[RESOLVED] Something funny happing with percentages

ali_p

I have a table which contains a field for a cost for a work of art. Basically when the user enters data I add 10 percent to the cost field and the amount plus 10 percent gets entered into the database. This is fine.

However my problem is when I try to do the reverse - try to minus the 10 percent when displaying the cost field. So the user enters 1000 into the form field 1100 gets entered into the database. But when i do the following

$totalPrice = $cost; (coming out of the DB)
$discount = round(($totalPrice * 0.1));
$pricenonopercent = ($totalPrice - $discount);

Instead of getting 1000 I get 990

I really don't get this.

Thanks

ixalmida

The error is in your math:

100 minus 10% equals 90

You want this calculation:

100 times 100 divided by 110 (i.e. - the original percent) equals 90.909090...

Check the math: 90.91 times 1.1 equals 100.001
(there's a tiny remainder because I rounded)

Here's how it would look in PHP:

$original_price = ($total_with_tax * 100) / 110;

dagon

the percentages is not a fixed number, its relative to the value, so you cant simply subtract the same percentage from a different number.

1100/(1.10)
or
$totalPrice / (1 + PERCENTAGE)

ixalmida

Note...the equation can be simplified further:

$original_price = $total_with_tax/ 1.1;

Just be careful with your percentages. You need to think in terms of the original sales tax. So if the tax percentage was 9.765%, then the markup was 109.765 percent. So your divisor would be 1.09765

ali_p

Thank you, this all makes perfect sense. Of course the figure minus 10 percent is not the same as the figure plus 10 percent.
I'll get around this by adding another table field which holds the commission price, so I'll have the original price and the commission price.

Thanks again for the logic