Find the closest SAE (fractional inch) socket or wrench size for any metric bolt. Fit quality indicator, cross-reference table, and size-difference analysis.
This page helps when the fastener is metric but the available tools are SAE. Instead of guessing which fractional-inch socket might fit, it shows the closest SAE size, the exact size gap, and whether that substitution is likely to be safe or risky. That is important because a near match can still round the fastener if the fit is too loose.
It is most useful in mixed-toolbox situations such as automotive repair, garage work, salvage jobs, and older equipment where the correct metric tool is missing. The cross-reference table and fit indicator make it easier to choose a substitute without relying on trial and error. It is especially useful when you are working on older equipment and the original metric tool is missing. That saves time when you are deciding whether to keep looking for the exact socket or use the nearest inch size.
Use it when you need a practical socket or wrench match rather than a plain mm-to-inch conversion.
A near match is not always a safe match. This page helps mechanics and DIY users judge whether an SAE substitute is close enough to use or likely to round the fastener. It is especially useful when you are working on older equipment and the original metric tool is missing.
inches = mm ÷ 25.4 Closest SAE = nearest fractional inch size to the result Δ mm = metric size − SAE equivalent in mm
Result: Closest: 1/2" (12.70 mm), Δ +0.30 mm
13 mm ÷ 25.4 = 0.5118". Closest SAE is 1/2" (12.70 mm), which is 0.30 mm smaller. This is a yellow-zone fit — acceptable but may slip under heavy torque.
Many vehicles use both metric and SAE fasteners. Japanese and European cars are primarily metric; older American vehicles are SAE. Modern US trucks and SUVs may have both. Home mechanics often reach for the nearest socket that "feels right," but a loose fit damages fasteners and can cause injury if a wrench slips.
Some substitutions work reliably: 19 mm → 3/4" (only 0.05 mm gap), 22 mm → 7/8" (0.23 mm), 8 mm → 5/16" (0.06 mm). Others are dangerous: 10 mm → 3/8" has a 0.47 mm gap — enough to round a bolt head. Always check the fit quality before applying torque.
If you frequently work on metric vehicles, invest in a metric socket set. A quality 10-piece metric set (8–22 mm) costs $20–$50 and prevents the fastener damage and frustration that come from using approximate SAE substitutes.
Society of Automotive Engineers, the organization associated with standard US fractional-inch tool and fastener sizing. In practice, it refers to the inch-based fractional socket and wrench sizes used in the US.
Close but not exact. 10 mm = 0.3937", while 3/8" = 0.375" (9.53 mm). The 0.47 mm gap may cause slipping.
None are exact. The best fits are: 19 mm ↔ 3/4" (Δ 0.05 mm) and 8 mm ↔ 5/16" (Δ 0.06 mm).
Only if the fit difference is small (< 0.15 mm). For high-torque applications, always use the correct metric tool.
Both systems produce standard hardware sizes in similar ranges. The increments just don't line up perfectly because one is base-10 and the other is fractional.
8, 10, 12, 13, 14, 17, 19, 21, and 22 mm cover a large share of common automotive fasteners. That is the size set most people end up checking first in a mixed toolbox.