Rounding & Significant Figures

JavaScript’s own (2.675).toFixed(2) returns "2.67", not "2.68", because 2.675 has no exact binary floating-point representation and is actually stored as something microscopically below it. This tool never converts the input to a float at all: it parses the digits as text and rounds them with exact integer arithmetic, so the answer matches what the digits actually say.

Enter a number, pick a rounding mode, and both a decimal-place result and a significant-figures result appear side by side, so you can compare the two without retyping.

Worked Example: The 2.675 Rounding Trap

Click Load Sample and the field fills with 2.675, decimal places set to 2, significant figures to 3, mode Half Up. The decimal-place card reads 2.68, correct because the digit immediately after the second decimal place is a 5 with nothing following it. Press Load Sample again for 9.995 at 3 significant figures: the digits 999 round up to 1000, which is one digit too many for 3 significant figures, so the result carries into the next order of magnitude and displays as 10.0.

Format Notes: The 4 Rounding Modes

• Half Up (the everyday default): a trailing 5 with nothing after it always rounds away from zero, so 1.5 becomes 2 and -1.5 becomes -2.
• Half Down: the same trailing 5 rounds toward zero instead, so 1.5 becomes 1 and -1.5 becomes -1. Rare outside specific financial contracts that specify it explicitly.
• Half to Even (banker’s rounding): a trailing 5 rounds to whichever neighbor is even, so 1.25 rounds to 1.2 but 1.35 rounds to 1.4. Used in accounting and statistics because it doesn’t bias a large sum of roundings consistently upward the way Half Up does.
• Truncate: every digit past the cutoff is simply dropped, regardless of value, so 1.999 at 2 decimal places is 1.99, not 2.00.

Edge Cases: Zero, Negatives and the 15-Digit Cap

A negative number that rounds to exactly zero, like -0.001 at 2 decimal places, displays as 0.00 rather than the misleading -0.00. Decimal places and significant figures each accept whole numbers from 0 to 15; anything outside that range shows “Decimal places must be a whole number from 0 to 15.” or the matching significant-figures message, rather than silently clamping to the nearest valid value. Input that isn’t a plain decimal number, like a fraction or an expression, shows “Enter a plain decimal number, like 2.675 or -14.2.”