Par

How long does it take to double your money?

Store 1 going in and 2 coming out, and solve for n.

Problem

At 7% a year, how many years does it take to double an investment?

Keystrokes

  1. fFINx≷y
    0.00

    Start clean.

  2. 7i
    7.00

    7% a year.

  3. 1CHSPV
    −1.00

    Any amount in, here 1.

  4. 2FV
    2.00

    Twice that amount out.

  5. n
    11.00

    The number of years, rounded up to a whole year.

  6. 2gLN%T1.07gLN%T÷
    10.24

    The exact answer, ln 2 ÷ ln 1.07.

Answer

It takes 10.24 years, so the money has doubled by the end of year 11.

Why n rounds up

A financial calculator treats n as a count of whole periods, so it rounds up to the first period in which the goal is met. For the exact figure, divide logarithms: gLN%T is the natural logarithm.

The rule of 72

72 divided by the rate in percent is a quick estimate: 72 ÷ 7 is about 10.3 years, close to the exact 10.24. It works best between about 4% and 12%.

Questions

What about tripling?

Store 3 in FV instead of 2.

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