What Size Is the Nut on a Car Battery?

On most passenger cars with top-post batteries, the terminal clamp nut/bolt takes a 10 mm wrench or socket. GM-style side-post batteries typically use an 8 mm hex-head bolt (with 5/16-18 threads). Some vehicles use 12 mm or 13 mm metric hardware—or 7/16 in or 1/2 in SAE—for terminal clamps and battery hold-downs, so it’s smart to verify before wrenching.

Why Battery “Nut Size” Varies

When people ask about a car battery’s nut size, they usually mean the hex size on the terminal clamp fastener. However, there are two common places you’ll encounter nuts or bolts on a battery: the terminal clamps (which secure the cables to the posts) and the hold-down hardware (which secures the battery to the tray). Sizes differ by automaker, battery type (top-post vs. side-post), and region (metric vs. SAE).

Common Sizes by Battery and Hardware Type

Top-Post Battery Terminals (Most Cars)

Top-post batteries—the kind with visible positive and negative posts on top—use a clamp tightened by a small bolt and nut. On many Asian and European vehicles, as well as numerous American models, a 10 mm wrench fits the clamp hardware. Depending on the vehicle and aftermarket parts, you may encounter 12 mm or 13 mm metric, or older SAE sizes like 7/16 in or 1/2 in.

Side-Post Battery Terminals (Common on GM)

Side-post batteries use a bolt that threads horizontally into the battery case. The head is typically 8 mm hex, with a 5/16-18 thread. Do not over-tighten these; the threaded insert is in the battery case, and overtightening can crack the case or strip the insert.

Battery Hold-Down/Retainer Hardware

The hold-down bracket or J-bolt nuts that secure the battery to the tray are very often 10 mm on many cars, though 12 mm and 13 mm are also common. Some trucks and older vehicles may use 1/2 in SAE. As with terminal hardware, verify on your specific vehicle.

Typical Socket/Wrench Sizes You Might Need

If you’re preparing tools for a battery service, carrying a small range of sockets covers most scenarios. Here are the sizes that most commonly fit battery terminal and hold-down hardware.

• 10 mm: Most common for top-post terminal clamps and many hold-down nuts
• 8 mm: Common for GM-style side-post terminal bolts
• 12 mm and 13 mm: Sometimes used for clamps and hold-downs
• 7/16 in and 1/2 in: SAE sizes you may encounter on older or domestic vehicles

Having 8–13 mm metric and 7/16–1/2 in SAE sockets on hand will usually ensure you can remove both the terminals and the hold-down without a second trip for tools.

How to Confirm the Correct Size on Your Vehicle

If you’re unsure which size your car uses, a quick check avoids rounding fasteners or over-tightening. Follow these simple steps to identify the correct nut or bolt size for your battery hardware.

1. Inspect the terminals: Determine if you have top-post (vertical posts) or side-post (bolts into the side of the battery).
2. Test-fit sockets: Start with 10 mm on top-post clamps or 8 mm on side-post bolts; step up or down as needed for a snug fit.
3. Measure if necessary: Use a caliper or measure across the flats of the hex to confirm the size if sockets feel close.
4. Check the manual or service data: Some OEMs publish terminal and hold-down sizes and torque specs.

A snug, slip-free fit ensures you don’t round the fastener, especially important on corroded terminals or when space is tight near the battery.

Torque and Safety Notes

Battery hardware is easy to over-tighten. Following typical torque ranges helps prevent damage and ensures reliable electrical contact.

• Top-post clamp pinch bolt: about 5–7 N·m (44–62 in·lb)
• Side-post terminal bolt: about 12–15 N·m (106–133 in·lb), often specified near 13 N·m
• Hold-down bracket hardware: commonly 7–12 N·m (62–106 in·lb), but check your vehicle’s spec
• Safety practice: Always disconnect the negative (–) terminal first and reconnect it last; avoid touching positive to ground with tools

Using a small torque wrench is ideal, but if you’re working by feel, tighten until secure and then add only a small additional turn—never force battery fasteners.

Summary

Most car battery terminal clamp nuts/bolts use a 10 mm wrench on top-post batteries, while GM-style side-post batteries typically use an 8 mm hex bolt (5/16-18 thread). Hold-down hardware is often 10 mm but can be 12 mm, 13 mm, or SAE sizes like 7/16 in or 1/2 in. Verify on your specific vehicle, use proper torque, and follow safe disconnection procedures to avoid damage or injury.

What size nut driver for car battery?

A 10mm wrench is the most common size, fitting the hexagonal nuts typically welded on battery terminal covers. Using the wrong wrench size can lead to stripped nuts, damaged terminals, and potentially hazardous situations when working with your vehicle’s electrical system.

What size socket is needed for a car battery?

This means you’ll need a crescent wrench, an adjustable wrench, vise pliers or a socket wrench. Most battery terminals will come with either 10 mm or 12 mm (0.4 or 0.5 inches) socket-sizes. Grab another pair of adjustable pliers to hold the bolt head in place as well.

What size are car battery nuts?

Car battery nuts vary in size, but 10mm and 5/16 inch are very common for top post terminals, while 3/8 inch is often used for side terminal bolts. It’s best to use a wrench or socket that fits the specific nuts on your vehicle’s terminals to avoid damage. 
Common Sizes for Top Post Terminals

• 10mm: is the most frequent size for the hexagonal nuts on top posts, especially on vehicles like Toyotas. 
• 5/16 inch: is another common size, often seen with corresponding 5/16-18 bolts. 

Common Sizes for Side Post Terminals 

• 3/8 inch: is frequently used for side terminal bolts, such as those found on General Motors vehicles.

Important Considerations

• Wrench Size: Opens in new tabUse the correct size wrench to prevent stripping the nut or damaging the terminal. 
• Vehicle-Specific: Opens in new tabThe exact size can differ based on your car’s make and model. 
• Hold-Down Bolts: Opens in new tabThe bolts that secure the battery in place may be a different size from the terminal nuts. 
• Corrosion Resistance: Opens in new tabSome terminals are made from corrosion-resistant materials like tin-plated brass or zinc-plated steel. 

What size nut do I need to remove a battery?

The most common sizes for these terminals are 10 mm and 13 mm. These tools help in loosening the nuts that secure the battery cables. Always start by disconnecting the negative terminal.