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 heavier sets.
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
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 are displayed.
How do I deal with my special dropped
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
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 tunings.
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
I checked your values against the ones
given by [insert famous string maker], and they're
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 data and Max's formula with some funky
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.
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.
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
Back to the Home page