CODEWITHSUNDEEP
ruby-on-railssumdecimalnumeric
Jerry
Jerry

Reputation: 198

Sum of values with added percentage keeps giving wrong result

Ik keep getting wrong amounts when I want to sum prices and then add VAT to it. I have the following situation:

Let's say I have 4 products and I want them show them on a invoice. Each products has the same price of 2.50.

I have the following code:

@products = Product.find([1,2,3,4])

On the invoice I have:

@products.each do |product|

  %p
    Price without VAT:
    = number_to_currency(product.price) --> Gives € 2,50

  %p
    Price with 21% VAT: 
    = number_to_currency(product.price / 100 * 121) --> Gives € 3,03

This shows like this:

Now I want to add a total line. I have tried something like this:

 - sum = @products.sum( &:price ) --> Gives 10
%p
  Total with 21% VAT: 
  = number_to_currency(sum / 100 * 121 ) --> Gives € 12,10 instead of € 12,12

What I try, I keep getting a total price including VAT on € 12.10 instead of € 12,12. (4 x € 3,03 = € 12,12)

I have the price in my database as: 

t.decimal "price"

The price is stored as 2.5

Who can help me out with this?

Upvotes: 0

Views: 136

Answers (1)

Andrey Deineko
Andrey Deineko

Reputation: 52357

One product's correct VAT amount:

(BigDecimal.new("2.5") / 100 * 121).to_f #=> 3.025

4 products correct VAT amount:

(BigDecimal.new("10") / 100 * 121).to_f #=> 12.10

Options:

  1. don't round up and then you can calculate the total VAT from total sum
  2. Round up, but then calculate total VAT as sum of VAT amounts per product

Applying what's said above, 1 product with VAT:

(product.price / 100 * 121).round(2)

all products with VAT:

products.sum { |p| (p.price / 100 * 121).round(2) }

Upvotes: 3

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