A combination wrench set pairs a box end with an open end in matched sizes, and US sets sell in SAE fractional inches, metric millimeters, or both in one tray.
That is the most common way a home mechanic ruins a fastener that was never the problem.
Wrench sizes are not interchangeable between the two systems, and the sizes inside a set are not random. Knowing which fasteners you actually turn tells you which set to buy before you spend anything.
What Does SAE Mean On A Wrench?
SAE stands for the fractional inch sizes used on older US-made equipment, lawn tractors, and mower decks. The sizes get written as fractions, and that is the whole clue.
Mixed sets from GearWrench and Milwaukee currently list SAE coverage running from 1/4 in. up to 1 in., stepping through 5/16 in., 11/32 in., 3/8 in., 7/16 in., 1/2 in., 9/16 in., 5/8 in., 11/16 in., 3/4 in., 13/16 in., 7/8 in., and 15/16 in. Milwaukee’s 30-piece metric/SAE combination wrench set carries SAE pieces from 1/4 in. through 15/16 in.
Metric Wrench Sizes And Where They Show Up
Metric wrenches are measured in millimeters and dominate most equipment built outside the US. A mixed set commonly covers 6 mm through 19 mm, with larger sets extending to 24 mm, 30 mm, or 32 mm.
The pairing chart below is the one worth keeping on your phone. It shows the SAE and metric sizes that sit close enough to be confused, which is where damage happens.
| SAE Size | Closest Metric | Watch Out For |
|---|---|---|
| 1/4 in. | 6 mm | Slips easily on small bolts |
| 5/16 in. | 8 mm | Common on brake fittings |
| 3/8 in. | 10 mm | Most-confused pair of all |
| 7/16 in. | 11 mm | Feels tight but is not |
| 1/2 in. | 13 mm | Near-match, rounds corners |
| 9/16 in. | 14 mm | Use the right one or stop |
| 5/8 in. | 16 mm | Deceptively close |
| 11/16 in. | 17 mm | Widest gap in the chart |
| 3/4 in. | 19 mm | The classic mismatch |
How To Choose The Right Wrench Set
Build the set around the fasteners you turn most, then add the other system once you work on both domestic and imported equipment. Almost nobody starts out needing both.
Three things decide it:
- What you own.
- How often you turn two systems. If you own one of each, a mixed set saves a second purchase later.
- Whether the full range matters. GearWrench’s 28-piece 6-point set covers SAE 1/4 in. to 1 in. and metric 6 mm to 19 mm, while its 44-piece 12-point long pattern set stretches further.
One honest note on coverage: most mixed sets skip some sizes on purpose and include the ones people actually reach for. A 17 mm gap in a set lineup is normal, not a defect.
If you want a shortcut, our tested combination wrench set picks cover the sizes worth paying for.
When you are ready to check exact size lineups, GearWrench’s 28-piece set listing shows how current product pages spell out included sizes rather than following a separate sizing chart.
FAQs
Can I use an SAE wrench on a metric bolt?
You can sometimes turn it, but you should not. Sizes like 3/8 in. and 10 mm sit close enough to grip, and that closeness is what rounds the bolt corners under load. Pick the matching system instead, or you risk a fastener that no longer takes any wrench.
Is it better to buy metric or SAE first?
Buy the system that matches your equipment, not the one that is more popular. Buying both at once only makes sense if you regularly work on each.
Why do mixed sets skip some sizes?
Makers build sets around the sizes people use most, so a lineup can jump from 5/8 in. to 11/16 in. while leaving out a size nobody asks for. That gap is a design choice, not a missing piece, and it keeps the set lighter and cheaper without hurting real work.
References & Sources
- GearWrench. “28-Piece 6-Point Combination SAE/Metric Wrench Set.” Lists included SAE 1/4 in. to 1 in. and metric 6 mm to 19 mm sizes.
- Milwaukee Tool. “30PC Metric & SAE Combination Wrench Set.” Shows a mixed set with SAE sizes from 1/4 in. through 15/16 in.