How to I calculate an amount from the price + tax?

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I'm trying to calculate american Tax. The store in Florida is causing me some headaches.

There is a state tax of 6% and a city tax of 1% but only on the first $5000

It's easy to work out the tax from the price:

C#

var total = taxRates.sum(t=>t.rate * Math.Min(t.MaxAmount??Price, Price))

This works with all tax rates and types. UK tax for eg. will only have 1 tax rate in the TaxRates list and the MaxAmount will be null.

He's the issue. I have the amount including tax and I have to come up with a one-size-fits-all method of calculating the Price ex. Tax.

Any ideas?

What I have tried:

I currently work out the threshold of the rate and iterate over the rates:

I.E. consider the following example for Florida:

Max Amount       Rate
5000             1%
null             6%
null             1%   (a fake locality tax for this demo)

the first $5000 will be taxed as 8% total. I work out the amount each 'tier' is taxed and and create the tax bands as so:

Max Amount       Rate
5000             8%
null             7%   (rates with the same maxamount are aggregates)

I can now tell the band threshold:

Rate     Threshold          Before Tax
8%       5400               5000
7%       decimal.MaxValue   null

so I can now iterate through this list to find the actual tax amount.

Say we have $2700. This is not higher than the first tax band so it it all taxed at 8%. 2700 / 1.08 = $2500

10000: This is over 5400 so 5400 is taxed at 8%:
4600: this is within the next band so all is taxed at 7%
= 5000 + (4600 / 1.07)

It works but it very clunky.

Posted 1-Nov-18 23:08pm

Andy Lanng

Updated 2-Nov-18 1:22am

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Comments

Richard MacCutchan 2-Nov-18 5:38am

How does 2700 plus 8% tax equal 2500? Similarly your last calculation does not look correct.

Andy Lanng 2-Nov-18 6:44am

first:
2500 + 8% = 2700
so
2700 / 1.08 = 2500, which is what I wrote
second :
so 5400 of the total was the 5000 charged at 8%. That means that 5400 of the 10000 has been accounted for.
this leaves 4600 to be accounted for in the next tax bracket

Richard MacCutchan 2-Nov-18 9:55am

I thought you were starting with the net amounts to calculate the tax.

Andy Lanng 2-Nov-18 10:56am

starting with the price + tax. I need to get the price. If I know the tax (i.e. 8%) then to get the price only I need to divide it by 1.08.
The issue is that when there is a limit to how much the 1% tax applies to, the total tax % will be somewhere between 7 & 8 %

Richard MacCutchan 2-Nov-18 11:54am

In my experience of shopping in the US they always show the net price, and add the tax at the till. Is Florida different?

Andy Lanng 5-Nov-18 3:47am

Florida is no different. My client is.

Richard MacCutchan 5-Nov-18 4:02am

I always used to think that without clients our job would be so much easier.

Andy Lanng 6-Nov-18 8:48am

^_^

MadMyche 2-Nov-18 8:48am

Is this for a physical store or will there be shipping involved?

Andy Lanng 2-Nov-18 10:52am

Thankfully no shipping ^_^

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#realJSOP · Answer 1 · 2018-11-02T01:23:00

C#

double amt = 5400d;
double stateTax = 6d;
double cityTax = 1d;
double cityTaxableAmnt = 5000d;
double stateTaxableAmt = double.MaxValue;

double stateTaxAmt = Math.Min(amt, stateTaxableAmt) * (stateTax * 0.01);
double cityTaxAmt  = Math.Min(amt, cityTaxableAmt) * (cityTax * 0.01);
total = amt + stateTaxAmt + cityTaxAmt;

I'll leave it as an exercise for the programmer to make this code work regardless of the state or country. Also, I would use decimal types in production code because floating point math is just an approximation, where decimal math is much more precise..