Fractional Inches, Millimeters, and Sandpaper Grit Explained

One international inch equals exactly 25.4 millimeters. Convert a fraction to a decimal, then multiply by 25.4. That calculation tells you size; it does not prove that a close metric and inch socket, thread, arbor, bit, or collet can be substituted.

Sandpaper grit is another labeled system. Higher numbers generally mean finer abrasive within the same grading system, but a FEPA P grade and a CAMI grade with the same number are not automatically identical.

Fractional inch

U.S. tools and hardware commonly use fractional inches such as 1/4, 3/8, 1/2, and 3/4 inch.

To convert a fraction to decimal inches:

decimal inches = numerator ÷ denominator

Examples:

• 1/4 = 0.25 inch
• 3/8 = 0.375 inch
• 1/2 = 0.5 inch
• 5/16 = 0.3125 inch

Keep enough precision for the decision. A tape-measure layout can tolerate a different display precision than a collet, bearing, thread, or machined fit.

Decimal inch

Decimal inch expresses the same inch dimension in base-ten form. It is useful for calculation and comparison with a technical drawing or caliper.

Do not round early. Convert from the exact fraction, then round only the displayed result to a sensible number of decimal places.

Millimeter (mm)

The millimeter is one-thousandth of a meter. Metric sockets, fasteners, bits, and dimensions commonly use mm.

The exact relationship is:

1 inch = 25.4 mm

mm = inches × 25.4

inches = mm ÷ 25.4

Useful exact conversions

Inch size	Exact millimeters	Practical display
1/16 in	1.5875 mm	1.59 mm
1/8 in	3.175 mm	3.18 mm
1/4 in	6.35 mm	6.35 mm
5/16 in	7.9375 mm	7.94 mm
3/8 in	9.525 mm	9.53 mm
1/2 in	12.7 mm	12.7 mm
3/4 in	19.05 mm	19.05 mm

The “practical display” column is rounded for reading. Use the exact value when a calculation requires it.

Rounding and fit

A rounded metric value can look close to an inch size without being interchangeable.

Examples of decisions where conversion alone is insufficient:

• A metric socket on an inch fastener
• A metric bolt in an inch thread
• A router shank in a collet
• A blade arbor hole on a saw
• A drill bit or bearing in a precision fit

Match the exact specification and system. A close fit can round a fastener, damage a collet, create runout, or fail to clamp securely.

Sandpaper grit

Grit is a standardized abrasive-particle size designation. Within one grading system, higher numbers generally identify finer abrasive and lower numbers coarser abrasive.

Grit number alone does not predict:

• Finish quality on every material
• Removal rate with every sander
• Abrasive life
• Scratch pattern after skipped steps
• Dust hazard

Abrasive material, backing, open/closed coat, tool motion, speed, pressure, workpiece coating, and dust loading also matter.

FEPA P-grit

A P prefix identifies the FEPA coated-abrasive grading system. FEPA standards specify particle-size distributions rather than one exact particle diameter for every grain.

Keep the P attached to the number in product copy. P180 is not simply “180” with optional decoration.

CAMI grit

CAMI is a U.S. coated-abrasive grading convention. Norton publishes a comparison showing that FEPA P grades and CAMI grades should not be assumed identical, particularly at finer grades.

Do not create an exact universal equivalence from a rough conversion chart. Buy within the finish system or abrasive manufacturer sequence when consistency matters.

How to choose a grit sequence

1. Identify the material and existing coating.
2. Follow the coating, finish, repair, or abrasive manufacturer's starting recommendation.
3. Use the coarsest grit needed without creating unnecessary deep scratches.
4. Progress through the stated steps rather than jumping blindly to a very fine grade.
5. Keep the grading system visible in the sequence.
6. Match disc/belt size, backing, attachment, and maximum speed to the sander.
7. Control dust and use hazard-appropriate PPE.

Sanding wood, paint, filler, metal, masonry, or unknown old coatings can create different hazards. Identify the material before selecting controls or respiratory protection.

Common mistakes

• Treating rounded inch and metric sizes as interchangeable.
• Rounding before completing the conversion.
• Mixing thread systems because diameters look close.
• Treating P180 and 180 CAMI as automatically identical.
• Assuming the highest grit creates the best finish without surface preparation.
• Using a damaged disc or the wrong backing/pad/speed.

Frequently asked questions

Is 13 mm the same as 1/2 inch?

No. One-half inch is exactly 12.7 mm. Whether a 13 mm item can substitute depends on the application; for sockets, threads, collets, and precision interfaces, use the exact size.

Why does some sandpaper have a P before the number?

The P identifies FEPA coated-abrasive grading. Keep it with the number when comparing products.

Does higher grit always mean a better finish?

Higher grit is finer within a named system, but finish also depends on material, preceding scratches, abrasive type, tool motion, pressure, and the finish process.

After checking the exact size and system, browse all current tools. For an unclear bit, socket, pad, disc, or accessory, ask product support.