<p>$B-V$ corresponds to the base 10 logarithm of a flux ratio.</p>

<p>$$B-V = -2.5 \log \left(\frac{f_B}{f_V}\right)$$</p>

<p>So trying to guess what you are trying to calculate, it is the percentage change in the blue to visible flux ratio?</p>

<p>In which case the percentage change is
$$ p = \frac{ 10^{-(B-V)_2/2.5} - 10^{-(B-V)_1/2.5}}{10^{-(B-V)_1/2.5}}\times 100$$</p>

<p>A percentage change can of course be negative.</p>

CC BY-SA 3.0
2016-03-07T21:47:04.960Z
2016-03-07T21:47:04.960Z
ProfRob
<p>I need to calculate a percentage difference of a B-V colour index between its estimated and actual value. So I tried doing this by <em>difference between values/actual value x 100</em>. However as B-V values can be on either side of zero this did not give representative answers.</p>

<p>So if anyone knows a way of calculating percentages of values close to zero or on either side of zero their help would be appreciated.</p>

<p>Thanks     </p>

<star><amateur-observing>
How to calculate B-V colour index value percentage difference
Edwin Shepherd
# How to calculate B-V colour index value percentage difference

(Asked by: username_0 on 2016-03-07)

I need to calculate a percentage difference of a B-V colour index between its estimated and actual value. So I tried doing this by *difference between values/actual value x 100*. However as B-V values can be on either side of zero this did not give representative answers.

So if anyone knows a way of calculating percentages of values close to zero or on either side of zero their help would be appreciated.

Thanks

**Tags:** star, amateur-observing

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## 1 answers

### Answer by username_1 on 2016-03-07. Score: 3 | Accepted answer

$B-V$ corresponds to the base 10 logarithm of a flux ratio.

$$B-V = -2.5 \log \left(\frac{f\_B}{f\_V}\right)$$

So trying to guess what you are trying to calculate, it is the percentage change in the blue to visible flux ratio?

In which case the percentage change is
$$ p = \frac{ 10^{-(B-V)\_2/2.5} - 10^{-(B-V)\_1/2.5}}{10^{-(B-V)\_1/2.5}}\times 100$$

A percentage change can of course be negative.


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