One tool can take all the guesswork out of measuring: a
caliper. It's the ultimate in accuracy—for a woodworker, anyway—and
it's easy as pie to use. A caliper reads out one precise measurement,
clear and simple. Most calipers read in thousandths of an inch. They're
designed for machine shops, not wood shops. Who measures wood in
thousandths? Musical instrument makers, maybe, but not the rest of us.
Fractions are what we want. A new generation of $30 to $50 calipers
designed for woodworkers replaces thousandths with sixtyfourths. With
one of these gems, you can easily make a measurement to the nearest
1/64 in. And you can go smaller yet, by reading between the lines, to
about the thickness of a heavy sheet of paper. That's plenty good
enough for wood, which can expand or contract that much each day with a
change in humidity. Let's take a look at all the jobs a caliper can do,
no matter what kind of scale it has. Then we'll weigh the pros and cons
of five kinds of calipers—basic slide, vernier, decimal dial, decimal
digital and fractional dial—you'll find in woodworking catalogs.
Finally, we'll tell you about the clear winner, a $30 beauty that will
pay for itself the first time you avoid a costly mistake.
Width of a Dado
A caliper measures an inside dimension,
too, with small “reverse” jaws that can span a gap as narrow as 1/16
in. A caliper is a perfect tool for setting up dado blades. Using the
reverse jaws, measure the width of a dado you've made; then use the
other jaws to measure the thickness of any shims you must add to or
subtract from the dado blades.


Thickness The most common use for a
caliper is to measure an outside dimension, such as the thickness of
your stock as it comes out of the planer. A caliper is a lot easier to
read than a ruler for this everyday job. You can also measure the
precise width of a board you've cut in order to adjust the ripfence
scale on your tablesaw. 
Depth of a Hole
A small bar extends from the end of every caliper
for measuring the depth of a hole. This bar may not fit in very
smalldiameter holes, however. The width of different calipers' bars
varies from 1/8 in. to just over 1/4 in.
Depth of a Rabbet Many folks are surprised to learn the back of a
caliper can measure the width and depth of a rabbet or dado. In
addition, whenever you use a zeroclearance throat plate on your
tablesaw, you can measure the height of your blade or dado set with the
back of the caliper. This is much easier than standing a ruler on end.



Read a vernier scale by using the zero to measure whole inches and
sixteenths. Then find where two lines meet. This is the number of
smaller fractional units to add, in this case, 1/128in. units.
A vernier caliper is really clever. It has a sliding scale that
replaces the dozens of tiny lines you would find on a finely divided
ruler. The sliding scale has widely spaced markings that actually read
“in between” the 1/16in. lines. This makes small measurements much
easier to read. Here are three simple steps to using a vernier scale.
First, read the number to the left of the zero mark on the sliding
scale, including sixteenths (here, it's 1in.). Next, find the line on
the sliding scale that exactly lines up with any line on the fixed
scale (here, it's the fourth line from zero). This indicates that the
caliper reads 4/128 (or 1/32) past the 1in. mark. If the fifth mark on
the sliding scale lined up better with a different line on the fixed
scale, the reading would be 5/128 past the 1 in. mark. The last step is
to add the whole numbers, sixteenths and 1/128in. measurements.
Turn the dial so it reads “zero” when measuring a mortise. All calipers can be zeroed out at any position.


Measure the tenon. The number on the dial is exactly how much bigger or smaller the tenon is than the mortise.

A caliper can easily tell you the exact difference in thickness or
width between two pieces of wood, without requiring that you add or
subtract nasty fractions. This is a great way to fit a tenon into a
mortise, a tongue into a groove or a plywood board into a dado.


Basic Slide • Reads in fractions • Smallest division is 1/16 in. • Costs $5 to $12 Pros A
slide caliper is inexpensive and small enough to easily slip into a
pocket. It doesn't have any spiky points to catch on fabric. Most
models read in both inches and millimeters. Cons A slide caliper
cannot read the depth of a rabbet or a hole. It cannot be zeroed out to
make comparison measurements (see page 41). Its markings are often hard
to read. Some models have dimension lines cut in brass or steel, rather
than inkedin lines. Our Favorit Model Hartville Tool (800) 3452396, www.hartvilletool.com 4in. brass caliper, #85879, $8.



Decimal Dial • Reads in thousandths • Smallest division is 0.001 in. • Costs $16 to $30 Pros A
decimal dial caliper is very precise (0.001 in. is onethird the
thickness of a dollar bill). By comparison, a basic slide's smallest
division of 1/64 in. is about 0.016 in., or 16 divisions on a decimal
dial caliper. Roughly speaking, a decimal caliper is 16 times more
accurate than a basic slide caliper. Cons Few woodworking
situations call for this level of accuracy. When you're making two
pieces that must fit together, such as a mortise and tenon, the feel of
the fit is more reliable than measuring in thousandths. Our Favorit Model Woodcraft Supply (800) 2251153, www.woodcraft.com 6in. dial caliper, #01R62, $30.



Vernier • Reads in fractions • Smallest division is 1/128 in. • Costs $13 to $20 Pros With
a vernier caliper, you get plenty of accuracy for the least amount of
money. A vernier caliper is named after a twopart scale that reads
both coarse and fine measurements (see “How To Read a Vernier,” page
40). A vernier caliper's simple slide mechanism is virtually
indestructible, unlike a dial or digital device. Cons A vernier
scale is not intuitive. You have to learn how to read it and remember
how it works each time you use it. A vernier caliper cannot make
comparison measurements. Most vernier calipers read in thousandths of
an inch, not in fractions. Our Favorit Model Lee Valley (800) 8718158, www.leevalley.com 6in. fractional vernier caliper, #88N70.01, $17.



Electronic Digital • Reads in thousandths • Smallest division is 0.001 in. • Costs $35 and up Pros A
digital caliper is foolproof and very accurate. It displays one clear
number. There's no need for counting lines or mentally adding numbers
together, as with a decimal dial caliper. For comparison measurements,
you simply push a button to zero the caliper. A short 4in. model is
amazingly compact and lightweight. Cons Nobody makes an
electronic caliper that reads in fractions—yet. All of them read in
thousandths, and going from decimals to fractions can really be
awkward. You need a conversion chart or an engineer's head for
remembering decimal equivalents. Our Favorit Model Lee Valley (800) 8718158, www.leevalley.com Electronic caliper: 6in., #88N62.56, $35 4in., #88N62.54, $33. 


Fractional Dial • Reads in sixteenths and hundredths • Smallest divisions are 1/64 in. and 1/100 in. • Costs $30 to $50 Pros A
fractional dial is clearly the best type of caliper for woodworking.
Fractions are our language! This model is our handsdown favorite
because it's so easy to read. We prefer the smaller 4in. version shown
here because it fits right into an apron pocket. Cons Of the many types of fractional dial faces, some are easier to read than others (see “Types of Fractional Dials,” at right). Editor's Choice Highland Hardware, (800) 2416748, www.highlandhardware.com 4in. caliper, #168077, $30 6in., #065008, $37.

A lot of companies have jumped on the fractional caliper bandwagon,
but not all their tools are easy to read. Our favorite type, the top
dial, has every commonly used fraction up to an inch clearly marked on
the outside of the dial. Each unmarked line is 1/64 in. The dial below
our favorite isn't as userfriendly. You must count lines in between
the major markings to find, for example, the 5/16in. mark. The bottom
dial is only marked in 1/64in. increments, and they're on the inner
ring of the dial. Which one is 5/16 in. here? You can go crosseyed, or
make a major mistake, trying to figure out both these dials. The key to
quickly reading any dial is to treat it like the face of a clock. One
revolution is 1 in.; a quarter of the way is 1/4 in. Each major
division is 1/16 in.
Best: 1/16in. Fractions


Less good: 1/8in. Fractions

Less good: 1/64in. Fractions