Belt Tension Calculator
Pick your belt section and enter pulley diameters and center distance to get span length, the 1/64-inch-per-inch deflection and the force a new or used belt should need to reach it. Or convert a sonic tension meter frequency (Hz) to belt tension, or work out T1 and T2 from transmitted power.
Tension a V-belt by deflection: measure the span length t between the pulley contact points, push one belt at mid-span by t/64 (1/64 inch for every inch of span), and compare the force needed with the table value for your belt section and small pulley diameter.
Use the Belt Tension Calculator
Looking for bicycle spoke tension? Use the spoke tension calculator, or the spoke length calculator to size spokes from rim ERD.
Belt & drive
Target deflection force (per belt)
How to set it
- Take up slack until the belt is snug, then run the drive briefly to seat it.
- Stop the drive. With a tension gauge, push one belt at the middle of the span until it moves the deflection shown.
- New belt: aim for the run-in force. Used belt: the normal force. Adjust center distance and re-check; keep pulleys aligned.
- Re-check a new belt after about 24 hours of running.
V-belt deflection force chart
Typical force (lb) to deflect one belt 1/64 inch per inch of span. Multi-rib J, L, M values are per rib. 1 lb = 4.448 N. Your belt manufacturer's table governs.
| Section | Small pulley (in) | New belt (run-in) | Used belt (normal) |
|---|---|---|---|
| A | 3.0–3.6 | 3.375 lb | 2.25 lb |
| A | 3.8–4.8 | 4.25 lb | 2.875 lb |
| A | 5.0–7.0 | 5.125 lb | 3.375 lb |
| AX | 3.0–3.6 | 4.125 lb | 2.75 lb |
| AX | 3.8–4.8 | 5 lb | 3.25 lb |
| AX | 5.0–7.0 | 6 lb | 4 lb |
| B | 3.4–4.2 | 4 lb | 2.625 lb |
| B | 4.4–5.2 | 6 lb | 4 lb |
| B | 5.4–9.4 | 7.125 lb | 5.25 lb |
| BX | 3.4–4.2 | 5.25 lb | 3.5 lb |
| BX | 4.4–5.2 | 7.125 lb | 4.75 lb |
| BX | 5.4–9.4 | 9 lb | 6 lb |
| C | 7.0–9.0 | 11.25 lb | 7.5 lb |
| C | 9.5–16.0 | 15.75 lb | 10.5 lb |
| CX | 7.0–9.0 | 13.5 lb | 9 lb |
| CX | 9.5–16.0 | 17.5 lb | 11.75 lb |
| D | 12.0–16.0 | 24.5 lb | 16.5 lb |
| D | 18.0–22.0 | 33 lb | 22 lb |
| E | 21.6–27.0 | 48 lb | 32 lb |
| 3V | 3.4–4.2 | 6 lb | 4 lb |
| 3V | 4.2–10.6 | 7 lb | 5 lb |
| 3VX | 2.2–3.65 | 7 lb | 5 lb |
| 3VX | 4.12–10.6 | 8 lb | 6 lb |
| 5V | 7.1–10.9 | 16 lb | 8–12 lb |
| 5V | 11.8–16.0 | 20 lb | 10–15 lb |
| 5VX | 4.4–10.9 | 18 lb | 10–14 lb |
| 5VX | 11.8–16.0 | 22 lb | 12–18 lb |
| 8V | 12.5–17.0 | 36 lb | 18–27 lb |
| 8V | 18.0–22.4 | 40 lb | 20–30 lb |
| J | 1.32–1.67 | — | 0.4 lb per rib |
| J | 1.77–2.2 | — | 0.5 lb per rib |
| J | 2.36–2.95 | — | 0.6 lb per rib |
| L | 2.95–3.74 | — | 1.7 lb per rib |
| L | 3.94–4.92 | — | 2.1 lb per rib |
| L | 5.2–6.69 | — | 2.5 lb per rib |
| M | 7.09–8.82 | — | 6.4 lb per rib |
| M | 9.29–11.81 | — | 7.7 lb per rib |
| M | 12.4–15.75 | — | 8.8 lb per rib |
Sonic tension meter
For banded or multi-rib belts use the mass of the whole belt width. Data sheet values beat the typical presets.
Formula
T = 4 · m · L² · f²
T in N, m belt mass in kg/m, L span in m, f in Hz. Frequency grows with the square root of tension, so doubling tension raises the frequency by about 41%.
Drive Parameters
Fill in the drive parameters and click Calculate
Detailed Breakdown
Formulas Used
Quick answer
Tension a V-belt by deflection: measure the span length t between the pulley contact points, push one belt at mid-span by t/64 (1/64 inch for every inch of span), and compare the force needed with the table value for your belt section and small pulley diameter. A new belt is set to the higher run-in force, a used belt to the normal force. With a sonic meter, tension = 4 × belt mass per metre × span² × frequency².
Examples
| Belt, small pulley | Force: new / used belt | 20 in span deflects |
|---|---|---|
| A, 3.8–4.8 in | 4.25 lb / 2.9 lb | 0.31 in |
| B, 5.4–9.4 in | 7.1 lb / 5.25 lb | 0.31 in |
| C, 9.5–16.0 in | 15.75 lb / 10.5 lb | 0.31 in |
| 3V, 4.2–10.6 in | 7 lb / 5 lb | 0.31 in |
| 5V, 7.1–10.9 in | 16 lb / 8–12 lb | 0.31 in |
Choose your belt section and enter the pulley sizes and center distance to get the span, deflection and target force.
Summary
Most people checking belt tension want to know how tight to set a belt on a real drive. The standard field method is force-deflection: the span length is t = √(C² − ((D − d)/2)²), the belt is pushed sideways at mid-span by t/64, and the force needed is compared with the value for the belt section and the small pulley diameter. New belts are set to the higher run-in value because they stretch and seat during the first hours. Pushing a belt sideways by 1/32 of half its span takes about one sixteenth of its tension, so the deflection force also estimates static tension (about 16 × force). Sonic tension meters measure the span's natural frequency instead, and tension follows from T = 4·m·L²·f². For drive design, the Euler-Eytelwein tab computes effective, tight-side and slack-side tensions from power, belt speed, friction and wrap angle. Table values are typical published figures; your belt maker's chart governs.
How it works
- Open the Deflection force tab and select your belt section (A–E, AX–CX, 3V, 5V, 8V, 3VX, 5VX or a J, L, M multi-rib belt).
- Enter the small and large pulley diameters and the center distance in inches or millimetres.
- Read the span length t, the deflection t/64 and the target force for a new (run-in) and a used (normal) belt.
- Push one belt at mid-span with a tension gauge until it moves the deflection distance, then adjust the center distance until the gauge force falls in the target range.
- With a sonic tension meter, open the Sonic frequency tab, enter span and belt mass per metre, and convert the measured frequency to tension or a target tension to frequency.
- For drive design, the Drive T1/T2 tab computes effective, tight-side, slack-side and centrifugal tension from power, speed, friction and wrap angle.
Use cases
- Setting the correct tension on a new or replacement V-belt for a motor, fan, pump or compressor drive.
- Checking a used belt during maintenance with a pencil-type tension gauge.
- Converting a sonic tension meter reading in Hz into belt tension in N or lbf.
- Finding span length and belt length from pulley diameters and center distance.
- Estimating static belt tension from a measured deflection force.
- Sizing drive tensions T1 and T2 and the tension ratio for flat and V-belts.
- Teaching the force-deflection and Euler-Eytelwein belt methods in mechanical engineering courses.