Most engine oil drain plugs use a 14 mm hex socket, which is the standard on many modern cars. Some vehicles require other sizes such as 12 mm, 17 mm, or 19 mm, and a few use an internal Allen recess. Always verify with your owner's manual or service guide for your exact make and model.
Understanding what you’re dealing with
Drain plugs come in different designs across automotive and household equipment. The tool you need depends on the head design (hex, six-point, or Allen) and the across-flats measurement. The following guide outlines the sizes you’re most likely to encounter on typical vehicles.
Head types you may see
Oil pans, transmissions, and coolant drains may use hex-head bolts, bolts with a recessed Allen socket, or Torx heads. Identifying the head type helps you choose the right socket or key.
Common socket sizes you might encounter
Before listing the sizes, note that exact size depends on the vehicle. The following sizes appear most often on passenger vehicles and light trucks.
- 14 mm across-flats hex: The standard for the majority of modern engine oil drain plugs. Use a 14 mm socket; some require a 12-point variant if space is tight.
- 12 mm: Less common but found on some smaller engines or specific makes. A 12 mm socket may be needed.
- 17 mm and 19 mm: Larger sizes used on certain European or high-performance engines and some transmission drains. Check the manual if you encounter space constraints.
- Allen/hex recessed (6 mm or 8 mm): Some drain plugs use an internal hex; an Allen wrench or a deep socket in the correct size is required.
Having a small set that covers 12–19 mm, plus a few Allen keys, is a practical approach for most DIYers. If a plug seems to require an unusual size, consult the vehicle’s service documentation or the parts counter for the exact spec.
How to determine your exact size
Check the owner's or service manual
The definitive source for your specific vehicle, oil type, and drain plug size is the manufacturer’s documentation. Look up the maintenance section for the engine oil drain plug specification.
Inspect the drain plug head
Visually identify whether the head is a hex (across flats), a recessed Allen (internal), or Torx. This determines whether you need a standard socket, an Allen key, or a Torx bit.
Measure or test carefully
If you’re unsure, start with the most common size (14 mm) and move up or down in 1 mm increments, or try standard SAE sizes if you suspect a non-metric fit. Do not force a socket onto a plug to avoid rounding or stripping. If space is tight, a thin-walled or deep socket may help.
If you can’t determine the size, bring the plug or the oil drain plug with you to an auto parts store; staff can confirm the exact size for your vehicle.
Practical tips and safety
Always use the correct size socket to prevent damage, apply the correct torque when reinstalling (as per manual), and replace any washer or gasket as recommended. For older vehicles, follow the manufacturer’s guidance on anti-seize or thread sealant, if applicable.
Summary
There is no universal answer to drain plug size because it depends on the vehicle, the plug head design, and whether the plug uses a standard hex, a recessed Allen, or a Torx. The most common size for car engine oil drains is 14 mm, but you may encounter 12 mm, 17 mm, or 19 mm, and some plugs require an Allen key. Always verify with the manual or measure the plug head, and keep a small socket set on hand to cover the possibilities.