SAE to Metric Calculator

How to use this calculator

Select your input size and click Convert to see the exact equivalent and nearest standard size.

SAE to Metric tab: Choose a SAE fraction from the dropdown (1/4” through 1”) or type a decimal inch value directly. The calculator shows the exact metric equivalent in millimeters, the nearest standard metric wrench/socket size, and the adjacent sizes above and below.

Metric to SAE tab: Enter a metric size in millimeters. The calculator shows the exact decimal inch equivalent, the nearest SAE fraction, and adjacent metric sizes.

Size Reference tab: A complete side-by-side table of all common SAE sizes and their metric equivalents. Useful as a quick reference to keep open while you work.

The conversion formula

SAE to metric conversion is straightforward arithmetic. There are exactly 25.4 millimeters in one inch, and this ratio is exact by international definition.

The challenge is not the math itself but finding the nearest available standard size. Metric wrenches and sockets come in specific sizes (6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 24, 25mm and so on). SAE sizes are fractions of inches (1/4, 5/16, 3/8, 7/16, 1/2, 9/16, 5/8, 11/16, 3/4, 7/8, 1 inch).

These two systems were designed independently and do not align neatly. The nearest metric size to a given SAE size is usually within 0.5mm but is essentially never an exact match except for a few coincidences.

Complete SAE to metric conversion reference

This is the definitive table for the most common wrench and socket sizes in both systems.

SAE Fraction	Decimal Inches	Exact mm	Nearest Metric	Difference
1/4”	0.2500	6.350	6 mm	-0.35 mm
5/16”	0.3125	7.938	8 mm	+0.06 mm
3/8”	0.3750	9.525	10 mm	+0.47 mm
7/16”	0.4375	11.113	11 mm	-0.11 mm
1/2”	0.5000	12.700	13 mm	+0.30 mm
9/16”	0.5625	14.288	14 mm	-0.29 mm
5/8”	0.6250	15.875	16 mm	+0.12 mm
11/16”	0.6875	17.463	17 mm	-0.46 mm
3/4”	0.7500	19.050	19 mm	-0.05 mm
13/16”	0.8125	20.638	21 mm	+0.36 mm
7/8”	0.8750	22.225	22 mm	-0.22 mm
15/16”	0.9375	23.813	24 mm	+0.19 mm
1”	1.0000	25.400	25 mm	-0.40 mm

The “Difference” column shows how far off the nearest metric size is from the exact SAE equivalent. Values under 0.5mm typically provide a usable fit for most non-critical applications.

When cross-compatibility matters (and when it does not)

Not all fastener work is equal. The acceptable tolerance for a metric/SAE size mismatch depends entirely on what you are tightening.

Low-stakes applications where a close match is fine: Drain plugs, decorative trim screws, non-critical body panels, furniture hardware. For these, a slightly loose fit is tolerable if you are careful.

Applications where exact size is required: Suspension components, brake caliper bolts, wheel lug nuts, engine head bolts, and any fastener with a specified torque value. Using a slightly wrong size socket on a high-torque fastener risks rounding the bolt head, which creates a much bigger problem than the original job.

When to never substitute: Any fastener on a safety-critical system (brakes, steering, suspension, wheel attachment). Always use the correct size tool for these applications.

The common wisdom in automotive repair is simple: if you are doing the job right, use the right size. Collect both SAE and metric sets. Combination sets are now inexpensive enough that there is no reason to improvise on anything that matters.

SAE vs metric: which system is used where

Understanding which system applies to your vehicle tells you which tools to reach for first.

Modern cars from any manufacturer (post-1995): Metric fasteners for essentially everything. Ford, GM, Chrysler, Toyota, Honda, BMW, Mercedes, and every other brand uses the metric standard for vehicle fasteners. The US auto industry adopted metric standards decades ago.

Pre-1980s American vehicles: SAE fasteners throughout. Classic muscle cars, vintage trucks, and American iron from this era use SAE sizing for most body, engine, and drivetrain fasteners.

1980s-1990s American vehicles: Mixed. GM and Ford were transitioning during this period. You may encounter both SAE and metric fasteners on the same vehicle, sometimes on the same component.

European vehicles: Fully metric since the mid-20th century. British vehicles from before the 1970s may use Whitworth fasteners, which are neither SAE nor metric.

Farm equipment and industrial machinery: Often SAE, though this varies widely by manufacturer and era. John Deere equipment uses a mix.

Plumbing and pipe fittings: SAE is still common for water supply lines in the US. Thread sizes like 1/2”, 3/4”, and 1” nominal pipe are standard SAE.

The most common metric socket sizes for car work

If you are building a socket set for automotive work, these are the sizes you will reach for most often.

The “always have these” metric sizes:

• 8mm: spark plug boots, smaller trim screws, brake line fittings
• 10mm: the most common socket size on any modern car, used for hundreds of fastener types
• 12mm: brake calipers, engine mounts, various brackets
• 13mm: often the closest metric to 1/2” SAE, used widely on American-built cars
• 14mm: drain plugs on many vehicles, suspension components
• 15mm: wheel lug nuts on smaller vehicles (Honda, Subaru, Mazda)
• 17mm: wheel lug nuts on many vehicles, caliper slide pins
• 19mm: wheel lug nuts on trucks and larger vehicles
• 21mm: wheel lug nuts on full-size trucks, axle nuts
• 22mm: common axle nut size

Less common but important:

• 24mm: larger axle and CV joint nuts
• 27mm, 30mm, 32mm, 36mm: large suspension components, axle shafts

For most passenger car work, an 8-piece metric set (10mm through 19mm) plus a 21mm covers 90% of jobs. A full 6mm-32mm set handles virtually everything.

Why 10mm is “the most important socket”

The 10mm socket has an almost mythological status in the automotive mechanic community. It is genuinely the most commonly needed size on modern cars and the size most frequently lost or misplaced.

On a typical modern sedan, 10mm fasteners appear on:

• Battery terminals and hold-down brackets
• Engine covers and valve covers
• Air intake components
• Brake line brackets
• Plastic trim clips throughout the interior
• Various grounding bolts
• Smaller body panels and bumper brackets

The joke among mechanics is that 10mm sockets disappear at a rate proportional to how much they are needed. If you buy only one spare socket, buy a 10mm.

In SAE equivalency, the closest size to 10mm is 3/8” (9.525mm), but a 3/8” socket on a 10mm bolt is 0.475mm undersized. Under torque, this mismatch can round the bolt head. Always use a 10mm for 10mm bolts.

When to use an impact wrench vs hand ratchet

Socket size matching matters even more with impact wrenches because the torque involved is dramatically higher than hand tools.

A hand ratchet used with a slightly wrong-sized socket at low torque may slip without damaging the fastener. An impact wrench applying 250 ft-lb through a slightly wrong socket will round the fastener head in under a second.

Impact wrench rules:

• Always use impact-rated sockets (marked with “impact” or “CR-V” steel designation)
• Always use the exact correct size
• Never use standard chrome sockets with an impact wrench; they can shatter

High-torque hand tool rules:

• For anything torqued above about 50 ft-lb, use the exact socket size
• For torque wrench work, exact fit is required to get accurate readings
• Breaker bars and long-handle ratchets amplify force, making exact sizing more critical

For the typical home mechanic doing basic oil changes, brake jobs, and general maintenance, the size chart in this calculator is mostly a reference tool. But for anyone working on suspension, drivetrain, or engine components, using the correct size tool every time is non-negotiable.

Building a tool set that covers both systems

If you work on a range of vehicles, having both SAE and metric tools is not optional. Here is how to think about building a practical set.

For a starter set on modern cars: A metric socket set from 8mm to 24mm (1/4” drive for small stuff, 3/8” drive for general work, 1/2” drive for heavy torque) handles virtually everything on any car built after 1990. Add a 1/2” drive 21mm for most wheel lug nuts and you are covered for daily driving needs.

For working on older American vehicles: Add SAE sockets from 3/8” through 1-1/4” and SAE wrenches in the same range. Pre-1980 American cars use SAE for nearly every fastener.

For farm equipment and small engines: Many small engines (lawnmowers, generators, pressure washers) use a mix of SAE and metric. Checking your owner’s manual before starting a job tells you which system applies.

Combination wrenches vs socket sets: For tight spaces where a socket cannot reach, combination wrenches (open end on one side, box end on the other) in both SAE and metric fill the gap. A 10-piece metric combination wrench set covering 8-19mm is a practical starting point.

Drive sizes explained: Socket sets come in 1/4”, 3/8”, and 1/2” drive sizes, referring to the square fitting that connects to the ratchet. Smaller drives are for lighter work (trim screws, small bolts), larger drives for heavier torque. A 3/8” drive set covers about 80% of automotive work. The 1/2” drive is needed for wheel lug nuts and heavy suspension components.

Bolt grades and why size is only part of the story

Converting between SAE and metric size tells you which wrench to use, but it does not tell you the strength of the fastener itself. Bolt grades define the tensile strength and dictate the proper torque specification.

SAE bolts are graded 2, 5, and 8, with grade 8 being the strongest. They are identified by radial marks on the bolt head: no marks for grade 2, three marks for grade 5, six marks for grade 8.

Metric bolts use a property class system: 4.6, 8.8, 10.9, and 12.9. The most common automotive metric fasteners are 8.8 (similar to SAE grade 5) and 10.9 (similar to SAE grade 8). The property class is stamped on the bolt head.

Why does this matter for SAE to metric conversion? Because using a metric fastener where an SAE fastener was specified (or vice versa) in a safety-critical application can create a strength mismatch, even if the physical size is similar. Thread pitch also differs between systems, so SAE and metric bolts are not interchangeable even when they look similar in size.

When replacing a fastener, always match the grade or property class, the thread pitch, and the head size. The SAE-to-metric conversion table gives you the head size. The grade marks and thread pitch determine whether the replacement fastener is truly equivalent.