Question 1

How do I calculate belt length for two pulleys?

Accepted Answer

For an open belt, the standard engineering formula is L = 2C + (π/2)(D₁ + D₂) + (D₂ − D₁)²/(4C), where C is the center distance and D₁, D₂ the pitch diameters. This calculator uses the exact tangent-line version of that formula — L = 2C·cos φ + r₁(π − 2φ) + r₂(π + 2φ) with φ = arcsin((r₂ − r₁)/C) — which stays accurate for large ratios and short centers where the shortcut formula drifts. Both reduce to the same numbers on everyday drives: a 2-inch and a 4-inch pulley on 12-inch centers need about a 33.6-inch belt.

Question 2

Does a crossed belt change the belt length?

Accepted Answer

Yes — meaningfully. In a crossed (figure-8) drive both pulleys carry extra wrap, and the difference term flips sign: L = 2C + (π/2)(D₁ + D₂) + (D₂ + D₁)²/(4C). The same 2-inch/4-inch drive on 12-inch centers that needs ~33.6 inches open requires roughly 36 inches crossed — about 2.4 inches more. Crossed belts also reverse the driven pulley's rotation and rub where the strands cross, so they suit light-duty drives; the calculator reports the crossing-point distance along the shaft line so you can check clearance.

Question 3

What is the difference between pitch length, outside length and inside length?

Accepted Answer

A V-belt is a wedge of rubber and cords with real thickness, so 'the belt length' depends on where you measure. Pitch length is measured at the cord line (the neutral axis that doesn't stretch) — it's the number catalogs are built around. Outside length runs longer than pitch by roughly 2π times the distance from the top cover to the cords (about 0.65 in for an A-section); inside length runs shorter by about 2π times the remaining thickness (about 1.3 in for A). Buying rules follow the section: classical A/B/C/D belts are sold by inside length, narrow 3V/5V/8V by pitch length, metric SPZ–SPC by ISO datum length. The calculator outputs all three so you can match whichever label the seller prints.

Question 4

Should I enter outside diameter (OD) or pitch diameter (PD)?

Accepted Answer

Whichever you actually have. Calipers on a pulley give you OD; a catalog or a stamped marking usually gives PD. For a V-belt pulley the two differ by twice the pitch-line offset of the belt section (e.g., an A-section pulley's PD is about 5.6 mm smaller than its OD). Select the measurement basis in the calculator and the conversion happens inside the math — guessing wrong shifts the belt length by π times that error, which is one of the most common reasons a purchased belt 'doesn't fit'.

Question 5

Can I mix units — inches for pulleys and millimeters for center distance?

Accepted Answer

That's the default workflow here, not a workaround. Every input field (D₁, D₂, center distance, belt length) carries its own independent unit dropdown — mm, cm, m, in or ft — and the engine converts everything to a common base before computing. Outputs have their own switchable unit too, so you can dimension a metric drawing, order belts from a US supplier in inches, and read every intermediate value in whichever system you think in, without touching a conversion chart.

Question 6

How do I size a timing belt by teeth instead of millimeters?

Accepted Answer

Timing belts are sold by tooth count, so this tool lets you enter pulley teeth (T₁, T₂) plus the belt pitch (GT2 2 mm, GT3, 5M, MXL, XL, L, H…) directly. Each pulley's pitch diameter is derived as d = p × T / π, the exact belt length is computed, then divided by the pitch to give the required tooth count — and rounded to the nearest tooth count that is actually stocked (even counts for metric pitches). Because a stocked belt rarely matches the exact count, the calculator also solves the adjusted center distance that makes the real belt fit, which tells you how much slot or tensioner travel your mounting needs.

Question 7

Why is the belt I calculated not the belt I can buy?

Accepted Answer

Manufacturers make belts in standard steps — whole inches of inside length for classical A/B/C/D, inch-based pitch designations for 3V/5V/8V, and an ISO length series (530, 560, 600, 630… mm) for SPZ–SPC. A calculated 42.3-inch A-belt doesn't exist; the nearest stock belts are A42 and A43. The calculator maps every V-belt result to the closest stock designation and prints the exact delta (e.g. −0.2 in). A belt slightly long tensions fine; a belt slightly short must be forced over the pulleys, so treat small negative deltas with suspicion.

Question 8

What wrap angle do pulleys need?

Accepted Answer

Traction lives in the contact arc. The rule of thumb: at least 120° of wrap on the smaller pulley (and never below 90°). Wrap on the small pulley is 180° − 2φ where φ depends on the diameter difference and center distance, so big ratios on tight centers starve the small pulley of grip. The calculator reports both wrap angles live and warns below 120° — the fixes, in order of cost, are a longer center distance, a smaller ratio, or an idler/tensioner that pushes the belt onto the small pulley.

Question 9

How fast is my belt moving, and does it matter?

Accepted Answer

Belt speed = π × pitch diameter × RPM. It matters because V-belts are limited to roughly 30 m/s (about 5,900 ft/min): beyond that, centrifugal force lifts the belt out of the sheave groove and capacity collapses. A 2-inch pulley at a typical 1750 RPM motor runs ~4.7 m/s — comfortably safe — while the same belt on an 8,000 RPM spindle at 75 mm is pushing 9.4 m/s, still fine. The calculator reports belt speed in m/s, ft/min, km/h or mph and warns when you cross the practical ceiling. For timing belts, speed is less limiting than tooth mesh frequency — above ~10,000 RPM check the manufacturer's curve.

Question 10

What ratio can one belt stage handle?

Accepted Answer

Practical single-stage V-belt ratios top out around 8:1 (and bottom at 1:8) because the small pulley's wrap angle and the arc of approach both deteriorate as the diameters diverge. Timing belts are similar in practice, though small-pitch GT profiles handle higher ratios on short centers better than classical sections. If you need more, stage the reduction through a jackshaft — two 4:1 stages give 16:1 with healthy wrap everywhere. The calculator flags ratios beyond the practical band automatically.

Question 11

How tight should a belt be tensioned?

Accepted Answer

Tension is set by belt deflection, not feel: press at mid-span with roughly the recommended force and the belt should deflect about 1.6 mm per 100 mm of span (1/64 inch per inch) for V-belts, per typical manufacturer tables. Too loose and it squeals and slips under load pulses (glazing the sidewalls); too tight and the bearings pay for it — a common failure on small motors with short shafts. After fitting a 'slightly long' matched belt, plan on a tensioner or slotted mount to take up the difference the part-number matcher reports.

Question 12

How accurate is this belt calculator?

Accepted Answer

The geometry is exact: tangent-line belt length, wrap angles, crossing point and the closed-form center-distance solution are computed from your numbers without the small-angle shortcuts that drift on tight centers. The V-belt thickness offsets use published datum dimensions for each section (A/B/C/D, 3V/5V/8V, SPZ/SPA/SPB/SPC) and are accurate to about a millimeter; real manufacturing tolerances add ±2–3 mm anyway. Timing-belt tooth availability is rounded to commonly stocked counts, so always confirm the exact designation with your supplier before ordering.

Question 13

Is there a V-belt length calculator app I can use offline?

Accepted Answer

Yes — this page is a free, install-free V-belt length calculator app that runs in any phone, tablet or desktop browser and keeps working offline once loaded. The same tool doubles as a metric belt length calculator (mm, cm, m) and an inch-based one (in, ft), with every input field carrying its own unit selector. Under the hood it applies the V-belt length calculation formula for A/B/C/D, 3V/5V/8V and SPZ–SPC sections with pitch, outside and inside lengths, plus a dedicated timing pulley belt length calculator mode that converts pulley teeth and pitch into the exact tooth count to order.

Section	Family	Top width	Thickness	OD − PD / pulley	Outside − pitch	Pitch − inside	Sold as (≈1000 mm)
A	classical	13 mm	8 mm	5.6 mm	+17.6 mm	−32.7 mm	A38
B	classical	17 mm	11 mm	7.4 mm	+23.2 mm	−45.9 mm	B38
C	classical	22 mm	14 mm	9.4 mm	+29.5 mm	−58.4 mm	C37
D	classical	32 mm	19 mm	12.6 mm	+39.6 mm	−79.8 mm	D36
3V	narrow	9.5 mm	8 mm	5.6 mm	+17.6 mm	−32.7 mm	3V390
5V	narrow	16 mm	13.5 mm	9.0 mm	+28.3 mm	−56.5 mm	5V500
8V	narrow	25.5 mm	17.5 mm	11.8 mm	+37.1 mm	−72.9 mm	8V800
SPZ	metric	9.7 mm	8 mm	5.0 mm	+15.7 mm	−34.6 mm	SPZ1000
SPA	metric	12.7 mm	10 mm	6.6 mm	+20.7 mm	−42.1 mm	SPA1000
SPB	metric	16.3 mm	13 mm	8.6 mm	+27.0 mm	−54.7 mm	SPB1000
SPC	metric	22.5 mm	18 mm	12.0 mm	+37.7 mm	−75.4 mm	SPC2000

Profile	Pitch (mm)	Pitch (in)	PD of a 20T pulley	Typical use
MXL	2.032	0.0800	12.94 mm	Small hobby drives, printers, encoders
XL	5.080	0.2000	32.34 mm	Small hobby drives, printers, encoders
L	9.525	0.3750	60.64 mm	Light industrial, conveyors, older CNC
H	12.700	0.5000	80.85 mm	Light industrial, conveyors, older CNC
GT2	2.000	0.0787	12.73 mm	3D printers, CNC, robotics (FRC) — zero-backlash
GT3	3.000	0.1181	19.10 mm	3D printers, CNC, robotics (FRC) — zero-backlash
GT5	5.000	0.1969	31.83 mm	3D printers, CNC, robotics (FRC) — zero-backlash
3M	3.000	0.1181	19.10 mm	HTD power transfer (3 mm)
5M	5.000	0.1969	31.83 mm	HTD power transfer (5 mm)
8M	8.000	0.3150	50.93 mm	HTD power transfer (8 mm)
14M	14.000	0.5512	89.13 mm	HTD power transfer (14 mm)

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