What is 0.4 as a fraction?

0.4 as a fraction is 2/5. Write the digits over 10 to get 4/10, then divide top and bottom by 2.

What is 0.4 as a percentage?

0.4 is 40%. Multiplying a decimal by 100 moves the point two places to the right.

How many millimetres is 0.4 of an inch?

0.4 inches is 10.16 mm, using the exact definition 1 inch = 25.4 mm.

0.4 as a fraction

Fraction2/5lowest terms

Decimal0.4

Percent40%

How to get there

0.4 has 1 digit after the point, so it is that many tenths: write the digits over 10 — 4/10.
4 and 10 share a greatest common divisor of 2. Divide both by it: 2/5.

40 of 100 squares — 0.4 of the whole, which is also 40%.

As a measurement

Read as inches, 0.4″ is 10.16 mm exactly. The nearest marks on a rule:

Rule	Nearest mark	Inches	mm	Difference
16ths	3/8″	0.375	9.525	−0.635 mm
32ths	13/32″	0.40625	10.3188	+0.1588 mm
64ths	13/32″	0.40625	10.3188	+0.1588 mm

Common questions

What is 0.4 as a fraction?: 0.4 as a fraction is 2/5. Write the digits over 10 to get 4/10, then divide top and bottom by 2.
What is 0.4 as a percentage?: 0.4 is 40%. Multiplying a decimal by 100 moves the point two places to the right.
How many millimetres is 0.4 of an inch?: 0.4 inches is 10.16 mm, using the exact definition 1 inch = 25.4 mm.

The same number, other ways

2/5Everything about the fraction: equivalents, the bar model, the number line.40%The same value as a percentage, with its own derivation.Decimal → fraction chartThe whole lookup table, printable.Nearest fractionMatch 0.4″ against any rule, with the exact error.

Where this comes from. Exact rational arithmetic on the values shown — fractions are pairs of integers until display, and inch–metric conversion uses 25.4 mm to the inch, exact by definition since 1959. See the full methodology.