Calculate V-belt or flat belt length for two-pulley and crossed-belt drives. Find speed ratio, driven RPM, belt speed, and contact angle.
The **Belt Length Calculator** determines the exact belt length needed for a two-pulley drive system — the most common power transmission setup in machinery, HVAC, automotive, and industrial equipment. Enter the pulley diameters and center-to-center distance, and the calculator returns the required belt length, speed ratio, driven pulley RPM, belt linear speed, and wrap (contact) angles.
Proper belt length and adequate contact angles are essential for reliable power transmission. If the contact angle on the smaller pulley drops below 120°, the belt is likely to slip. This calculator warns you and provides a visual contact-angle rating.
Choose between open (standard) and crossed-belt configurations. The built-in presets cover common machinery setups, and the reference tables let you quickly compare belt lengths and speeds across different center distances and pulley sizes. Whether you are replacing a worn belt or designing a new drive, this tool gives you the numbers you need in seconds. It keeps the geometry and drive-speed implications together so you can check whether a proposed pulley pair is practical before you order parts.
Use this calculator to estimate belt length, wrap angle, and speed ratio before ordering parts or checking whether a pulley combination is likely to slip. It is especially useful when a replacement belt needs to match an existing drive without changing the center distance or pulley set. That makes it easier to compare a few pulley layouts before buying a standard belt size.
Open Belt: L = 2C + π(D+d)/2 + (D−d)²/(4C) Crossed Belt: L = 2C + π(D+d)/2 + (D+d)²/(4C) Speed Ratio: i = D / d Belt Speed: v = π D n / 60 000 [m/s] (D in mm, n in RPM) Contact Angle (small): θ ≈ 180° − 60(D−d)/C
Result: 1 061 mm belt, speed ratio 2.5 : 1, 700 RPM driven, 18.3 m/s belt speed
A 200/80 mm pulley pair with 300 mm center distance requires a 1 061 mm belt. The smaller pulley spins at 700 RPM with a safe 156° contact angle.
Belt length depends mainly on pulley diameters and center distance, but wrap angle is just as important because it determines how much surface contact the belt has on the smaller pulley. A drive can have the correct length and still perform poorly if wrap is too small.
This calculator is most useful during replacement selection and early layout work. It helps you compare whether a small change in pulley diameter or shaft spacing will force a different standard belt length or reduce the available contact angle below a comfortable operating range.
Catalog belt lengths, tensioning range, pulley groove profile, and service factor still matter after the geometry is solved. Treat the result as the drive-layout starting point, then confirm against the manufacturer series you actually intend to buy.
A common rule of thumb is to keep at least about 120 degrees of wrap on the smaller pulley to maintain traction. Less wrap increases the chance of slip, especially under load.
Measure from the centerline of one pulley shaft to the centerline of the other shaft. That distance is the straight-line spacing between the shaft centers.
Open drives are standard. Crossed belts reverse rotation direction and increase contact angle but wear faster, so they are usually chosen only when the reverse rotation is needed.
Choose the nearest standard belt length and then adjust center distance with the take-up mechanism or motor slide if the drive allows it. If there is no adjustment range, recheck the pulley pair before ordering.
Width does not change the nominal pitch length, but it does affect power capacity, pulley groove choice, and allowable tension. Two belts can share the same length and still be incompatible if their section or groove profile is wrong.
Many standard V-belts are happiest below roughly 30 m/s, while specialized belts can run faster if the pulley system is designed for it. Higher belt speeds increase heat and centrifugal loading, so the practical limit depends on the belt construction.