Calculatrice d’arrondi
The detailed guide below is currently available in English.
Choosing a rounding mode
Rounding to a place replaces the number with the nearest value that has nothing beyond that place, but exact halfway cases need a tie rule. Half up sends 2.5 to 3 and is what most people learn first. Half down sends it to 2. Half even — banker’s rounding — looks at the digit before the tie and picks the even neighbor, so 2.5 rounds to 2 but 3.5 rounds to 4; statistics and finance prefer it because ties are equally likely to round up or down, canceling drift. Rounding "up" or "down" here means toward positive or negative infinity, which is useful when an estimate must never fall short of a real quantity.
Questions fréquentes
Why does 2.675 rounded to hundredths give 2.67, not 2.68?
Binary floating-point stores 2.675 as a hair below the true value, so it is not actually a halfway case. A tiny correction tolerance handles exact halves like 2.5, but numbers that look like ties in decimal may genuinely not be.
What is the difference between rounding and truncating?
Truncating just deletes digits past the place (2.9 → 2), while rounding picks the nearest value (2.9 → 3). The "down" mode here matches truncation for positive numbers.