Sometimes close enough is close enough. But when precision matters, turn to calipers.
Choose your readout. From bottom to top: This 4" tool reads in increments of .0005" and .01mm. The 6" models shown include an analog calipers that provides increments of 1u204464" and .01"; digital in .001" and .1mm; analog that reads in .001".

Because calipers physically contact the surfaces being gauged, they deliver hair-splitting measurements you can't get from sighting across the markings on a ruler or tape measure. Here's how to choose and use calipers.

Understanding the tool

Typical calipers open a little more than 6", although 3" and 4" models provide enough capacity for most woodworking tasks and stow easier in a shop apron. The display, whether an analog dial or digital LCD readout, shows measurements in fractional (164 "), decimal (increments to .001"), and/or metric (.1 or .01mm). Fractional readings are most useful for woodworking, but decimal and metric scales can simplify math. Markings on the bar, below, provide quick reference, including whole-inch increments.

Know how many inches you have. For measurements over 1", read full-inch increments on the bar, and add the precise fractional measurement shown on the dial. This measurement reads 1 1⁄8".

On digital calipers, the LCD screen provides a fast and precise read. And you can convert measurements from fractions to metric (and to decimal imperial [.001"] on some models) with the push of a button. On the other hand, analog calipers, which we prefer, allow you to eyeball gradations between marked increments, and never have a dead battery.

Taking a measurement

Whether dial or digital, all calipers work in the same fashion. First, close the jaws. If the display doesn't read zero, adjust the calibration. For a dial model, loosen the bezel clamp screw, if it has one, and rotate the bezel until the zero on the scale aligns with the pointer and retighten the screw. For digital versions, simply press the zero reset button.

Before taking measurements, brush the surfaces of the workpiece clean to prevent sawdust or other debris from affecting your reading. Then gently touch the jaws to the workpiece, photo's below. Forcing the jaws or thumbwheel can distort the caliper and throw off the reading. If you want to preserve the measurement after removing the tool from the workpiece (to compare to another workpiece, for example), tighten the jaw lock to prevent the jaws from moving.

Measure outside dimension. Close the caliper's jaws gently on each face of the workpiece, and read the measurement on the display.
Gauge inside dimension. Hold the calipers perpendicular to the surface, and apply light pressure to the thumbwheel to find the widest span of the hole or opening.
Find step distance. Place the flat end of the calipers on one face, and the step contact on the rear of the sliding head on the other surface. To avoid tilting the calipers, press the bar against the shoulder between the surfaces.
Determine depth. Rest the end of the calipers on the edge of the hole. Extend the depth rod and take your reading when it touches the bottom of the hole.

In addition to determining dimensions, calipers serve as a layout tool, photo below.

Lay out lines parallel to an edge or end. Open the jaws the desired dimension and tighten the jaw lock. Place the step contact against the edge of the board and mark along the end of the calipers.

While you can spend north of $100 on calipers, you can find a quality pair suitable for woodworking tasks for $20–$40.


What's the difference?
To quickly find the difference between the thickness of two pieces, or the depth of two holes, measure the thinner or shallower of the two first, below. Tighten the jaw lock and zero the scale, inset. Now, loosen the lock and measure the thicker or deeper item, below. The display shows the difference between the two, inset. Fast, accurate, and no math required!