Quarter inches are fine, and 12 or more carries into feet.
Size finder
What size do I need?
Enter the clear span you have to cover, then the tool returns the smallest nominal size that reaches it for your species, grade, and spacing - reverse-looked-up from the published tables.
If nothing reaches, spacing is usually the cheapest thing to change: 16 in vs 24 in on center shows what each size gains and loses across the four published spacings. Remember that the clear span is measured between the faces of the supports, not the length of the board - see how to read a span table.
Smallest size that reaches your span
2x8 Douglas Fir-Larch at 16″ o.c. spans 12′-9″, which covers 12′-0″.
2x8spans 12′-9″
A 2x8 Douglas Fir-Larch at 16″ o.c. covers your 12′-0″ span (published max 12′-9″). Smaller sizes in this table fall short.
Source: International Residential Code (IRC), 2018 - Table R502.3.1(2) - Floor Joist Spans, 40 psf Live Load. Published by International Code Council.
Edition: These values are transcribed from the IRC 2018 tables. The ICC has since published IRC 2021 and IRC 2024, and the edition in force varies by jurisdiction - confirm the table and edition adopted by your local building department before relying on a span.