PHP - Floating Number Precision

php floating-point precision floating-accuracy
dcmoody picture dcmoody · Sep 16, 2010 · Viewed 44.2k times · Source 
$a = '35';
$b = '-34.99';
echo ($a + $b);

Results in 0.009999999999998

What is up with that? I wondered why my program kept reporting odd results.

Why doesn't PHP return the expected 0.01?

Answer

NullUserException picture NullUserException · Sep 16, 2010

Because floating point arithmetic != real number arithmetic. An illustration of the difference due to imprecision is, for some floats a and b, (a+b)-b != a. This applies to any language using floats.

Since floating point are binary numbers with finite precision, there's a finite amount of representable numbers, which leads accuracy problems and surprises like this. Here's another interesting read: What Every Computer Scientist Should Know About Floating-Point Arithmetic.

Back to your problem, basically there is no way to accurately represent 34.99 or 0.01 in binary (just like in decimal, 1/3 = 0.3333...), so approximations are used instead. To get around the problem, you can:

  1. Use round($result, 2) on the result to round it to 2 decimal places.

  2. Use integers. If that's currency, say US dollars, then store $35.00 as 3500 and $34.99 as 3499, then divide the result by 100.

It's a pity that PHP doesn't have a decimal datatype like other languages do.

Related questions

How can I make sure a float will always be rounded up with PHP?

I want to make sure a float in PHP is rounded up if any decimal is present, without worrying about mathematical rounding rules. This function would work as follows: 1.1 to 2 1.2 to 2 1.9 to 2 2.3 to 3 2.8 to 3 I know the round() function exists …

Read More
PHP String to Float

I am not familiar with PHP at all and had a quick question. I have 2 variables pricePerUnit and InvoicedUnits. Here's the code that is setting these to values: $InvoicedUnits = ((string) $InvoiceLineItem->InvoicedUnits); $pricePerUnit = ((string) $InvoiceLineItem->PricePerUnit); If I …

Read More
Converting a number with comma as decimal point to float

I have a list of prices with a comma for a decimal point and a dot as the thousand separator. Some examples: 12,30 116,10 1.563,14 These come in this format from a third party. I want to convert them to floats and add …

Read More