Spotlight: AccuMath 400B Slide Rule
Introduction
I purchased an AccuMath 400B slide rule at Antique Station, an antique store in Oro Grande just north of Victorville, CA.
I am impressed of the larger markings on the slide rule. The slide rule very easy to read, yet has a lot of scales for calculations including powers, roots, trigonometry, and logarithms.
The Scales
The slide rules have the following scales:
Top Frame (stationary):
S: Sine of angles in degrees. A = sin(S°) ÷ 100, S° = arcsin(A × 100)
K: Cube and cube root scale associated with scale D. K = D³, D = ³√K
A: Square and square root scale associated with scale D. A = D², D = √A
Slide:
B: Square and square root scale. Scale is from 1 to 100 and it is the same scale as Scale A.
CI: Reciprocal of scale C.
C: Multiplication and division scale from 1 to 10. Same as scale D.
Bottom Frame (stationary):
D: Multiplication and division scale from 1 to 10.
L: Logarithmic and exponential scale associated with D. L = log D, D = 10^L. (base 10 logs)
T: Tangent of angles in degrees. D = tan(T°) ÷ 10, T° = arctan(D × 10)
Note: A lot of slide rules associate the S scale with the C/D scales, but for this particular design, the A scale is used instead.
The Back Side
U.S./Metric Conversions, Equivalents, and Settings (i.e. 1 in mercury = 1.133 ft water)
Fractions up to 64ths and their decimal equivalents (i.e. 47/64 = 0.734375)
Trigonometric Identities, Right Triangles, Law of Sines and Cosines
Example Calculations
Keep your exponents in mind! N = mantissa * 10^exponent
Hairline: the plastic cursor you move around, Slide: the center piece of the slide rule you move around
4 × 2 = 8
Slide C right to match C: 1, D: 4
Move cursor right to C = 2
Read down to D: 8
3 × 9 = 27
Rearrange to 9 × 3. Slide C left to match C:1, D: 9
Move cursor left to C = 3
Read down to D: 2.7
Multiply by 10 since we slide C left: 2.7 × 10 = 27
12 × 33 = 396 (Approximate method)
Rearrange to 33 × 12. Slide B left to match: B: 1, A: 33
Move cursor left to B = 12
Read on A: the cursor is very close to 4.
Multiply by 100 since we moved B left. Result: approx 400.
54 ÷ 9 = 6
Move Slide B to match: A = 54, B = 9
Move cursor left to B = 1
Read A = 6
√81 = 9, 9^3 = 729 (approximate)
On A, slide cursor to 81.
Read on D: 9 (square root)
Read on K: in between 720 and 730. (so ≈725?)
Note that the scales are limited.
√144 = 12 (√(144 ÷ 100 × 100) = √1.44 × √100 = √1.44 × 10)
On A, slide cursor to 1.44.
Read on D: 1.2
Multiply by 10. Result: 12.
Logarithms: D and L scales. L = log D, D = 10^L
log 3 ≈ 0.47712
On D, slide cursor to 3.
Read on L: about 0.47
10^0.55 ≈ 3.54814
On L, slide cursor to 0.55
Read on D: about 3.54
Tangent of Angles in degrees: D and T scales. D = tan(T°) ÷ 10, T° = arctan(10 × D)
Tan(25°) ≈ 0.46631 (approximate)
On T, slide cursor to 25.
Read on D: about 4.65. Divide by 10 for a result of approximately 0.465
arctan(0.7) ≈ 34.99202 (approximate)
On D, slide to 7 (0.7 × 10)
Read on T: close to 35. Result: approximately 35°
Sine of Angles in degrees: A and S scales (for this model!) A = sin(S°)÷100, S° = arcsin(A×100)
sin(30°) = 0.5
On S, slide cursor to 30.
Read on A: 50. Divide by 100 for a final answer of 0.5
arcsin(0.2) ≈ 11.53700°
On A, slide to 20. (0.2 × 100)
Read on S: slightly after 11.5, for an approximation of 11.5°.
Reciprocal: CI and D scales. When the slide is at its home position, that is C = 1 and D = 1 are aligned:
1/CI = D
Find 1 / 4 = 0.25
Slide D to 4.
Read CI at 2.5 and divide by 10. Result: 0.25
Source
Instruction Manual for the AccuMath 400B.
Page 1: https://www.sliderule.ca/stermf.jpg
Page 2: https://www.sliderule.ca/stermb.jpg
Eric’s Slide Rule Page, last updated December 1, 2002. Accessed June 27, 2026.
Best,
Eddie
All original content copyright, © 2011-2026. Edward Shore. Unauthorized use and/or unauthorized distribution for commercial purposes without express and written permission from the author is strictly prohibited. This blog entry may be distributed for noncommercial purposes, provided that full credit is given to the author.
