If you’ve ever stared at a bolt or ruler and wondered whether 3/8 inch actually means 9.5 mm, you’re not alone. This gap between imperial and metric measurements trips up DIYers, mechanics, and makers almost every time it shows up on a tool or hardware shelf. This guide cuts through the confusion with the exact conversion, a comparison to common metric bolts, and a practical chart so you know exactly what fits before you buy or tighten.

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3/8 inch equals: 9.525 mm · Closest metric: 9.5 mm · Vs 8 mm: 1.525 mm larger · 1/4 inch equals: 6.35 mm · 1/2 inch equals: 12.7 mm

Quick snapshot

1Confirmed facts
2What’s unclear
  • Exact washer fit depends on specific manufacturer tolerance
  • Thread pitch compatibility varies by application standard
3Common equivalents
4What to watch

Three key measurements anchor every inch-to-metric comparison: the decimal value, the exact millimeter result, and the nearest standard metric bolt size.

Label Value
Exact conversion 9.525 mm
Formula 3/8 × 25.4
8 mm equivalent 0.315 inch (smaller)
10 mm equivalent 0.394 inch (larger)
Decimal form 0.375 inch

What is 3/8 inch in mm?

The math behind this conversion is straightforward. One inch equals exactly 25.4 millimeters, a definition set by international agreement. Multiply 3/8 (or 0.375) by 25.4 and you get 9.525 millimeters — no rounding, no approximation, just the exact result.

Conversion formula

  • Formula: 3/8 × 25.4 = 9.525 mm
  • Step 1: Convert fraction to decimal: 3 ÷ 8 = 0.375
  • Step 2: Multiply by 25.4: 0.375 × 25.4 = 9.525
The takeaway

Industrial conversion charts from Aspen Fasteners conversion chart list 9.5 mm as 0.3740 inches — a difference of just 0.001 inch from the true 0.375 inches. Most charts round to 9.5 mm for practical use, which works fine in many applications but can matter in precision work.

Exact value and rounding

The distinction between “9.525 mm” and “9.5 mm” matters in fastener applications. Precision-engineered parts from Fasteners Etc measurement guidance advise using calipers for exact measurement before purchasing hardware. Rounding to 9.5 mm introduces a 0.025 mm error — small enough for general tasks, significant enough in close-tolerance assemblies.

The implication: always verify the tolerance requirement of your specific project before rounding.

Is 3/8 the same size as 8mm?

No — this is one of the most common mix-ups in the workshop. A 3/8 inch bolt measures 9.525 mm in diameter, while an 8 mm bolt measures exactly 8 mm. The gap between them is 1.525 mm, which is roughly the thickness of a credit card.

Direct comparison

  • 3/8 inch = 0.375 inch = 9.525 mm
  • 8 mm = 0.315 inch = 8 mm
  • Difference: 1.525 mm (0.060 inch)

Difference in measurements

On a ruler, 8 mm falls between 5/16 inch (7.94 mm) and 3/8 inch (9.525 mm). No standard fractional inch marking equals exactly 8 mm. This is why metric and imperial fasteners don’t swap directly — they evolved from different systems with different nominal sizes.

The catch

Thread conversion tables from TR Fastenings thread geometry guide explicitly warn that metric and imperial sizes are not directly interchangeable, even when their diameters are close. Always check thread pitch along with diameter.

What this means: a 3/8-16 UNC bolt will not thread into an M10 hole, even though their diameters are similar. The threads-per-inch count differs from the metric pitch.

Is 3/8 closer to M8 or M10?

A 3/8 inch bolt at 9.525 mm sits between an M8 (8 mm) and an M10 (10 mm). The closer match is M10 — it’s only 0.475 mm larger than 3/8 inch, while M8 is 1.525 mm smaller.

Bolt size equivalents

  • 3/8 inch: 0.375 inch / 9.525 mm
  • M8: 8 mm / 0.315 inch
  • M10: 10 mm / 0.394 inch

Tolerance considerations

The FastenerMart reference chart confirms that M10 at 10 mm (0.3937 inches) serves as the standard metric replacement for 3/8 inch bolts in many applications. This isn’t a perfect match — it’s 0.475 mm larger — but within typical assembly tolerances for many uses.

Why this matters

For mechanics and fabricators: an M10 bolt will usually pass through a 3/8 inch hole with clearance, but the reverse (3/8 inch through an M10 hole) depends on hole tolerance. MW Components fastener conversion guide notes that 3/8-16 UNC converts to M10x1.5 for coarse-thread applications, and 3/8-24 UNF also maps to M10x1.5 for fine-thread variants.

The trade-off: M10 offers slightly more material strength but may require a tap for threads in soft material where a 3/8-16 UNC tap would suffice.

How many mm is 3/8 inch?

To answer directly: 3/8 inch equals 9.525 mm. The calculation uses the inch-to-millimeter definition of 25.4 mm per inch, applied to the decimal value 0.375.

Step-by-step conversion

  1. Divide the numerator by the denominator: 3 ÷ 8 = 0.375
  2. Multiply the result by 25.4: 0.375 × 25.4 = 9.525
  3. Result: 9.525 millimeters
Conversion helper

For quick reference without a calculator: the fraction 3/8 sits roughly halfway between 1/4 (6.35 mm) and 1/2 (12.7 mm) on the fractional scale. On a metric ruler, it aligns closest to the 9.5 mm marking.

Common fractional equivalents

For anyone working with fractional inch hardware, here’s how 3/8 inch fits into the standard ruler scale:

Fraction Decimal Millimeters
1/4 inch 0.250 6.35 mm
5/16 inch 0.313 7.94 mm
3/8 inch 0.375 9.525 mm
7/16 inch 0.438 11.11 mm
1/2 inch 0.500 12.7 mm

The pattern: each 1/16 inch step equals 1.5875 mm. This means 3/8 inch (6/16) is 6 × 1.5875 = 9.525 mm, and 7/16 inch (7/16) lands at 11.11 mm.

For anyone switching between systems regularly, memorizing that 1/16 inch = 1.5875 mm gives you the full range without needing conversion tools.

Will an M10 bolt fit in a 3/8 washer?

Probably yes, with a caveat. Standard 3/8 inch washers have an inner diameter around 9.5 mm (the nominal bolt size), while an M10 bolt is 10 mm in diameter. The M10 bolt is 0.5 mm larger than the nominal 3/8 inch opening.

Washer inner diameter

According to the Aspen Fasteners conversion chart, standard washers for the 9.5 mm range typically align with M10 washers rather than exact 3/8 inch ID washers. Many manufacturers produce 3/8 washers with slightly oversized IDs to accommodate both imperial and near-metric bolts. For a quick conversion, you can refer to this 132 kW in PS umrechnen.

Fit tolerances

  • Nominal 3/8 ID washer: ~9.5 mm
  • M10 bolt diameter: 10 mm
  • Clearance gap: -0.5 mm (interference fit in theory)

The reality depends on the specific washer. Some 3/8 washers have IDs of 10 mm or slightly more to provide clearance for the M10. Others hold closer to 9.5 mm, creating an interference fit where the M10 won’t pass through without force.

What to watch

Before ordering washers for mixed imperial-metric assemblies, measure the actual inner diameter with calipers. Charts from Engineered Parts conversion guide confirm that washer compatibility varies by manufacturer and material. A loose fit around an M10 in a 3/8 washer reduces load distribution and may compromise the joint.

The implication: don’t assume interchangeability. For critical applications, match washers to bolt standards — use M10 washers with M10 bolts, and 3/8 washers with 3/8 bolts.

Fractional to Metric Conversion Table

Six common fractional inch sizes map to their exact millimeter equivalents across the range relevant to most fasteners and hardware.

Fractional inch Decimal Millimeters Nearest metric bolt
1/8 inch 0.125 3.175 mm M3 (3 mm)
1/4 inch 0.250 6.35 mm M6 (6 mm)
5/16 inch 0.313 7.94 mm M8 (8 mm)
3/8 inch 0.375 9.525 mm M10 (10 mm)
7/16 inch 0.438 11.11 mm M11 (11 mm)
1/2 inch 0.500 12.7 mm M12 (12 mm)

The pattern across this range: each fractional step lands consistently between its metric neighbors. 3/8 inch sits squarely between M8 and M10, closer to the latter, which explains why M10 serves as the go-to metric replacement in practice.

How to Convert 3/8 Inch to MM

Two methods work for this conversion — one with a calculator, one with mental math once you know the key constants.

Method 1: Calculator method

  1. Take the fractional value: 3/8 = 0.375
  2. Multiply by 25.4: 0.375 × 25.4
  3. Result: 9.525 mm

Method 2: Quick estimate using 1/16

  1. Know that 1/16 inch = 1.5875 mm (a constant)
  2. Count 1/16 increments: 3/8 = 6/16
  3. Multiply: 6 × 1.5875 = 9.525 mm

The second method works without a calculator and builds intuition. Monster Bolts conversion table recommends this approach for anyone frequently switching between systems.

Quick reference

For workshop use, memorize two anchor points: 1/4 inch = 6.35 mm and 1/2 inch = 12.7 mm. 3/8 inch is the arithmetic midpoint: (6.35 + 12.7) ÷ 2 = 9.525 mm.

“3/8 inch equals exactly 9.525 mm. If a chart shows 9.5 mm, it’s rounded — the exact value matters in precision work.” — McFeely’s fastener distributor

For anyone working between imperial and metric systems regularly, building a habit of verification — checking actual measurements with calipers rather than trusting nominal sizes — prevents mismatched parts and failed assemblies.

Confirmed facts

  • 3/8 inch = 9.525 mm (exact)
  • 3/8-16 UNC converts to M10x1.5
  • 3/8-24 UNF also converts to M10x1.5
  • Nearest metric bolt: M10 (10 mm)
  • Difference to 8 mm: 1.525 mm

What varies by context

  • Washer ID tolerance — check actual measurement
  • Thread pitch compatibility — UNC vs metric pitch
  • Application-specific fit requirements

“Always use calipers for exact measurement before purchasing hardware. Charts give approximate values; your project tolerances determine what’s acceptable.” — Fasteners Etc conversion chart guidance

Bottom line: Mechanics and fabricators who need exact fastener fits should trust 9.525 mm over rounded 9.5 mm when tolerance matters. Using M10 hardware where 3/8 inch is called for usually works, but mismatched washers risk compromised joints in critical applications.

Related reading: 100 ml to oz converter

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Frequently asked questions

Is 8mm a 1/4 inch?

No. 8 mm equals 0.315 inch, while 1/4 inch equals 6.35 mm. 8 mm is closer to 5/16 inch (7.94 mm) than to 1/4 inch. The 1/4 inch bolt is M6 (6 mm), not M8.

Is 1 cm 3/8 inch?

No. 1 cm equals 10 mm or 0.394 inch. 3/8 inch at 9.525 mm is slightly smaller than 1 cm. The difference is about 0.475 mm — roughly half a millimeter.

Is 3/8 thread the same as 10mm?

Not exactly. 3/8-16 UNC and 3/8-24 UNF both convert to M10x1.5 in thread pitch terms — the diameter is similar, but the thread pitch differs. UNC/UNF uses threads-per-inch (TPI), while metric uses pitch in millimeters. A metric M10x1.5 has a 1.5 mm pitch; 3/8-16 UNC has roughly 1.58 mm pitch (16 TPI). They are not interchangeable without a tap or die.

Why is 3/8 inches bigger than 10 mm when it is supposed to be smaller?

3/8 inch (9.525 mm) is actually smaller than 10 mm, not bigger. The confusion likely comes from the rounding: charts listing 3/8 as 9.5 mm versus 10 mm makes the numbers look close, but 9.525 mm < 10 mm. 3/8 inch is 0.475 mm smaller than M10.

What does 3/8 equal to in metric?

3/8 inch equals exactly 9.525 mm. In bolt terms, it maps closest to M10 (10 mm). For quick reference: 1/4 inch = 6.35 mm, 3/8 inch = 9.525 mm, 1/2 inch = 12.7 mm.

How are washers identified and measured?

Washers are identified by inner diameter (ID), outer diameter (OD), and thickness. A “3/8 washer” has an ID of approximately 3/8 inch (9.5 mm) to fit a 3/8 inch bolt. Metric washers use millimeter dimensions. Always measure the actual ID with calipers — nominal sizes vary by manufacturer and standard (USS, SAE, DIN, ISO).

Is 3/8 inch to mm on a ruler?

Yes. On a fractional inch ruler, 3/8 inch is a marked line — the seventh hash mark from the inch start, if counting 1/16 increments. On a metric ruler, the closest marking is 9.5 mm. The true value of 9.525 mm falls slightly past the 9.5 mm line, about a third of the way to 10 mm.