On the internet you will find many spoke length calculators. I don't see much point in reinventing wheels, so here's a link to a page of them by Damon Rinard. These calculators are very fine in the office, but not so handy in the workshop or if your chosen components are not to be found amongst the listed options. So here's a simple pencil-and-paper method that will work very well for whatever kind of wheel you want to build.

The Spoke Length Chart

The chart opposite is a graphical representation of the hub flange and spoke where they meet. The centre of the hub is at the bottom left corner, with curves at 10mm intervals from this point representing hub size in terms of spoke-hole circle diameter. Certain commonplace diameters, i.e. Shimano small flange and Campagnolo large flange, are indicated by bold curves.

Lines radiate from the hub centre at different angles according to the total number of spokes and number of crossings (i.e. the number of times a spoke crosses another on the same side of the wheel). The point of intersection between one of these Number × Crossings lines and a given Hub Diameter (i.e. spoke-hole circle) indicates the position of the spoke head. Note that some spoke patterns share the same line.

The threaded, rim end of the spoke is somewhere up the left-hand axis, way off the top of the chart; and spoke length is given by the gently curved lines at 2mm intervals, centred at that same off-the-chart rim point. Note that the numbers for these Spoke Length contours increase as you move down the chart.

The coloured lines illustrate the use of this chart to determine the basic spoke length (294mm) for a 622mm (700C) rim built 36×3 on a 55mm hub.

Download charts for 559 (26in MTB) and 622 (700C) diameter wheels in pdf format (75KB).

Advice on crossings

A line radiating from the rim point (off the top of the chart) and labelled Spoke Angle 5° indicates an average spoke alignment for a typical tangent-spoked bicycle wheel. Radial spokes (×0) are inclined at zero degrees and coincide with the left side of the chart.

Unless the hub flange is specially reinforced it is best to avoid radial or near-radial spoke angles, as such patterns are apt to tear chunks out of light alloy hub flanges! Very large spoke angles (10° or more) on the other hand, cause problems at the rim: the spoke thread may be bent and ultimately break where it enters the nipple, which can tilt only so far.

If possible, choose a number of crossings that give a spoke alignment between 3° and 7°. In other words: your spoke head should be not less than half-way from the left side of the chart to the 5° line, or more than that distance beyond it.

Although 36×4 and 48×5 are depicted on the chart, these patterns are not recommended as they can result in spokes crossing adjacent spoke heads. This may distort spoke alignment and/or prevent replacement of a broken spoke without removing another first.

Rim factors

The charts give a basic spoke length, for a rim that is quite shallow by modern standards. So your actual spoke length will generally be somewhat shorter (or rarely longer), depending upon the actual depth of your chosen rim. To be precise, it depends upon the distance below the bead seat (shelf inside the rim where the tyre bead sits) to the tip of the spoke (level with the nipple slot in a tensioned wheel). This is dimension b on the rim cross-section opposite.

This diagram also depicts the distance f to the outer edges of the rim, which is an easier to measure and only slightly less precise way to compare rims, since rim flange height is standardised at 6mm (±0.5mm) above the bead seat.

It's easiest of all (and least precise but usually good enough) to compare overall rim height, h on the diagram.

The chart assumes a spoke tip 4mm below the bead seat, or 10mm below the flanges, or an overall rim height of 14mm. Most modern rims are deeper, so you will usually have to subtract a rim correction factor, r from the basic spoke length.

r = b – 4 or = f – 10 or = h – 14.

For example: a lot of popular rims measure 19mm overall, 19 – 14 = 5, in which case subtract 5mm from the figure yielded by this chart.

Other wheel sizes

Charts are provided for rims of 622mm bead diameter (commonly called "700C") and 559mm (the 26in mountain-bike size). These charts can be used to predict the lengths of spokes in other, slightly different sizes, by comparing bead diameters. Depending upon whether the actual bead diameter is greater or less than the chart diameter, you should add or subtract a size correction, s.

Refer to the page on Tyre Sizes to discover the bead diameter, d of your actual wheel.

s = (d – chart diameter) ÷ 2

For example: a 27×1¼ wheel has a 630mm bead diameter, (630 – 622) ÷ 2 = 4, so for this wheel size add 4mm to the basic spoke lengths yielded by the 622 Chart, before subtracting (or possibly adding) a rim factor.