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I have read through questions like Calculate logarithms by hand and and a section of the Feynman Lecture series which talks about calculation of logarithms. I have recognized neither of them useful for my purpose which is to quickly calculate logarithms of base $10$ upto $4$ digit accuracy
(I believe 4 is the goldilocks number in this case) .

I wish to find things like $\log_{10}(2) \approx 0.3010$ quickly without using a calculator or log table. Why? Because I want to be free from carrying them around and losing them all day. Plus, they're not always available when I need them (you can guess why). My main purpose is to approximate the answers of very large and very small results of time consuming calculations. Logarithms make that job much easier for me. For example,

$$\frac{87539319}{1729} \approx 10^{7.942 - 3.237} = 10^{4.705} = 5.069*10^4$$

According to Wolfram (Yup, I'm that lazy) the answer is, $50630.0\overline{283400809716599190}$. Yes, I've over estimated by around $60$ but thanks to a log table, I did that approximation as fast as it took Wolfram to load the precise answer in my browser. But, without a log table, dividing itself would have me executing an iterative convergence just to find the multiples.
(1729*2 = too low, 1729*8= too high ... this must be so intuitive for most of you)

So, a quick approximation method for logarithms would be really helpful to me.


Also, a good way to find antilogs will be nice as well. I just realized that I can't compute decimal powers. $$\Large 10^{0.3010} = 10^{0.3}*10^{0.001} = \sqrt[10]{1000} * \sqrt[1000]{10} = \text{Calculator Required}$$ I checked " How to calculate a decimal power of a number" but alas, the thing which came closest to what I needed required a calculator for an intermediate step. Defeats the purpose, I know.
If I can't find the antilog, the whole point of having a quick way to find the logarithm would be lost.

I hope you can help.

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They're a little before my time, but I think some of the larger/more detailed slide rules could do something like this. –  Antonio Vargas Mar 4 at 4:36
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Also, you might find something interesting in this thread. –  Antonio Vargas Mar 4 at 4:51

1 Answer 1

Jacques Laporte has a page explaining some algorithms that work digit by digit. For other functions (e.g. trigonometric and hyperbolic) there is the class of CORDIC algorithms. Such algorithms were used in the first HP calculators, as they need very modest hardware.

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