HP 20S: The July 2026 Program Collection

HP 20S: The July 2026 Program Collection

Triangulation

d = l * sin(α) * sin(ß) ÷ sin(α + ß)

Store in the following registers before calculation:

R1 = measure of angle A

R2 = measure of angle B

R3 = length l from point A to B

Solve:

R4: distance

Code:

01: LBL A; 61, 41 ,A

02: DEG; 61, 23

03: RCL 1; 22, 1

04: SIN; 23

05: ×; 55

06: RCL 2; 22, 2

07: SIN; 23

08: ÷; 45

09: (; 33

10: RCL 1; 22, 1

11: +; 75

12: RCL 2; 22, 2

13: ); 34

14: SIN; 23

15: ×; 55

16: RCL 3; 22, 3

17: =; 74

18: STO 4; 21, 4

19: RTN; 61, 26

Examples

Example 1:

Inputs: R1 = 60°, R2 = 50°, R3 = 10

Output: R4: 7.05990377592

Example 2:

Input: R1 = 30°, R2 = 80°, R3 = 27.5

Output: R4: 14.4101446626

Source:

“Triangulation (surveying)” Wikipedia. https://en.wikipedia.org/wiki/Triangulation_(surveying) (last edited October 24, 2025). Retried March 17, 2026

Payment: Continuous Compounding

PMT = PV * (e^r – 1) ÷ (1 – e^(-r * t))

Store in the following registers before calculation:

R1 = t: number of payments

R2 = r: periodic interest rate (as a decimal)

R3 = PV: present value

Solve:

R4: PMT: present

Code:

01: GTO B; 61, 41, b

02: RCL 3; 22, 3

03: ×; 55

04: (; 33

05: RCL 2; 22, 2

06: e^x; 12

07: -; 65

08: 1; 1

09: ); 34

10: ÷; 45

11: (; 33

12: 1; 1

13: -; 65

14: (; 33

15: RCL 1; 22, 1

16: ×; 55

17: RCL 2; 22, 2

18: ); 34

19: +/-; 32

20: e^x; 12

21: ); 34

22: =; 74

23: STO 4; 21, 4

24: RTN; 61, 26

Examples

Example 1:

Input: t: 36, r: 0.10 ÷ 12, PV: 5,000.00

Output: PMT: 161.434037378

Example 2:

Input: t: 60, r: 0.05 ÷ 12; PV: 26,349.56

Output: PMT: 497.374636585

Electrical Engineering: System Temperature to Noise Figure

When an amplifier is activated, two ways to express noise are the noise temperature (in Kelvin) and the noise figure (in decibels, dB). Using a reference temperature of 290 K, when the noise temperature (T) is known, the noise figure (F) can be calculated by:

F = 10 * log((T + 290) ÷ 290)

If temperature is given in degrees Celsius (°C), then the formula for noise figure becomes:

F = 10 * log((T°C + 563.15) ÷ 290)

since T = T°C + 273.15.

The following program assumes that temperature is given in degrees Celsius.

Store in the following registers before calculation:

R1 = T°C; temperature in degrees Celsius

Solve:

R2: F: noise figure

Code:

01: LBL C; 61, 41, C

02: RCL 1; 21, 1

03: +; 75

04: 5; 5

05: 6; 6

06: 3; 3

07 . ; 73

08: 1; 1

09: 5; 5

10: =; 74

11: ÷; 45

12: 2; 2

13: 9; 9

14: 0; 0

15: =; 74

16: LOG; 51, 13

17: ×; 55

18: 1; 1

19: 0; 0

20: =; 74

21: STO 2; 21, 2

22: RTN; 61, 26

Examples

Example 1:

Input: T = -160 °C (store in R1)

Output: F: 1.43068666235 dB

Example 2:

Input: T = 15°C

Output: F: 2.99642532081 dB

Source:

Ball, John A. Algorithms for RPN Calculators John Wiley & Sons: New York. 1978. ISBN 0-47-03070-8. pp. 266-267

Moderate Exercise: Target Heart Rate Range

The following equations calculate the target heart range for a typical person engaging in moderate exercise. According to the particle by Jenna Fletcher (see Source), the American Heart Society (AHA) states the person is engaged in moderate exercise when their heart rate is 50% to 70% of their maximum heart rate. The maximum heart rate is 220 minus the person’s age.

Disclaimer: This is not medical advice, any questions should be discussed with a doctor or health professional.

The moderate range is determined by:

high = (220 – age) * 0.7 = low * 1.4

low = (220 – age) * 0.5 = high * 5/7

For high intensity, use the range 70% to 85%.

Code:

01: LBL D; 61, 41, d

02: +/-; 32

03: +; 75

04: 2; 2

05: 2; 2

06: 0; 0

07: =; 74

08: ÷; 2

09: 2; 2

10: =; 74

11: R/S; 26

12: ×; 55

13: 1; 1

14: . ; 73

15: 4 ; 4

16: =; 74

17: RTN; 61, 26

Examples

Example 1:

Input: Age 49 (my age at the time of this blog)

Output: low: 85.5, high: 119.7

Example 2:

Input: Age 25

Output: low: 97.5, high: 136.5

Source:

Fletcher, Jenna. “What a target heart rate is and how to calculate it”. Medically Reviewed by Debra Sullivan Ph. D. Medical News Today. January 22, 2024. https://www.medicalnewstoday.com/articles/target-heart-rate-calculator March 16, 2026.

Percentile in a Range

The program calculates the percentile of x in the range [a, b].

percentile = (x – a) ÷ (b – a) * 100%

Store before calculating:

R1 = a, R2 = b, R3 = x

Code:

01: LBL E; 61, 42, E

02: ( ; 33

03: RCL 3; 22, 3

04: - ; 65

05 RCL 1; 22, 1

06: ) ; 34

07: ÷; 45

08: ( ; 33

09: RCL 2; 22, 2

10: - ; 65

11: RCL 1; 22, 1

12: ) ; 34

13: ×; 55

14: 2; 2

15: 10^x; 51, 12

16: =; 74

17: RTN; 61, 26

Examples

Example 1:

Input: R1 = 10, R2 = 50, R3 = 30 (30 in [10, 50])

Output: 50 (50%)

Example 2:

Input: R1 = 85.5, R2 = 117.5, R3 = 110 (110 in [85.5, 117.5])

Output: 76.5625 (76.5625%)

Programming all five of these programs will fill the program space of the HP 20S entirely (99 steps)!

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.

Spotlight: AccuMath 400B Slide Rule

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.

Swiss Micros DM32: Estimating Earth’s Acceleration at Latitude

Swiss Micros DM32: Estimating Earth’s Acceleration at Latitude

Introduction

Earth’s gravitational force is usually set a constant of 9.80665 m/s², usually shortened to 9.8 m/s² or 9.81 m/s² in publications such as physics text books. However, in reality gravity on Earth is not constant. There are many ways to calculate (estimate) the gravitational acceleration depending where you are on Earth. Gravity depends on many factors including latitude (degrees North or South) and the elevation. The blog focuses on the effect of latitude on Earth’s gravity.

The is part of the Acceleration Due to Gravity table from the Desk Ref book (see the Source section). The column for m/s² is added.

Degrees Latitude (North or South)	Gravity Acceleration (cm/s²)	Gravity Acceleration (m/s²)
0 (Equator)	978.0327	9.780327
15	978.3786	9.783786
30	979.3249	9.793249
45	980.6199	9.806199
60	981.9178	9.819178
75	982.8698	9.828698
90	983.2186	9.832186

[Glover, Young, pg. 587]

There are many ways to estimate the gravitational acceleration depending where you are on Earth. Gravity depends on many factors including latitude (degrees North or South) and the elevation.

Earth’s gravity tends to be at the strongest at the poles. However, gravity weakens at higher elevations, where we are further away from the center of the planet.

Gravity Estimate – (Univ. of Illinois)

The formula that is presented by The Grainger College of Engineering Physics Van [Univ. of Illinois] is a simple but pretty accurate estimation of gravity:

g = g_45 – 1 / 2 * (g_poles – g_equator) * cos(2 * latitude * π ÷ 180)

where:

g_poles = 9.832 m/s²

g_45 = 9.806 m/s²

g_equator = 9.78 m/s²

lat = latitude, north or south

2 * latitude is converted to radians. (as it is multiplied by π ÷ 180)

Simplifying the equation leads to:

1 / 2 * (g_poles – g_equator) = 1 / 2 * (9.832 – 9.78) = 0.026

2 * latitude * π ÷ 180 = latitude * π ÷ 90 (in radians)

Then:

g = 9.806 – 0.026 * cos(latitude * π ÷ 90)

(in radians)

DM32 Program: Gravity Estimate

Input L as D.MS (degrees/minutes/seconds) format.

E01 LBL E
E02 RAD
E03 INPUT L
E04 →HR
E05 90
E06 ÷
E07 π
E08 ×
E09 COS
E10 0.026
E11 ×
E12 +/-
E13 9.806
E14 +
E15 STO G
E16 RTN

World Geodetic System 84 Ellipsoidal Gravity Formula

The formula is presented by the World Geodetic System (WGS): [Wikipedia]

g = Ge * ((1 + k * sin² L) ÷ √(1 – e² * sin² L))

L: latitude in decimal degrees

with the constants:

Ge = 9.7803253359 m/s²

k = 0.001931852652

e² = 0.0066943799901

Input L as D.MS (degrees/minutes/seconds) format.

DM32: WEG ‘84

G01 LBL G
G02 DEG
G03 INPUT L
G04 →HR
G05 SIN
G06 x²
G07 STO T
G08 0.001931852652
G09 ×
G10 1
G11 +
G12 1
G13 RCL T
G14 0.0066943799901
G15 ×
G16 -
G17 SQRT
G18 ÷
G19 9.7803253359
G20 ×
G21 STO G
G22 RTN

Table of Values

Sources

“Gravity of Earth” Wikipedia. (2026, January 31).

https://en.wikipedia.org/wiki/Gravity_of_Earth Retrieved March 9, 2026.

Grainger Engineering Office of Marketing and Communications. (answer written by Rebecca H.) (2016, November 21). “How gravitational force varies at different locations on Earth.” Illinois. https://van.physics.illinois.edu/ask/listing/64061. Retrieved March 10, 2026.

Glover, Thomas J. and Richard A. Young. Desk Ref. Sequoia Publishing, Inc. Anchorage, AK 4^th Edition. 2022 pg. 587

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.

HP 32SII: Volume of a Square Frustum (Three Approaches in RPN)

HP 32SII: Volume of a Square Frustum (Three Approaches in RPN)

The volume of a square frustum is:

v = height ÷ 3 × (s1^2 + s2^2 + √(s1^2 × s2^2))

which is based off the formula for the volume of general frustum:

v = height ÷ 3 × (base_area_1 + base_area_2 + √(base_area_1 × base_area_2))

Here are three approaches, which were all done with a HP 32SII calculator. The beauty of RPN is on display here. The three programs are labeled A, C, and D.

s1, s2: side of the square base h: height

Stack: Z: s1 Y: s2 X: h

Program A: (24 steps, 36 bytes) 

V = h/3 * (s1^2 + s2^2 + √(s1^2 * s2^2))

LBL A

STO H

R↓

x²

x<>y

x²

ENTER

ENTER

CLx

-

R↓

x<>y

R↓

×

√

R↓

+

R↑

+

RCL H

×

3

÷

RTN

Program C: (17 steps, 25.5 bytes)

Note: √(s1^2 * s2^2) = s1 * s2

V = h/3 * (s1^2 + s2^2 + (s1 * s2))

LBL C

ENTER

3

÷

R↓

×

LST x

x<>y

R↓

x²

x<>y

x²

+

R↑

+

×

RTN

Program D: (15 steps, 22.5 bytes) 

Note: s1^2 + s2^2 + s1 * s2 = s1^2 + s2^2 + 2 * s1 * s2 – s1 *s2 = (s1 + s2)^2 - s1*s2 

V = h/3 * ((s1 + s2)^2 - s1 * s2)

LBL D

3

÷

R↓

×

LST x

x<>y

R↓

+

x²

R↑

-

×

RTN

Examples

Example 1:

s1: 24

s2: 30

h: 5.8

Result: 4245.6

Example 2:

s1: 2.54

s2: 12.15

h: 10

Result: ≈ 616.4503

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.