August 2026 13 min read

V Belt Calculator

Free V-belt calculator. Compute V-belt pitch length, belt speed, wrap angles on both pulleys, and a preliminary number of belts for a given power and section (A/B/C/SP). Real open-belt geometry with V-belt wedge grip notes.

V Belt Calculator

V-Belt Drive Inputs

Unit System

SI — mm Imperial — in

Belt Section

Sets capacity estimate

Small Dia D (mm)

Driver pulley

Large Dia d (mm)

Driven pulley

Centre Distance C (mm)

Axis-to-axis

Driver Speed n (rpm)

Power (kW)

Power Unit

kW hp

Auto-calculate instantly

Belt Length

1,596.9mm

Section B (classical).

Wrap is excellent (≥150°).

Belt Speed

9.82m/s

Wrap (small)

165.6°

Wrap (large)

194.4°

Speed Ratio

2:1

Power / Belt

2.25kW

Belts Needed

Belt Speed

9.82m/s

π·D·n/60

Wrap (small)

165.6°

Driven grip limit

Wrap (large)

194.4°

On driven pulley

Speed Ratio

2:1

d/D

Power / Belt

2.25kW

Preliminary

Belts Needed

Preliminary est.

Result Summary

Section	B (classical)
Small Dia D	125 mm
Large Dia d	250 mm
Centre C	500 mm
Belt Length	1,596.9 mm
Belt Speed	9.82 m/s
Wrap (small / large)	166° / 194°
Speed Ratio	2 : 1
Power	5 kW
Preliminary Belts	3

Estimated Result — Uses open-belt geometry L = 2C + π(D+d)/2 + (d−D)²/(4C) and wrap angles θ_s = 180° − 2·asin((d−D)/(2C)), θ_l = 180° + 2·asin((d−D)/(2C)). Belt speed v = π·D·n/60. The belt count is a PRELIMINARY estimate: per-belt power scales linearly with small-pulley diameter and speed from a reference rating; always confirm against the manufacturer’s catalogue with service factor and arc-of-contact corrections.

Interactive Diagram

The V cross-section wedges into the sheave groove for high grip with low slip.

Visualizations

V-Belt Length Breakdown

1,000mm2C

589.1mmπ(D+d)/2

7.8mm(d−D)²/4C

Small-Pulley Wrap

θ_s166°

Formula

A V-belt drive uses the same centre/length geometry as a flat open belt, but the trapezoidal section wedges into the sheave so it transmits more power per width and resists slip. Length is two straight spans plus the arcs over both pulleys. The small (driver) pulley sets belt speed and limits wrap; the large pulley gets the complementary wrap.

L = 2C + π(D+d)/2 + (d−D)²/(4C)

Alternative Form

θ_s = 180° − 2·asin((d−D)/(2C)) , v = π·D·n/60

Variables

Symbol	Name	Unit	Description
L	Belt length	mm	Pitch length
C	Centre distance	mm	Axis-to-axis
D	Small (driver) dia	mm	Sets belt speed
d	Large (driven) dia	mm	Sets ratio
θ_s, θ_l	Wrap angles	°	Small / large
v	Belt speed	m/s	π·D·n/60

Explanation

Belt speed comes only from the driver pulley (D, n); the driven pulley turns slower by d/D. The wrap angle on the small pulley is the traction limit — keep it high with a large centre distance and modest d−D. The preliminary belt count divides power by a per-belt capacity that grows with pulley size and speed, then rounds up; real designs add a service factor and catalogue corrections.

Derivation

L = 2C + π(D+d)/2 + (d−D)²/(4C) (open belt). Wrap: θ_s = 180° − 2β, θ_l = 180° + 2β, β = asin((d−D)/(2C)). Belt speed v = π·D·n/60 (m/s, D in m). Preliminary per-belt power P_belt ≈ P_ref·(D/D_ref)·(n/1000); belts N = ceil(P_design / P_belt). Confirm with ISO 5290 / manufacturer tables.

Step-by-Step Calculation

Follow each step to understand how the result is calculated from your inputs.

Step 1

Belt pitch length.

L = 2C + π(D+d)/2 + (d−D)²/(4C)

D:125

d:250

C:500

Intermediate: L = 2·500 + π(125+250)/2 + (250−125)²/(4·500)

Result: 1,596.9 mm

Step 2

Belt speed from driver pulley.

v = π·D·n/60

n:1500 rpm

Intermediate: v = π·125·1500/60000

Result: 9.82 m/s

Step 3

Wrap angles.

θ_s = 180 − 2·asin((d−D)/(2C)) , θ_l = 180 + 2·asin((d−D)/(2C))

Intermediate: θ_s = 165.6°, θ_l = 194.4°

Result: wrap 166°/194°

Step 4

Preliminary belt count.

N = ceil(P / P_belt)

P:5 kW

P_belt:2.25 kW

Intermediate: P_belt = 1.5·(125/125)·(1500/1000)

Result: ~3 belts

Final Answer

1,596.9 mm

Worked Examples

See how the calculator works with different scenarios.

Scenario

Section B, D=125, d=250, C=500, n=1500, P=5 kW.

Inputs

D:125

d:250

C:500

n:1500

P:5

section:B

Calculation

L = 1000+π·375/2+125²/2000=1000+589+7.8=1597 mm; v=π·125·1500/60000=9.8 m/s; wrap=180−2·asin(125/1000)=165.7°; P_belt=1.5·(125/125)·1.5=2.25 kW → N=ceil(5/2.25)=3

Result

L≈1597 mm, 3 belts

Interpretation

A 3-belt B-section drive for 5 kW.

Understanding Your Result

Your Result: 1,596.9 mm

Typical range: Aim for small-pulley wrap ≥150° (≥120° minimum) and belt speed 5–25 m/s; yours: wrap 166°, speed 9.8 m/s. The belt count 3 is preliminary.

What This Means

A B (classical) V-belt drive with D = 125 mm, d = 250 mm and C = 500 mm needs a 1,597 mm belt running at 9.8 m/s, with 166° small-pulley wrap; preliminary ≈ 3 belts for 5 kW.

Why It Matters

V-belts are the workhorses of power transmission. Getting length, speed and wrap right avoids slip and premature failure; the belt count tells you how many to install as a matched set. Under-designed wrap or too few belts means slip, heat and short life.

Professional Interpretation

Confirm N against the B (classical) catalogue with a service factor (typically 1.2–1.6) and the actual arc-of-contact correction. Keep C ≈ 1.5–3× d. Match belts as a set; verify D meets the section minimum (125 mm).

Belt length

1,597 mm

Belt speed

9.8 m/s

Wrap

166° / 194°

Recommendations

•Wrap angle is acceptable.
•Confirm belt count with the manufacturer catalogue + service factor.
•Use a matched set; replace all belts together.
•Keep driver D ≥ section minimum (125 mm).
•Belt speed 5–25 m/s; avoid running at the extremes.

Educational Content

Introduction

The V-belt is the most common power-transmission belt because its wedge shape grips hard without slipping. This calculator returns the exact belt length, the belt speed, the wrap angles that govern grip, and a preliminary belt count for your chosen section and power.

Concept Explanation

Geometry is the open-belt formula L = 2C + π(D+d)/2 + (d−D)²/(4C), with D the driver (small) pulley and d the driven. Belt speed v = π·D·n/60; wrap θ_s = 180° − 2·asin((d−D)/(2C)) on the small pulley and θ_l = 180° + 2·asin(...) on the large. Section (A/B/C/SPA/B/C) sets capacity; per-belt power rises with pulley size and speed. The belt count is rounded up from power ÷ per-belt capacity.

Applications

▸Industrial machine drives

▸HVAC and pumps

▸Agricultural equipment

▸Woodworking machinery

▸Compressors

▸Replacement belt sets

Benefits

•Exact belt length
•Wrap & speed checks
•Preliminary belt count
•Multiple sections
•SI/imperial units

Limitations

•Preliminary belt count
•No service factor
•No catalogue correction
•Open belts only
•No tension/slip model

Best Practices

•Keep wrap ≥150° on small pulley
•Use matched belt sets
•Apply service factor 1.2–1.6
•Confirm with manufacturer tables
•Keep driver above section min dia

Common Misconceptions

Myth: V-belt length = flat formula.

Truth: Same geometry, but measure pitch length.

Myth: Belt count is exact here.

Truth: It is a preliminary estimate; verify catalogue.

Myth: Any centre works.

Truth: Centre sets wrap and grip.

Myth: V-belts never slip.

Truth: Overload or low tension still slips.

Practical Tips

•Belt 1,597 mm, 3 belts (prelim).
•Wrap 166° (small).
•Confirm belt count in the catalogue.
•Use a matched set; replace all together.
•Keep belt speed in 5–25 m/s.

Common Mistakes

Low wrap

Increase C or reduce d−D; add idler.

Trusting belt count

Apply service factor + catalogue.

Wrong length basis

Use pitch length, not top.

Small pulley too small

Respect section minimum diameter.

Mixed belt ages

Replace as a matched set.

Assumptions

This calculator makes the following assumptions to provide estimates:

Open belt: Same-direction drive.

Pitch length: Geometric length.

Preliminary count: Linear size/speed model.

One unit: mm or in consistently.

No service factor: Add for final design.

Limitations

Be aware of these limitations when using this calculator:

Estimate only: Catalogue confirms count.

No slip model: Ideal traction.

Open only: Crossed differs.

Single stage: No compound.

Frequently Asked Questions

Answers to common questions about this calculator.

What is a V-belt?

A V-belt is a trapezoidal belt that seats in a matching sheave groove. The wedge action multiplies the grip so it carries more power than a flat belt of the same width and resists slip.

What does this calculator give?

Belt pitch length, belt speed, wrap angles on both pulleys, and a preliminary estimate of how many belts (of a chosen section) you need for the power.

What is the V-belt length formula?

Geometry is the same as an open flat belt: L = 2C + π(D+d)/2 + (d−D)²/(4C). Measure pitch length at the neutral line, not the top.

What is belt speed?

v = π·D·n/60 (m/s) using the small (driver) pulley diameter D and its rpm n. All belts on the drive share this speed.

What is the wrap angle?

On the small pulley: θ_s = 180° − 2·asin((d−D)/(2C)); on the large: θ_l = 180° + 2·asin((d−D)/(2C)). More wrap = more grip.

Why is wrap on the small pulley critical?

The smaller pulley has less contact, so it limits traction and heat. Design for the highest possible small-pulley wrap (≥120°, ideally ≥150°).

How many belts do I need?

Divide the required power by the per-belt capacity (adjusted for pulley size and speed) and round up, then apply a service factor. This tool gives a preliminary count — confirm with the manufacturer catalogue.

What are belt sections A, B, C?

Standard cross-sections of increasing width/power: A (light), B (medium), C (heavy), D (very heavy); SPA/B/C are narrower "wedge" profiles with higher capacity.

What is a service factor?

A multiplier (1.1–1.8) covering shocks, starts, hours/day and environment; design power = transmitted power × service factor.

What is the speed ratio?

i = d/D (large/small). The small (driver) pulley turns faster; cap single belts around 4:1–5:1 before considering compound drives.

What is belt slip?

Under overload a V-belt can slip in the groove, wasting power and heating; enough wrap and tension prevent it.

How do I set tension?

Use the manufacturer’s deflection-force method; too loose slips, too tight overloads bearings. Automatic tensioners help.

What is the minimum pulley diameter?

Each section has a minimum (A≈75 mm, B≈125 mm, C≈200 mm); below it the belt bends too sharply and life drops.

What is the maximum belt speed?

Around 25–35 m/s for standard belts; above that centrifugal force reduces grip and wear rises.

How efficient is a V-belt drive?

About 90–98% for a well-designed single drive; slightly less than a gear but cheaper and shock-absorbing.

What is a matched belt set?

Belts in a multi-belt drive must be matched in length so they share load; do not mix old and new.

What about cogged V-belts?

Notched (cogged) belts bend easier on small pulleys and run cooler; they allow smaller minimum diameters.

When do I use a V-belt vs a gear?

V-belts for cost, shock absorption, quietness and long centre distances; gears for precise ratio, high torque and compactness.

What standards cover V-belts?

ISO 4184 / DIN 2215 for classical; ISO 5290 for wedge (SP) sections; RMA IP-20/22.

How do I choose the centre distance?

Typically 1.5–3× the large pulley diameter; enough for wrap but not so long it needs a tensioner or whips.

What is the correction term for?

(d−D)²/(4C) corrects length for the pulley-size difference; zero when equal.

Can I compute centre from length?

Yes — solve the quadratic 8C² + (2π(D+d)−4L)C + (d−D)² = 0 for the positive root.

What is the rated power per belt?

It rises with pulley diameter and speed (more surface, better grip) and falls with small pulleys; catalogues tabulate it per section.

Why does the estimate differ from a catalogue?

The catalogue adds arc-of-contact, length and service corrections; treat the count here as a first cut.

What is the belt top width vs pitch?

Top width is the visible width; pitch length sits at the neutral line inside the belt — use pitch for length.

How do I verify the result?

Re-run in centre mode: set C from your chosen standard belt and confirm wrap and speed are sensible.

What maintenance do V-belts need?

Check tension, alignment and wear; replace as a matched set; keep pulleys clean and grooves true.

What causes V-belt failure?

Insufficient tension/slip, small pulleys, misalignment, oil/heat, and shock overload. Proper design avoids most.

Is the belt-count estimate accurate?

It is a simplified preliminary figure (linear in pulley size and speed). Always confirm against the belt maker’s rating tables.

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Related Guides

Deepen your understanding with these educational resources.

ISO 4184 / 5290 — V-belts

V-belt standards.

Gates — V-belt design

Drive selection.

Glossary

Technical terms used in mortgage calculations explained in plain language.

V-Belt: Trapezoidal wedge belt.
Pitch Length: Length at the neutral line.
Wrap Angle: Contact arc (θ_s, θ_l).
Section: A/B/C/SP cross-size.
Service Factor: Design margin multiplier.
Belt Speed: π·D·n/60 (m/s).

Authoritative References

Trusted sources and standards referenced by this calculator.

ISO 4184 — V-belt lengths Gates — V-belt drive design Optibelt / Arntz — Technical manuals MIT OpenCourseWare — Mechanical design Belt drive reference (Engineering Toolbox)

Applicable Standards

ISO 4184ISO 5290 (SP)DIN 2215RMA IP-20/22

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Last Updated:

August 2026

Version:

2.1.0

References:

License:

Free to use for educational and professional purposes.