# Calculate non-linear fee in percent between to prices

I have this problem. Googling 2 hours with no answers found, but propably searching wrong words - im really not not a math guy but need this for a plugin I write:

A price for a product is 150 OR below and the fee is 50% = 75

A Price for a product is 3000 OR Above and the fee is 10% ( and staying there ) = 300

The fee in % between 151 - 2999 should decrease in a smooth curve (50% to 10%)

How do I get the % for example 1400 ?

Thanks for any help / Jonas

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depends on what kind of smooth curve. Is it a linear curve? –  Slungpue Apr 27 '14 at 11:11
As above "should decrease in a smooth curve (50% to 10%)", thanks for looking into my problem btw! Edit; my goal is to plain it out how closer to 3000 you get, if possible. –  Jonas Lundman Apr 27 '14 at 12:11
yeah, but there are litterally infinitely many different smooth curves, each of which will give a different answer. I can, for example, fit a third-degree polynomial to a curve which goes from 0.5 to 0.1, or I can also fit a straight line. –  Slungpue Apr 27 '14 at 13:02
Hi! Ok, I to far away of the words here but lets say we start easy with a linear, I suppose the amount 1425 (half of 3000) would endup with 13% something? I would like the % to be little higher here, so I endup with minimum amount 200 of fee over 1300... Means half is a little lower. –  Jonas Lundman Apr 27 '14 at 13:20
This is the table (bad) I gonna replace:150 = 75 ( ~ 50% ) 200 – 250 = 100 ( 50 – 40% ) 300 – 550 = 130 ( 43 – 23% ) 600 – 850 = 230 ( 38 – 27% ) 900 – 1250 = 250 ( 27 – 20% ) 1300 – 3000 = 300 ( 23 – 10% ) 3000 or more 10% (from 300) –  Jonas Lundman Apr 27 '14 at 13:31

Ok, if the smooth curve between 150 and 3000 is a straight line, we could start by finding the actual line $f(x) = ax+b$.

We have the two conditions $f(150) = 0.5$ and $f(3000)=0.1$, so we get the system of equations $$150a+b = 0.5\\ 3000a + b = 0.1$$ which has solutions $a=-0.000140351$ and $b=0.521053$, so the curve is described by $$f(x) = -0.000140351x + 0.521053$$

Now to find the fee for any price between 150 and 3000 you just plug it in as $x$. For $1400$ we get $-0.000140351*1400+0.521053=0.3245616$.

Edit: It kinda sounds like the normal curve would fit your preferences pretty well. Just playing around with the numbers , using the function $$f(x) = 0.1+0.4e^{-(x-150)^2/1000000}$$

for values between $150$ and $3000$, you get this curve:

Changing the number where I chose $1000 000$, you could get a flatter or steeper graph by increasing/decreasing the number.

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Wow, so greatful for this help! Well, I can see that a straight line produces to high % fee at mid costs. Like 36% at 1400, but prefer more like round 20% and rise faster 1300 - 150. Or, devide into 2 formulas and split at 1300 ( 150-1300, 1300-3000). But I don know how to "human" read the math and get "a" on my php script or calculator to experiment the thresholds. The last line was ok to use. –  Jonas Lundman Apr 27 '14 at 14:05
If I wanna change 3000a + b = 0.1 to 1600a + b = 0.1, how do I calculate that ( to get = −0.000140351 resp the b summary) ? –  Jonas Lundman Apr 27 '14 at 14:25
well, that's a simple linear system. You can use wolfram, for example –  Slungpue Apr 27 '14 at 15:46
Tanks for your time and this great turtorial and solution. I can now adjust my choices to end up with suitable calculations for my plugin! –  Jonas Lundman Apr 27 '14 at 15:52
check the edit. I was kinda bored so I played around with an exponential function. –  Slungpue Apr 27 '14 at 16:38