You could do it the other way. Say you want to know what $20 today would have bought in 1940. Take 20 and divide by 15.252 and you get 1.311, so $20.00 in 2008 dollars equals $1.31 in 1940 dollars.
Somebody ran a TV ad in 2005 claiming that cable rates had doubled in the last nine years. Is that a lot? Let's see. Nine years earlier is 1996. Looking at the lower table, the CPI for 2005 is 195.3 and the CPI for 1996 is 156.9. Divide 195.3 by 156.9 and you get 1.245. If your cable bill was $20 in 1996, your 2005 bill, adjusted only for inflation, should have been: $20 x 1.245, or $24.90. So yes, they were significantly out-pacing the rate of inflation. If your bill did increase 100% over those nine years, it wasn't solely becasue of inflation.
If this is too confusing, you can use the Bureau of Labor Statistics' Inflation Calculator.
So, have fun, amaze your friends, annoy your neighbors, but remember this is only an exhibition, this is not a competition, please no wagering.
Year Factor Year Factor Year Factor Year Factor 1913 21.568 1938 15.144 1963 6.978 1988 1.805 1914 21.353 1939 15.362 1964 6.888 1989 1.722 1915 21.141 1940 15.252 1965 6.779 1990 1.634 1916 19.590 1941 14.526 1966 6.590 1991 1.568 1917 16.682 1942 13.100 1967 6.393 1992 1.522 1918 14.141 1943 12.343 1968 6.136 1993 1.478 1919 12.343 1944 12.132 1969 5.818 1994 1.441 1920 10.676 1945 11.863 1970 5.503 1995 1.401 1921 11.929 1946 10.950 1971 5.272 1996 1.361 1922 12.710 1947 9.575 1972 5.108 1997 1.330 1923 12.487 1948 8.860 1973 4.809 1998 1.310 1924 12.487 1949 8.972 1974 4.331 1999 1.282 1925 12.202 1950 8.860 1975 3.969 2000 1.240 1926 12.064 1951 8.213 1976 3.753 2001 1.206 1927 12.272 1952 8.058 1977 3.524 2002 1.187 1928 12.487 1953 7.997 1978 3.275 2003 1.160 1929 12.487 1954 7.938 1979 2.941 2004 1.130 1930 12.786 1955 7.967 1980 2.591 2005 1.093 1931 14.048 1956 7.850 1981 2.349 2006 1.059 1932 15.586 1957 7.599 1982 2.213 2007 1.030 1933 16.425 1958 7.389 1983 2.144 1934 15.935 1959 7.338 1984 2.055 1935 15.586 1960 7.214 1985 1.984 1936 15.362 1961 7.141 1986 1.948 1937 14.828 1962 7.070 1987 1.880Data current as of March, 2008
Year CPI Year CPI Year CPI Year CPI 1913 9.9 1938 14.1 1963 30.6 1988 118.3 1914 10.0 1939 13.9 1964 31.0 1989 124.0 1915 10.1 1940 14.0 1965 31.5 1990 130.7 1916 10.9 1941 14.7 1966 32.4 1991 136.2 1917 12.8 1942 16.3 1967 33.4 1992 140.3 1918 15.1 1943 17.3 1968 34.8 1993 144.5 1919 17.3 1944 17.6 1969 36.7 1994 148.2 1920 20.0 1945 18.0 1970 38.8 1995 152.4 1921 17.9 1946 19.5 1971 40.5 1996 156.9 1922 16.8 1947 22.3 1972 41.8 1997 160.5 1923 17.1 1948 24.1 1973 44.4 1998 163.0 1924 17.1 1949 23.8 1974 49.3 1999 166.6 1925 17.5 1950 24.1 1975 53.8 2000 172.2 1926 17.7 1951 26.0 1976 56.9 2001 177.1 1927 17.4 1952 26.5 1977 60.6 2002 179.9 1928 17.1 1953 26.7 1978 65.2 2003 184.0 1929 17.1 1954 26.9 1979 72.6 2004 188.9 1930 16.7 1955 26.8 1980 82.4 2005 195.3 1931 15.2 1956 27.2 1981 90.9 2006 201.6 1932 13.7 1957 28.1 1982 96.5 2007 207.3 1933 13.0 1958 28.9 1983 99.6 1934 13.4 1959 29.1 1984 103.9 1935 13.7 1960 29.6 1985 107.6 1936 13.9 1961 29.9 1986 109.6 1937 14.4 1962 30.2 1987 113.6Then I take the current CPI (for March, 2008 it's 213.528) and divide that number by all of the CPIs to get the conversion factors in the chart at the top of the page. That's all there is to it. I acknowledge that there may be some flaws in this methodology, but we're just trying to have fun, not get to the moon. More information can be had at The CPI Home Page.