What is it?
The McDonald Patent Universal String Tension Calculator (MPUSTC) is a handy
calculator to figure string tensions in steel-string instruments. If you
plug in your scale length, string gauges and tuning, it will give you a readout
of the tension on each of the strings. This is useful when you're trying
to fine-tune a set of custom gauges, or when you're working out how far you
can push a drop tuning before it becomes unmanageable.
How good is it?
Pretty good. It has presets for a whole range of instruments, and you can alter
gauges and tunings to your heart's content. However, the database of string
mass values with which it operates is necessarily incomplete--think of all
the different types and gauges of string in the world! So we had to fudge
it here and there. The most serious limitation is that it can't handle gauges
above .056, tho' we hope to fix that if anyone ever sends us hard data for
What instruments does it work with?
At present: 6- and 12-string guitar, Nashville-strung guitar, mandolin, mandola,
unison-strung Irish bouzouki, long and short citterns. Drop tunings no problem.
We expect to add more as time goes on. It may be possible to use the MPUSTC
to figure the tension in other instruments than those on the preset list,
but we won't swear to it.
How do I use it?
Click on the pop-up, and choose the instrument and string set closest to what
you have in mind. The form changes to display corresponding scale, note,
gauge and string type values, and the associated string tensions are calculated.
If you want to fine-tune the default values, overtype the
scale length, gauge and the wound/plain popups. The MPUSTC may override
some of your settings, but you'll see alerts which explain why this is
so. Press Recalculate when you've finished, and the new tension values
How do I deal with my special dropped tuning?
Type the notes of your new tuning into the appropriate input boxes. (To enter
a C sharp, type: Csharp To enter a B flat, type: Bflat.) The
MPUSTC will take a guess at the frequency, working on the assumption that
your notes are five semitones or less from standard tuning. Go ahead and
Recalculate when the values are in place.
If you're trying a Michael Hedges tuning--sixth string
a fifth below normal, that sort of thing--you will exceed the MPUSTC's
range. Download Graham's helpful PDF and
you can learn how to do the necessary calculation by hand. That's also
a good trick to know if you're into mean temperament or quarter tone
Metric sucks. Can I work in Imperial?
Yup. When you enter your scale length, use the code "in", like this: 22.5in The
MPUSTC will respond appropriately. Note that, because the output is being converted,
there will likely be some small inaccuracy.
I checked your values against the ones given by [insert
famous string maker], and they're different. Why?
We don't know. Makers who offer tension charts seldom state the scale length,
of course, but even with a lot of experimentation we've found that our figures
typically differ by 5--10% from the quoted figures. For what it's worth, we're
using a classic formula and pretty good data.
Who made it?
The tension calculator uses a formula supplied by Max Krimmel of the Guild
of American Luthiers. String mass data was gathered by Graham McDonald using
equipment at the Powerhouse Museum in Sydney. Will Meister hooked up Graham's
How do I tell you about bugs or get you to extend the
range of supported instruments?
Comments to Graham or to Will. Remember that the MPUSTC is supplied free of
charge. We try to make it as good as we can, but you don't get custom work
unless you pay for it. Requests for additional instruments will be honoured
if you send:
i. full details including string gauges, note values, scale length and precise
tuning in Hz
ii. beer money.
Requests for additional string gauges will be ignored unless you are prepared
to supply hard data.
Graham McDonald is
an Australian luthier with an international reputation. He has written two books on instrument building, The Bouzouki Book and The Mandolin Project
as well as The Mandolin - a history.
Will Meister is an
English writer and Web guru who enjoys difficult projects. He also edits 63xc.com,
the offroad fixed gear site.
This calculator is made available under a Creative Commons Attribution-NonCommercial 4.0 International Licence
Back to the Home page