters, millimeters, or meters.
Common Pulley Ratio Results
These examples show how the ratio changes as the alternator pulley gets smaller or larger relative to the crankshaft pulley.
| Crankshaft Pulley Diameter | Alternator Pulley Diameter | Pulley Ratio | Meaning |
|---|---|---|---|
| 6 in | 3 in | 2.00 | Alternator turns 2 times per crankshaft revolution |
| 6 in | 2.5 in | 2.40 | Higher alternator speed |
| 7 in | 2.5 in | 2.80 | Common for faster charging at low engine speed |
| 5.5 in | 3 in | 1.83 | Lower alternator speed |
Diameter Unit Conversions
Use matching units when calculating by hand. The calculator handles these conversions internally.
| Unit | Equivalent in Inches | Equivalent in Millimeters |
|---|---|---|
| 1 in | 1 in | 25.4 mm |
| 1 cm | 0.393701 in | 10 mm |
| 1 mm | 0.0393701 in | 1 mm |
| 1 m | 39.3701 in | 1000 mm |
Example Calculations
Example 1: Calculate the alternator pulley ratio
You have a 6.5 inch crankshaft pulley and a 2.5 inch alternator pulley.
R = D_c / D_a
R = 6.5 / 2.5 = 2.6
The alternator pulley ratio is 2.6:1. This means the alternator turns 2.6 times for each crankshaft revolution.
Example 2: Calculate the alternator pulley diameter
You want a pulley ratio of 2.4 with a 6 inch crankshaft pulley.
D_a = D_c / R
D_a = 6 / 2.4 = 2.5 in
The alternator pulley diameter should be 2.5 inches.
FAQ
What does an alternator pulley ratio mean?
The alternator pulley ratio tells you how many times the alternator pulley turns for each turn of the crankshaft pulley. A ratio of 2.5 means the alternator turns 2.5 revolutions for every 1 crankshaft revolution.
Does a smaller alternator pulley increase alternator speed?
Yes. If the crankshaft pulley stays the same size, a smaller alternator pulley increases the ratio. A higher ratio makes the alternator spin faster at the same engine RPM.
Can the pulley ratio be calculated with different units?
Yes, but both pulley diameters must be converted to the same unit before dividing. For example, do not divide inches by millimeters directly. Convert both values to inches, millimeters, centimeters, or meters first, then calculate the ratio.
