Why start with eighths?

Because they are the first rule that contains the halves and quarters inside it: 4/8 is 1/2 and 2/8 is 1/4, which the reduced column and the bars make visible. Learn to read eighths and the finer rules are the same idea, halved again.

What is 3/8 as a decimal and a percent?

3/8 is exactly 0.375, which is 37.5%. Every eighth ends cleanly in decimals because 8 is 2 × 2 × 2.

Eighths, drawn and stated

The first serious rule, with each mark shown as a bar — the filled share IS the fraction — beside its reduced name, exact decimal, percent, and what it measures in millimetres.

Fraction	Reduced	Decimal	Percent	mm (as inches)
1/8	—	0.125	12.5%	3.175
2/8	1/4	0.25	25%	6.35
3/8	—	0.375	37.5%	9.525
4/8	1/2	0.5	50%	12.7
5/8	—	0.625	62.5%	15.875
6/8	3/4	0.75	75%	19.05
7/8	—	0.875	87.5%	22.225
8/8	1	1	100%	25.4

Ready for finer rules? 16ths · 32nds · 64ths — or see the marks in place on the ruler.

Common questions

Why start with eighths?: Because they are the first rule that contains the halves and quarters inside it: 4/8 is 1/2 and 2/8 is 1/4, which the reduced column and the bars make visible. Learn to read eighths and the finer rules are the same idea, halved again.
What is 3/8 as a decimal and a percent?: 3/8 is exactly 0.375, which is 37.5%. Every eighth ends cleanly in decimals because 8 is 2 × 2 × 2.

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.