E6-B keychain computer

This is a en E6-B sliding rule in the shape of a keychain. The diameter is about 1.7 inches and it's flat so it goes well in a keychain or inside a wallet.

It is made in plastic but might suffer wear or break if carried in a keychain with keys and other abrasive elements so maybe better for a slim keychain, the car keys or a wallet.

The sliding rule is used by pilots to perform manual calculations about fuel usage, time to destination and others it's basically a decimal logarithmic scale and can be used by non pilots to do multiplications, divisions, square roots, percentages, unit conversions and proportions.

To perform a multiplication you just align the first number from the inner scale with "10" in the outer scale, then look in the outer scale for the second number and the inner scale will show you the result.

The precision is not very high because of the size, you will not know about decimal values in divisions and your multiplications and proportions can be off by some small units. But it works very well for aproximations, estimates and to calculate the tip or split the bill at the restaurant. It's also great to start conversations and has a vintage element reminding of the times when calculators didn't exist and everybody had to use a sliding rule for calculations.

You can get it for $5 from PilotEssentials.