The E6B is a circular slide rule originally developed for aviation in the 1930s by Philip Dalton. It combines a logarithmic slide rule with specialized scales for flight calculations. Unlike a straight slide rule, its circular design allows infinite rotation without running off the scale. Pilots use it for speed-time-distance calculations, fuel consumption, wind correction, and unit conversions. This online version faithfully recreates the core logarithmic scales in an interactive digital format.

Step 1: Rotate the inner ring so that the inner "1" aligns with the first number (A) on the outer scale.
Step 2: Find the second number (B) on the inner scale.
Step 3: Read the value on the outer scale directly opposite B on the inner scale — that's A × B.
Example: To compute 2 × 3, align inner 1 with outer 2, then find inner 3 and read outer 6. This works because the logarithmic scales add lengths (log A + log B = log(A×B)).

On an E6B, the inner scale has a special "60" mark (representing 60 minutes per hour). To calculate distance: align the inner "60" with your speed on the outer scale. Then find your time on the inner scale — the outer scale reading gives the distance.
Example: At 120 mph for 30 minutes: align inner 60 with outer 12 (representing 120), find inner 30 (minutes), outer reads 6 → 60 miles. The ratio 60:speed creates the correct time-distance relationship.

Logarithmic scales convert multiplication into addition and division into subtraction. On a log scale, the physical distance from 1 to a value V is proportional to log(V). When you rotate the inner ring, you're adding or subtracting these log distances. This elegant mathematical property allows a simple mechanical device to perform complex calculations instantly — no batteries required, which is why pilots have relied on the E6B for nearly a century.

60 (Speed Index): Key reference for time-based calculations (60 minutes = 1 hour).
36 (Seconds index): Used for calculations involving seconds (3600 seconds/hour).
KM (1.609): Kilometer-to-mile conversion factor.
L (3.785): Liter-to-US gallon conversion.
NAUT (1.152): Nautical mile to statute mile conversion.
These preset marks save pilots from memorizing conversion factors and reduce calculation errors in flight.

This digital version provides approximately 3 significant figures of precision, matching the practical accuracy of physical E6B computers. The logarithmic scales are computed mathematically for precise angular placement. For critical navigation, the E6B is typically used for cross-checking electronic calculations. The digital format eliminates parallax errors that can occur when reading a physical slide rule, offering potentially clearer readings at the cursor position.