How to Use the Slide Rule
- Basic Multiplication: Set the left index (1) of scale C over the first number on scale D. Read the result on scale D under the second number on scale C.
- Division: Set the divisor on scale C over the dividend on scale D. Read the result on scale D under the left index of scale C.
- Using the Cursor: Drag the red hairline cursor to align values across scales for precise reading.
- Moving the Slide: Click and drag the middle section (scale C) to slide it left or right.
Tip: Slide rules work with significant figures, not decimal places. You must track the order of magnitude separately!
Try These Examples
Example 1: Multiply 2.5 × 3.2
1. Slide C scale so its left "1" aligns with "2.5" on D scale
2. Find "3.2" on C scale
3. Read the answer on D scale below it: 8.0
Example 2: Divide 8.4 ÷ 2.1
1. Find "8.4" on D scale
2. Slide "2.1" on C scale to align above it
3. Read answer on D scale under C's left "1": 4.0
A Brief History
The slide rule was the primary calculating tool for engineers and scientists for over 350 years, from its invention in the 1620s until the introduction of pocket calculators in the 1970s.
Notable uses:
- NASA engineers used slide rules to calculate trajectories for the Apollo missions
- The Empire State Building was designed using slide rules
- Albert Einstein owned and used a Nestler slide rule
- Until the 1980s, every engineering student carried one
The most popular models in the US were made by Keuffel & Esser (K&E) and Pickett. This simulation is styled after the classic K&E 4081.