In the table, you'll see a summary of the features of each listed SLC, along with selected notes. Each linked SLC in the table opens a new tab/window, according to your browser's preferences. You can, therefore, easily make side-by-side comparisons of the various SLCs, and verify calculations in one by repeating the calculation in another. In fact, since the underlying formulae of all SLCs should yield the same result(s), you could use the same comparative technique to familiarise yourself with the more elaborate, or 'challenging', SLCs.
This table is a work in progress, released under the CC license; you can crib from, or even copy, the table below, but the wiser course is to help arrange for a canonical version to be hosted somewhere...

Spoke-length calculators (SLCs) On-line

Login required?
Tutorial or FAQ?
Illustrated?
2D or 5D?1
Form-based?
JavaScript required?
Database-backed?
 

Notes

Benjamin J. Manthey's SLC

N N Y 2D Y* Y N SLC strives to be minimalist. Illustrations of the key measurements involved spread over entire length of page (some scrolling required). Adapts reasonably well to portable device screens.

Bike School's SLC

N Y Y 5D Y Y N SLC offering from United Bicycle Institute. Has pop-up explanatory text for each measurement, and separate measuring tutorial, which is excellent. If the hub is not symmetrical, user must iterate twice with this SLC--once for each side of the wheel. SLC can be used 'in reverse' i.e. from known spoke length to unknown rim diameter.
(A variant of this SLC, which allows simultaneous calculation of left- and right-side spoke lengths, can be found at BikeFAT.com.)

DT Swiss' SLC

N* Y* N 5D Y Y Y SLC, full-featured commercial offering intended for those in-the-know, has interface almost certainly intimidating to tyros. Calculates even for asymmetrical front wheels/hubs.
* Account creation allows calculations stored across sessions, but anonymous (one-time) use also possible.
* A pop-up compilation of tips and observations--evidently translated from German--is available. Certain of the sponsoring company's products are foregrounded within the SLC.

ebike's SLC

N Y N 5D Y Y Y* SLC, an offering of GRIN Technologies, divides the measurements inputs sensibly into hub-, rim-, and lacing-options sections. Very information-rich single-pager that provides clear definitions and discussion of the required measurements. Although intended for e-bike wheel builds (think: hub motors), page provides information and insights for advanced wheel-builders (e.g. SLC shows "tension ratio and spoke angle into the rim").
* Database data is hub motor-specific.

Edd SLC

N Y N 5D Y Y Y SLC adapts well to displays on portable devices. FAQ and very worthwhile how-to-measure tutorial. Although billed as 'easy to use', its interface is confused both by the on-screen layout of elements, and by a collision of the database-search and data-entry aspects of using it. However, user-contributed calculations are accepted for assimilation into database.

FreeSpoke (complete) SLC

N* Y N 5D Y N* Y SLC incorporates self-growing database with simple and full input forms available. Database of rim and hub dimensions, derived from Damon Rinard's Spocalc, is accessible (browse, filter, search). Versatility similar to DT Swiss' SLC. Slightly confusing interface, but very informative. A work-in-progress to be encouraged.
* Account and session info possible, but one-time use is, too.
* JavaScript not strictly required.

Leon Mitchell's SLC

N N N 2D Y Y N SLC is simple, but has sufficient information for ready use by anyone passingly familiar with the measurements required. Like many, the author bases his SLC on formula(e) from Jobst Brandt's book "The Bicycle Wheel".

Pro Wheel Builder's SLC

N* N* N 5D Y Y Y SLC, a commercial offering with a disorienting interface that melds database search and simple-use cases, is full-featured.
* Account and session info possible, but one-time use is, too (i.e. there's a disclaimer checkbox). There is a link to "Instructions" for the SLC--only on the navigation menu of the site's foyer page--but that link appears to be a place-holder...

SpokeLength.com SLC

N N N 2D Y Y N SLC is simplest-possible form (mouse-driven/touch-based, with only partial support for keyboard-based input). Accessible to anyone familiar with the measurements required. Form is dimensionally fixed, so doesn't adapt well to lowest-resolution displays on portable devices. SLC can be used for noting the effects of small changes (or errors) in dimensional inputs on the resulting calculated spoke length; changing any of the three basic measurements results in an immediately-updated result.

SpokeService's SLC

N N N 2D Y Y N SLC is simple--even spartan--but is accessible to anyone passingly familiar with the measurements required. The site, like a few others, also includes a Spoke Tension tool--useful for visualising tension distribution in a wheel.

Wheelpro's SLC

N Y* N 5D Y Y N SLC is generally capable, though not suited to more exotic wheel types (which the author acknowledges). Interface, overall, is a bit awkward. SLC can output a table of results for a range of number of spokes e.g. 28-36.
* Help is spread over several pages, linked from a glossary page. Site uses frames.
Author offers e-book "Professional Guide to Wheel Building" exclusively from the site.

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Spoke-length calculators

Login?
FAQ?
Illustrated?
Hubs?
Form?
JavaScript?
Database?
 

Notes

1. "2D" is to "5D" as simple is to complex: SLCs designated "2D" assume symmetrical hubs, which can be described using only a diameter and a width, while "5D" means the SLC anticipates the possibility that the hub/wheel itself is asymmetrical (in at least one sense--and perhaps several), so the process of measuring, and entering, the required values is more painstaking for "5D" than for "2D" SLCs. A reasonably clever user can use either type of SLC for any wheel-build.