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Articles: Guitar Fretboard Radius
What is fretboard radius?
Fretboard radius is simply the curve of the fretboard from side to side. This curve is to make the fretboard more comfortable for the fretting hand. The radius is described as a measurement such as 7.25", 9.5", 12" or 16". These measurements tell you that the fretboard is a segment of a circle or cylinder, which has a radius, or size of 7.25", 9.5", 12" or 16". The larger the radius, the larger the circle, and the flatter the fretboard will be.

In simple terms
Quite simply, the smaller radius (which is more curved) is more comfortable for playing (particularly barre) chords, while a larger, flatter radius is better for low action, single-note playing and bending. The other important characteristic of radius is that the flatter the radius, the lower the action can be. This is because when you bend a string on a lower part of the fretboard such as your first or second string, you are bending it towards the middle of the fretboard, which is higher than the edges, so the notes will tend to "fret out" - buzz against the higher parts of the fret, killing sustain.

Another important thing to note is that staggered polepiece pickups (like vintage style single-coils) were originally designed for vintage Fender guitars, which had a small 7.25" radius. These sometimes don't work well with flatter radii, making the middle strings jump out much louder, particularly the G string (which was also designed for wound strings). There is a workaround for this problem, which is to keep the pickups set lower (further away from the strings), where the string-to-string balance will even out. The downside to this workaround, is that you lose volume with the pickups set lower. You should also be aware that there are "modern vintage" pickups, which also have staggered polepieces

 

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A  fretboard that is a fixed width, but has a radius of 16" (green line) is a lot flatter than a fretboard which has the same width, with a 7.25" radius (blue line)

Which fretboard radius is best?
Which radius is "best" tends to be a personal thing, you should be asking "which is best for me?". Generally, most players are going to prefer something between a 9.5" and a 12". Many players who play more "lead-based" styles, or those needing lower action and more bending will probably be happier with larger radii such as 16" or even 20". Ultimately this is down to personal preference, which is largely based on the instruments we learned to play on. The best thing you can do is play as many different guitars as possible (like we need an excuse, right?), making note of which feel more comfortable to you. Bear in mind that other factors such as neck thickness, fretboard thickness, scale length and even finish do affect the feel of a neck, but the more time you spend playing different instruments, the easier it will be to identify each factor.
Examples of common radii
Guitar Radius
Classical guitar flat
Vintage Fender Stratocaster 7.25"
Modern Fender Stratocaster 9.5"
Guitars with LSR roller nuts 9.5"
Guitars with Floyd Rose locking nuts* 10"
Gibson guitars 10 - 12"
Ibanez guitars 12"
Jackson guitars 16"
Compound radius replacement necks
USA Custom guitars 7.25" - 9.5"
Warmoth 10" - 16"
* Applies to the original and Schaller Floyd Rose models but not necessarily "licensed" models
Conical radius fretboards
Conical radius fretboards (commonly and incorrectly called "compound" radius fretboards) are an attempt to get a "best of both worlds" between smaller and larger radii. These fretboards have a radius which changes, they start out more curved at the nut and gradually get flatter as you climb the neck, so each fret is a fraction flatter than the one before it. While a normal fixed radius is a segment of a circle or cylinder, a conical radius is actually a segment of a cone.

The idea is that players tend to bend higher up the neck and play chords lower down. The larger radius higher up on the neck lets you get the action down lower and still bend cleanly, while the smaller radius nearer the nut makes chording easier. Warmoth make replacement necks with a 10" - 16" conical (which they call "compound") radius, but my favourites are the USA Custom guitars, who make a 7.25" - 12". They also make the flatter 10" - 16" or even a 10" - 12" (on request).

Compound radius fretboards
As already noted, fretboards billed as compound are often not compound, but conical. A compound fretboard has one radius for part of the length, then changes abruptly to another radius for the next part, instead of the smooth, gradual  transition of a conical fretboard. This may be as simple as a 10" radius from the nut to the 12th fret, then changing to 16" for the remainder of the fretboard, or there could be three or four radii involved.

 

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A conical radius fretboard

Compound radius fret dressing
A trick often used to make smaller radius fingerboards better for bending is to dress the frets from about the 10th or 12th fret with a flatter radius. This essentially makes the fretboard compound without needing to remove the frets, dress the fingerboard and refret again. The difference in height between the common guitar radii is usually very small, so doing this properly means that very little of the fretwire needs to be removed, which usually leaves plenty of life in the frets. So if you have a vintage (or vintage reissue) Strat that you want to get a lower action without notes fretting out on bends, this is a great option.

String radius
While at first thought it might seem as though the curve of the strings should match that of the fretboard, this is not necessarily so. A flatter radius is usually possible, as bending the middle strings usually pulls the strings to the lower sides of the fretboard, where there is less chance of them fretting out. Try setting your nut radius to match your fretboard and your bridge radius to be slightly flatter - effectively giving your strings a compound radius. You will find this gives a much easier feel and a lower action.

Measuring radius
To measure radius it is necessary to have a simple but specialised tool: a radius gauge. Stewart McDonald have some nifty radius gauges that work without removing the strings and a set of understring radius gauges that are capable of measuring both the fretboard and the string radius. If you are into making your own tools, there are templates at Kinman Pickups.

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