Invented by Leon A. Greenberg, the Alco-Calculator is a cardboard slide-rule, which estimates one’s blood alcohol concentration given the quantity of alcohol consumed, body weight, and hours since beginning alcohol consumption. 

For a “device” that cost less than $2 when it was introduced in 1972, it gave very accurate estimates that were comparable to breath analysis devices. 
Leon A. Greenberg is well-known for his two inventions, the Alcometer and the Alco-Calculator.

Check out this instruction manual for information on how the rates of alcohol absorption and elimination were calculated.

As described in the Center of Alcohol Studies Publication catalog:

The alco-calculator is a handy slide rule which estimates blood alcohol concentration. The slide rule contains complete instructions to (1) estimate the amount (per cent) of alcohol in the blood after drinking known amounts of beverages; (2) estimate the minimum number of drinks consumed from the concentration of alcohol in the blood; (3) calculate and compare the alcohol values by the number of drinks of different types of beverages; (4) estimate the concentration of alcohol over time; and ( 5 ) estimate the time necessary for a particular concentration to fall to a desired level. One side of the slide calculates the blood alcohol values for men, the other side, for women. Includes revisions by David Lester.