How to use this calculator
- Pick the type. Choose male dovetail (measure over the pins) or female dovetail (measure between the pins).
- Enter the dovetail angle. Enter the included angle α measured from the base; 60° is most common.
- Enter the pin diameter. Enter the diameter D of the two equal pins or rods seated in the acute corners.
- Enter the flat width. Enter W — the wide base flat for a male dovetail, or the narrow throat opening for a female dovetail.
- Read the measurement. Read the target over-pin or between-pin measurement M and the pin constant k. Toggle metric/imperial in the header.
How it works
A dovetail slide is checked by laying a cylindrical pin or rod of known diameter
D into each of the two acute corners and measuring across them with a
micrometer. Each pin touches the sloped face, so its centre is offset from the corner
by a fixed amount that depends only on the dovetail angle. That offset is captured by the
pin constant:
k = 1 + cot(α/2)
where α is the included angle measured up from the base flat.
The over- or between-pin measurement then differs from the relevant flat width W
by D·k:
- Male dovetail (measure over the pins):
M = W + D·k, withWthe wide base flat. - Female dovetail / socket (measure between the pins):
M = W − D·k, withWthe narrow throat opening.
The two cases, in plain section view:
MALE (over pins) FEMALE / socket (between pins)
(O) (O) W = throat (narrow, top)
/ \ ┌── ↔ ──┐
/ measure \ \(O) (O)/ ← measure between
/ over → M \ \ /
/____________________\ \____/
W = base flat (wide) W = throat opening
M = W + D·k M = W − D·k
For the common 60° dovetail, cot(30°) = 1.7320508, so
k = 1 + 1.7320508 = 2.7320508 — the well-known
"add 2.732 × pin diameter" rule. Other angles use the general
k = 1 + cot(α/2).
Worked example
Verified against the live calculator
A male 60° dovetail with a 40 mm base flat, checked with
Ø10 mm pins. The pin constant is
k = 1 + cot(30°) = 2.7320508, so the over-pin measurement is
M = 40 + 10 × 2.7320508 = 67.3205 mm. The matching
female socket with a 40 mm throat would measure between the pins
M = 40 − 27.32051 = 12.6795 mm. Those are the numbers the calculator shows.
Frequently asked questions
How do you measure a dovetail with pins or rods?
Lay one cylindrical pin or rod of equal diameter into each acute corner of the dovetail, then measure across them with a micrometer. For a male dovetail you measure over the pins; for a female dovetail (socket) you measure between them. The measurement relates to the flat width by M = W ± D·(1 + cot(α/2)), where α is the included angle and D the pin diameter.
What is the 2.732 dovetail rule?
For the common 60° dovetail the pin constant is k = 1 + cot(30°) = 1 + 1.7320508 = 2.7320508. So the over-pin or between-pin distance is just the flat width plus or minus 2.732 times the pin diameter. That is why machinists remember it as the "add 2.732 × pin diameter" rule — it only holds at 60°; other angles use k = 1 + cot(α/2).
What is the difference between checking a male and a female dovetail?
A male dovetail is the projecting slide: the pins sit in the two acute corners against the wide base flat, and you measure over the pins, so M = W + D·k. A female dovetail (the socket or guideway) has the pins against the narrow throat opening at the top, and you measure between them, so M = W − D·k. The pin constant k is the same for both.
Which width do I enter for W?
W is the relevant flat width. For a male dovetail it is the wide flat at the base of the slide (the widest face). For a female dovetail it is the throat — the narrow opening at the top of the socket. Get this wrong and the over/between sign of the correction will be off.
What pin or rod diameter should I use?
Choose a pin diameter large enough that each pin seats fully on the sloped (angled) face and projects clear of the surface you measure to — but not so large that it rides up out of the corner, nor so small that it bottoms in the corner without touching the slope. The pin must contact the sloped face, not bottom on the corner, or the geometry behind the formula no longer holds.
Does this work in metric and imperial?
Yes — enter the pin diameter and flat width in mm or inches and the dovetail angle in degrees. Toggle SI/Imperial in the header; the angle stays in degrees in both systems.
Method & assumptions
- Follows the pin/rod inspection method in Machinery's Handbook, "Checking a Dovetail Slide": two equal cylindrical pins seated in the acute corners, measured over (male) or between (female) the pins.
- Both pins are exactly equal in diameter and lie square to the measuring faces; the faces are flat and the angle is uniform along the slide.
- The pin diameter must be chosen so each pin contacts the sloped face and projects beyond the surface you measure to — it must not bottom in the corner nor ride out of it, or the geometry behind
kno longer holds. αis the included angle from the base; 60° is most common. Outside 0°–180° the geometry is undefined and the result is reported as not-a-number.