FretFind2D
FretFind2D is a two dimensional fretboard design tool. FretFind2D doesn't just calculate fret spacing. It models the entire fretboard, strings and frets, as a system of line segments on a two dimensional plane. Because of this approach, it can design fretboards for instruments with multiple scale lengths and nonparallel frets as well as fretboards for instruments that play just or meantone scales.
 units
 scale length

 single

 fundamental scale length
 The scale length is the playing/speaking length of the string measured from the nut to the bridge. It is perhaps more properly twice the distance from the nut to the octave fret. The fundamental scale length is the length of a line drawn from the middle of the nut to the middle of the bridge. For single scale length instruments that line is the perpendicular bisector of both the nut and the bridge. I call this length "fundamental" because on a standard instrument with a narrow nut and a wide bridge the outer strings actually have a slightly longer scale length.
 multiple

 first string scale length
 The scale length is the playing/speaking length of the string measured from the nut to the bridge. It is perhaps more properly twice the distance from the nut to the octave fret. Enter the actual scale length of the first (traditional high E) string.
 last string scale length
 The scale length is the playing/speaking length of the string measured from the nut to the bridge. It is perhaps more properly twice the distance from the nut to the octave fret. Enter the actual scale length of the last (traditional low E) string.
 perpendicular fret distance

The perpendicular fret distance is the ratio of distances along the first and last string that fall on a line perpendicular to the midline of the neck. This is used to control the angle of the nut, frets and bridge.
Traditionally this property of nonparallelly fretted fretboards is measured by assigning a "perpendicular fret". "Perpendicular distance" avoids two problems with the "perpendicular fret" method. First, it is possible that no fret falls into this perpendicular position. With "perpendicular distance" we avoid fractional frets. Second, it is possible and even likely with nonequal temperament fretboards that as a fret crosses the fretboard it will fall at different ratios along the strings. With "perpendicular distance" we avoid complex calculations and have more predictable results.
A value of 0 results in a perpendicular nut. A value of 1 results in a perpendicular bridge. The default 0.5 results in a perpendicular octave fret. To calculate an appropriate value for any fret, simply divide the distance of the fret from the nut by the total length of the string. In twelve tone equal temperament the values look like this:
Fret P.D. Fret P.D. 1 0.05613 13 0.52806 2 0.10910 14 0.55455 3 0.15910 15 0.57955 4 0.20630 16 0.60315 5 0.25085 17 0.62542 6 0.29289 18 0.64645 7 0.33258 19 0.66629 8 0.37004 20 0.68502 9 0.40540 21 0.70270 10 0.43877 22 0.71938 11 0.47027 23 0.73513 12 0.50000 24 0.75000
 individual

Danger: Experimental!!!
 string scale lengths:
 The scale length is the playing/speaking length of the string measured from the nut to the bridge. It is perhaps more properly twice the distance from the nut to the octave fret. Enter the actual scale length of the each string.
 perpendicular fret distance

The perpendicular fret distance is the ratio of distances along the first and last string that fall on a line perpendicular to the midline of the neck. This is used to control the angle of the nut, frets and bridge.
Traditionally this property of nonparallelly fretted fretboards is measured by assigning a "perpendicular fret". "Perpendicular distance" avoids two problems with the "perpendicular fret" method. First, it is possible that no fret falls into this perpendicular position. With "perpendicular distance" we avoid fractional frets. Second, it is possible and even likely with nonequal temperament fretboards that as a fret crosses the fretboard it will fall at different ratios along the strings. With "perpendicular distance" we avoid complex calculations and have more predictable results.
A value of 0 results in a perpendicular nut. A value of 1 results in a perpendicular bridge. The default 0.5 results in a perpendicular octave fret. To calculate an appropriate value for any fret, simply divide the distance of the fret from the nut by the total length of the string. In twelve tone equal temperament the values look like this:
Fret P.D. Fret P.D. 1 0.05613 13 0.52806 2 0.10910 14 0.55455 3 0.15910 15 0.57955 4 0.20630 16 0.60315 5 0.25085 17 0.62542 6 0.29289 18 0.64645 7 0.33258 19 0.66629 8 0.37004 20 0.68502 9 0.40540 21 0.70270 10 0.43877 22 0.71938 11 0.47027 23 0.73513 12 0.50000 24 0.75000
 string width at the nut
 The string width at the nut is the distance along the nut from the center of the first string to the center of the last string. I'm using delta x distance (distance measured along a line drawn perpendicular to the neck's midline) because I think that is what you would feel as the width if you were playing an instrument with multiple scale lengths. It also makes the calculation easier. (Please note, FretFind will space the remaining strings equally between these two points.)
 string width at the bridge
 The string width at the bridge is the distance along the bridge from the center of the first string to the center of the last string. I'm using delta x distance (distance measured along a line drawn perpendicular to the neck's midline) because I think that is what you would feel as the width if you were playing an instrument with multiple scale lengths. It also makes the calculation easier. (Please note, FretFind will space the remaining strings equally between these two points.)
 fretboard overhang

 equal
 nut & bridge

nut bridge  first & last

last first  all

last first nut bridge

The fretboard overhang is the distance from the center of outer strings to edge of nut or bridge.
For fretboards with multiple scale lengths this is calculated as delta x distance,
distance measured along a line drawn perpendicular to the neck's midline.
There are four input modes for overhang.
 Equal:
 you enter a single value and the overhang will be constant.
 Nut & Bridge:
 allows you to specify one overhang at the nut and another overhang at the bridge.
 First & Last:
 allows you to specify one overhang for the first string and another for the last string.
 All:
 you specify an overhang for all four locations separately.
 calculation method

 equal (root 2)
 just (scala)

The calculation method determines how FretFind calculates fret placement.
There are two input modes.
 Equal:
 uses the X^{th} root of two, a standard method for calculating equal temperaments. You enter the number of tones per octave.
 Scala:
 uses a Scala SCL file which allows you to specify each scale step exactly in either ratios or cents. If you are interested in creating your own scale, please read this description of the Scala scale file format. Otherwise try a scale from the Scala scale archive, found at the very bottom of the Scala download page. You can learn more about Scala at the Scala home page.
 number of frets
 This is the number of frets you would like FretFind to calculate. The number of frets must be an integer.
 number of strings
 The number of strings must be an integer. If you change the number of strings be sure to update the tuning section below (only useful with nonequal temperament scales).
 tuning
 Enter the scale step (of the scale defined above) to which each string will be tuned. For example a standard guitar in the key of E would be tuned 0, 7, 3, 10, 5, 0. The first string is the string to the far right on the fretboard. This step is not important for the Equal calculation method. Entering a tuning for the Scala calculation method will very likely result in partial frets.
 Link
 Link to this design
The latest version in development is available on GitHub.