String spacing and fretboard radius
Two measurements define a neck across its width: where each string crosses the nut and the bridge, and how much the center of the fretboard rises with the radius. Enter the distance between the outer strings, center to center, and the gauges; the table shows each string's position and the clear gap between neighbors.
Fretboard radius arc height
How far the center of the board rises above the straight line between the edges at 43 mm (typical nut width) and 56 mm (typical width at the end of the neck), for the most used radii.
| Radius (in) | Radius (mm) | Arc at 43 mm | Arc at 56 mm |
|---|---|---|---|
| 7.25″ | 184.1 | 1.26 | 2.14 |
| 9.50″ | 241.3 | 0.96 | 1.63 |
| 10″ | 254.0 | 0.91 | 1.55 |
| 12″ | 304.8 | 0.76 | 1.29 |
| 14″ | 355.6 | 0.65 | 1.10 |
| 16″ | 406.4 | 0.57 | 0.97 |
| 20″ | 508.0 | 0.46 | 0.77 |
How to use it
- Tap an instrument to load starting gauges and measurements.
- Choose whether you're measuring at the nut or the bridge and enter the distance between the outer strings, center to center.
- Choose equal gap (same clear space between strings, accounting for gauge) or equal centers.
- For the radius, enter the radius at the nut and at the end of the board (the same for a single radius), the scale, frets and end width. Print the gauge at 100% and measure the 100 mm bar.
Help and explanations
For the radius, choose a single value (7.25″, 9.5″, 12″, 16″) or a compound one (10″–16″). The table gives radius, width and arc height along the neck, and the radius gauge prints at full size.
Equal centers or equal gap
With equal centers, the distance from one string to the next is always the same, so the clear space is smaller between the heavy strings. With equal gap, the calculator subtracts the gauges from the width and divides what's left. With 35 mm (1.38″) between the outer strings and a .010–.046 set, the difference on the middle strings reaches 0.6 mm.
At the bridge, spacing comes from the hardware: a modern Fender bridge has 2-1/16″ (52.4 mm) between the outer strings; vintage-style bridges use 2-7/32″ (56.4 mm).
Radius and arc height
The smaller the radius, the rounder the board: 7.25″ on vintage Fenders, 9.5″ on modern ones, 12″ on Gibsons; 16″ or more is nearly flat. A compound radius (10″ at the nut, 16″ at the end) is rounder where you play chords and flatter where you bend.
Arc height is how far the center of the board rises above the straight line between the edges: s = r − √(r² − (w/2)²). On a 9.5″ (241.3 mm) radius, that's 0.038″ (0.96 mm) across a 1.69″ (43 mm) width and 0.064″ (1.63 mm) across 2.2″ (56 mm). The nut and the bridge saddles have to follow that arc.
Tips
- Measuring edge to edge? Subtract half a gauge from each outer string.
- On a compound-radius board, the bridge saddles follow the radius at the end of the neck.
- Changed the radius or the nut? Check the string action; for fanned frets, use the multiscale fret calculator.
Frequently asked questions
What is the string spacing at the nut on a guitar?
On a 1-11/16″ (43 mm) nut, about 1.38″ (35 mm) between the centers of the outer strings, roughly 7 mm (0.28″) string to string, and just over 3 mm (1/8″) from each outer string to the edge.
What is Fender bridge string spacing?
On the modern bridge, 2-1/16″ (52.4 mm) between the outer string centers, about 10.5 mm (0.41″) string to string. Vintage bridges use 2-7/32″ (56.4 mm).
Which fretboard radius should I choose: 9.5″ or 12″?
7.25″ (184 mm) feels great for chords, but bends can choke with low action; 9.5″ and 12″ are the middle ground; 16″ (406 mm) is almost flat. A compound 10″–16″ gives you a bit of both.
What is fretboard arc height?
How much the middle of the board rises above the edges. Across 43 mm (1.69″): 1.26 mm on a 7.25″ radius, 0.96 mm on 9.5″, 0.76 mm on 12″ and 0.57 mm on 16″.
How do I measure fretboard radius?
Set a 9.5″ radius gauge across the board between two frets. If it rocks on the center, the board's radius is smaller; if it touches only at the edges and shows light in the middle, it's larger.