Question

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Python rounding error with float numbers

I have a rounding Problem in Python. If i calculate

32.50 * 0.19 = 6.1749999999999998

But this should be 6.175. If i round 6.1749999999999998 with 2 decimal places it correctly shows 6.18. So i can live with that.

But if i calculate this:

32.50 * 0.19 * 3 = 18.524999999999999

This should be 18.525. If i round the value 18.524999999999999 with two decimal places it shows 18.52.

It should show me 18.53. What am i doing wrong and how can i fix it ?

Answer 1

If you need exact arithmetic, you could use the decimal module:

import decimal
D=decimal.Decimal

x=D('32.50')*D('0.19')
print(x)
# 6.1750
print(x.quantize(D('0.01'),rounding=decimal.ROUND_UP))
# 6.18

y=D('32.50')*D('0.19')*D('3')
print(y)
# 18.5250
print(y.quantize(D('0.01'),rounding=decimal.ROUND_UP))
# 18.53