The bridge saddle location of the guitar, bass guitar, mandolin, ukulele and of most fretted instruments is moved a bit back from its nominal location. This change in location is called bridge saddle compensation. Without it, all fretted notes will play sharp to some extent, due to stretching of the string during fretting and also bending stiffness. Compensation effectively lengthens the string length of fretted notes. The longer string lengths make the pitch of the fretted notes flatter, thus compensating for the sharping effects. This page contains an online calculator which can be used to calculate compensation. The calculator provides individual string compensation values and also straight saddle compensation for instruments that feature a straight angled saddle. It contains data for a number of types of instruments and can be easily used to calculate compensation values for those instruments. The calculator can also be used to calculate compensation for non-standard instruments.
Initial appearance: July 17, 2014
Last updated: September 11, 2018
The scale length of an instrument is the distance between the edge of the nut and the nominal bridge saddle position. For guitars and other fretted instruments the placement of the frets is based on the scale length. See the page entitled Calculating Fret Positions on this site for more info on fret placement. For fretless instruments the bridge is generally located at the nominal bridge position. If the bridge was located here for fretted instruments though, the pitch of each note would be sharp and this sharpness would increase the farther up the neck you went. There are two major factors involved with this phenomenon. The first is that the strings of an instrument are positioned at some distance over the frets. The distance is small, but pressing a string down to a fret stretches it a little, which raises its tension a little, which raises its pitch. The other factor is that strings exhibit bending stiffness, and this increases the vibrating frequency of the string a bit, which also sharpens the pitch.
How much pitch is sharpened depends primarily on the stiffness of the string, or for wound strings, of the string's core. This stiffness is a function of the diameter of the string and also of the material it is made of. The solid nylon of the upper classical guitar strings and the nylon floss of the cores of the lower strings is not very stiff, so stretching the strings while fretting doesn't raise the pitch all that much. Consequently not much compensation is required. Contrast that with the solid steel of steel guitar strings, which is quite stiff. Fretting steel strings raises their pitch significantly and they require more compensation. No matter what the material, the bigger the diameter of the string (core), the stiffer it is, so large diameter solid strings will need more compensation than thinner ones.
The generally accepted solution to this sharping problem is to move the bridge saddle a bit farther away from the nut. This effectively increases the length of the string from fret to bridge saddle, which flattens the pitch and thus provides some compensation for the sharping effects described. Bridge-only compensation is by no means a mathematically perfect solution, but in practical terms it may very well provide a solution which is well within the limits of variability which is not under the builder's control (string mass variability, note-to-note fretting pressure variability1, etc.), and which is also within the limits of pitch perception for musicians in real musical contexts. Its persistent and widespread use may attest to its practical utility. Likewise the limited use of systems and devices claimed to improve intonation beyond what is provided by bridge saddle only compensation, even though such systems and devices have been available for quite some time, may indicate that bridge saddle only compensation is in fact functionally optimal. Recent research indicates that all popular compensation strategies are perceptually equally effective.2
Just to define the term: Bridge compensation is the distance the bridge (saddle) must be moved from the nominal bridge saddle position to achieve the desired compensatory results. For the most part compensation has historically been calculated empirically. An instrument was set up with the bridge (saddle) located at the nominal bridge position, and then it was moved back by increments until both the open string and the fretted octave were in tune. This is still the way it is done for instruments like electric guitars that have intonation adjustment available. For instruments with fixed bridges and saddles, like typical acoustic guitars, once compensation was figured out, the compensation distance(s) was recorded and future instruments were simply built with the bridge saddle(s) located at the derived position. It should be noted that each string will usually have its own compensation value. Most acoustic instruments will make use of a straight angled saddle though. Because a straight saddle rarely can be located to provide ideal compensation for all strings, the use of a straight saddle represents a mathematical compensation compromise. But here again, considering the high popularity of the straight saddle, this compromise may not be of (much) practical significance.
Given some information about the instrument and the strings that will be on it, it is possible to do an accurate job of estimating compensation. This is generally not something you'd want to do with pencil and paper as there is a lot of math. But it is an ideal application for an online calculator such as this, which will do all of that math for you.
This calculator makes use of compensation equations that are variants of those developed by industrial polymer physicist Sjaak Elmendorp. His equations are described in his American Lutherie article entitled "It's All About the Core, or How to Estimate Compensation".3 I highly recommend this article for background about compensation. More advanced users of this calculator will probably need to reference it, although basic users will not. The article fully describes the derivation of his compensation equations; it describes validation testing of the model and resultant accuracy specifications; and it describes measurement limitations which should be considered when making use of those equations. Most of this information will not be repeated here. If you are interested, see the article. By the way, even though this article is short, appeared in a popular press journal, and describes a very informal-appearing investigation, the author is a true scientist and the methodological rigor of the experiments and the reporting of same is exemplary.
The calculator outputs offsets from the nominal bridge saddle location to the compensated bridge saddle location for each string, and also for a straight bridge saddle. Refer to the following diagram.
The calculator has been designed so that it can be used for simple compensation estimation for a number of standard instruments, and also for much more advanced use. Basic use is pretty straight forward:
Select an instrument from the drop down menu at the top of the calculator and click on the "Fill" button. This will fill the subsequent forms with data for a typical configuration of that kind of instrument.
(optional) Modify common parameter values for scale length and action as needed. Note that all fields accept numeric data only. Values are in inches. Note also that average action values for the instrument should be specified. If you make any modifications here, click on the "Propagate" button. This will update the subsequent forms with your changes.
Scroll down past the tabs containing the data for each string to the section entitled "Calculate Results". Click on the "Calculate" button.
A compensation value for each string will appear in the red results fields below. Each of these values is an offset from the nominal bridge saddle location to the compensated bridge saddle location for that string. Also included are straight saddle offsets for the first and last strings. These are also offsets from the nominal bridge saddle location. Connecting these two with a straight line represents the compensated straight saddle. If you are using the calculator to derive compensation for a classical guitar or other instrument that has a straight saddle that is set perpendicular to the centerline of the fretboard, take the difference between the two straight saddle offset values, divide that in half, and add that result to the smaller of the two straight saddle offset values. The result is the compensation offset for the saddle.
|Status of calculations:|
|String#||Compensation||Straight Saddle Compensation|
Selecting an instrument from the menu fills in the forms with data for a typical instrument of that type - typical scale length, typical action and typical string set. Adjusting any of the common parameters and propagating them modifies the form data. But it is the string data forms that the calculator uses to estimate compensation. The data in the forms can be modified directly to further refine the compensation estimation. Describing a new instrument from scratch is detailed in the following section. This section contains information on typical modifications to the string data forms you may want to make.
A number of steel string guitars with straight saddles provide separate compensation for the B string. Likewise a number of classical guitars that make use of a straight saddle will provide separate compensation for the B and/or G string. This is because these strings need enough compensation to make them way out of line of a straight line saddle. If you intend to use such a saddle on your instrument, you can specify that the calculator should not consider certain strings in its straight saddle calculations. On the tab for those strings, un-check the box labeled "Include this string in straight saddle compensation calculations?" This will result in more accurate straight saddle calculations for the other strings. That increase in accuracy may be perceptible.
People don't have very good pitch perception in the bass range (and it is even worse in the double bass range) so it may be advantageous to exclude the lowest string from guitars and basses from straight saddle compensation calculations. The resulting compensation for those strings probably won't result in perceivable differences, and compensation accuracy of the remaining strings will be improved, possibly perceptibly.
As you can see from the parameters in the tabs, most of the information needed to estimate compensation relates to the instrument's strings. So if you want to use the calculator to estimate compensation for an instrument that does not appear in the drop down menu, most of the work involved in doing that will be in describing the instrument's strings. In an ideal world all of this information would be readily available from string manufacturers, but in fact little of it actually is. The very first thing you'll want to do if at all possible is to get or compose a set of strings from D'Addario. Why? Because D'Addario is the only string manufacturer that provides unit weight and tension data for most of the strings in their product line.
Specifying an instrument involves filling out the parameter fields in the forms as described below. If you are going to spend the time to do this for a common instrument you would be doing the world a great favor if you send your completed forms to me so that I can include this instrument in the drop down menu. Thanks. Be sure to include some information on the instrument and string set, and please provide your email address so I can contact you with any questions. Here are the parameters and how to fill these fields in.
The compensation equations used by this calculator are variations of those specified in . The article also shows how to derive material MOE and gives examples for nylon and nylon floss. These materials and other polymers will exhibit nonlinear stress/strain curves and thus their elastic moduli will be functions of tension. The calculator makes use of the variable elasticity data for nylon and nylon floss from the article. Gut also exhibits variable elasticity, and values are calculated from my own experiments. Straight saddle compensation estimation is done by simple linear regression, using a simple incremented x-value per string.
Mottola, R.M. “Same-Fretted-Note Intonation Variability of the Steel String Acoustic Guitar” OSFPreprints, 2018
Mottola, R.M. “Blind Listening Evaluation of Steel String Acoustic Guitar Compensation Strategies” OSFPreprints, 2017
Elmendorp, Sjaak “It's All About the Core, or How to Estimate Compensation” American Lutherie, #104