Post Hole Auger Buying Guide: Bit Sizes, Drives & Depth

Setting a fence line, hanging a gate, planting a windbreak, or pouring deck footings all go faster when you match the right post hole auger to your machine and your ground. But the questions pile up quickly: what bit diameter, earth teeth or rock teeth, a PTO or hydraulic drive, and how deep can you actually get before you hit frost or bedrock?

This guide is written for small-farm owners, ranchers, homesteaders, and compact-tractor and skid-steer owners who want to buy once and dig clean holes. We'll walk through auger bit sizing, drive selection, earth versus rock bits, hex sizes, hydraulic flow, and depth — in plain English, with honest trade-offs.

Whether you're shopping a post hole digger for a tractor or a skid steer auger, the goal is the same: enough torque, the right bit, and a hole your posts drop straight into.

What size auger do I need for fence posts?

The most common question buyers ask is simply "what size auger for fence posts?" The sizing rule most fence crews use is straightforward: the hole should be roughly three times the diameter of the post. That leaves room for backfill or concrete to grab the post and keep it from heaving or leaning. Set posts in tamped soil and you can lean toward the smaller end; set them in concrete and go up a size so there's a proper collar around the post.

Auger bits are sold in common diameters — 6", 9", 12", and 18" being the everyday sizes — with larger 24"+ bits available for footings and piers. Here's how they line up with real jobs:

When in doubt, a 9" bit is the single most versatile size for farm and ranch fencing. If you know you'll build a lot of corners and gates, keep a 12" on hand too — swapping bits is quick once your drive is mounted.

Earth bits vs. rock & heavy-duty bits: what if I hit rock?

Not all auger bits cut the same ground. This is where a lot of first-time buyers get burned.

• Earth / dirt bits use a fishtail point or single pilot with cutting blades. They fly through loam, clay, sand, and soft soil, and they're the most affordable choice.
• Rock & heavy-duty bits add carbide-tipped teeth, a rock pilot, and heavier flighting so they can chew shale, caliche, frozen ground, roots, and rocky mixes without folding over.

"Hitting rock" is the number-one frustration on tractor forums, and the honest answer is that no auger loves solid bedrock. What a good rock-and-earth combination bit does is let you keep working in the rocky, root-bound, and hardpan soils that stop a plain dirt bit cold. Most quality drives use a shear pin or shear bolt that snaps to protect the gearbox if the bit catches — carry spares. When you jam on a boulder, back the bit out, clear it, and re-drill; on truly rocky sites, an excavator's crowd force or a rock bar helps break things loose so the auger can finish the hole.

PTO vs. hydraulic vs. 3-point: choosing an auger drive

The bit does the cutting, but the drive decides how much torque and down-pressure you have. Pick the drive around the machine you already own.

A note on hydraulic flow and hex sizes. For any hydraulic auger drive, match the drive's flow rating to your machine — a low-flow drive on a low-flow mini, a standard-flow drive on your skid steer. With augers, torque matters more than speed: a drive that spins fast but stalls in clay won't finish holes. On the connection end, the 2" hex is the common standard between skid-steer auger drives and their bits (larger drives may use a 2 9/16" hex or a round shaft with a pin). Before you buy, confirm your bit's hex matches your drive's output shaft — a 2" hex bit needs a 2" hex drive.

How deep should the hole be — and where do extensions come in?

Two rules govern depth. First, a good rule of thumb is to set at least one-third of the post's total length below ground — a 6-ft-tall fence usually means a 2–3 ft hole. Second, and just as important in cold country, the base of the post should sit below the frost line so seasonal freeze-thaw can't heave it out. Frost depth ranges from near zero in the deep South to 40"–48" or more across the northern states; your local building department can confirm the number for your county.

Most auger bits offer roughly 3–4 ft of usable flighting. When you need to go deeper — beating a hard frost line, setting tall corner posts, or drilling footings — a drive extension adds length between the drive and the bit. Keep the hex size consistent (a 2" hex extension pairs with a 2" hex drive and bit) and remember that deeper holes need more down-pressure and a steady, patient feed so you don't bind the bit.

Common mistakes to avoid

• Oversizing the bit for the machine. A big bit on a low-flow drive just bogs and stalls. Size the bit to your torque and flow, not the other way around.
• Running an earth bit in rocky ground. If your county is known for rock, caliche, or hardpan, buy a rock-and-earth bit from the start.
• Ignoring hex/shaft compatibility. The most common return we see is a bit that won't fit the drive. Match hex to hex.
• Forgetting to call 811 first. Always have underground utilities located before you dig — it's free and it's the law in most states.
• Buying on price alone. Thin flighting and soft teeth wear fast. Heavier steel and carbide pay for themselves over a long fence.

Featured post hole auger picks

Stinger 6"–48" Auger Bit — 60" Rock & Earth (2" Hex)

$1,050 — carbide rock-and-earth teeth, 60" flighting, available in diameters from 6" up to 48".

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Stinger Auger Drive Extension — 2" Hex (2–8 ft)

$405 — add depth to beat the frost line and set tall posts; 2" hex, available in 2–8 ft lengths.

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Both are built for skid-steer and compatible hydraulic drives running a 2" hex. Not sure a Stinger bit fits your loader? Check our tractor & attachment compatibility guide or just ask us.

Frequently Asked Questions

What size auger do I need for a 4x4 fence post?

For a standard 4x4 wood post, a 9" auger bit is the go-to size — it leaves room for concrete or tamped backfill around the post. If you're just tamping in firm soil, a 6" bit can work for line posts, while corner, gate, and brace posts usually call for a 12" bit.

Can a post hole auger dig through rock?

A rock-and-earth bit with carbide teeth will grind through shale, caliche, roots, and rocky mixes, but no auger drills solid bedrock. When you hit a large rock, back the bit out, clear it, and re-drill; an excavator's crowd force or a rock bar helps break stubborn stone loose so the auger can finish the hole.

What's the difference between a 3-point PTO and a skid steer auger?

A 3-point PTO post hole digger runs off your tractor's PTO and needs no hydraulics, making it great for long farm fence lines — though its down-pressure is limited to the auger's own weight. A skid steer auger uses hydraulic flow for more torque and down-force, which makes it faster and better in hard or mixed soils.

How much hydraulic flow do I need for a skid steer auger?

Most standard-flow skid steers run 17–24 gpm, which suits the majority of auger drives and bits from 6" to 18". Compact and mini machines are low-flow (roughly 9–13 gpm) and need a low-flow drive. With augers, torque matters more than raw speed, so match the drive to your machine's flow rating.

How deep should a fence post hole be?

A good rule of thumb is to bury at least one-third of the post's length, and to set the base below your local frost line so freeze-thaw won't heave it. Frost depth runs from near zero in the South to 48" or more up north. A drive extension lets you reach that depth when a standard bit runs short.

What is a 2" hex on an auger?

The 2" hex is the common hexagonal connection between a skid-steer auger drive and its bit or extension. It's a standard size, so a 2" hex bit pairs with a 2" hex drive and a 2" hex extension. Always confirm your drive's output shaft before ordering a bit.

Related reading: Augers & Drilling collection · Tractor Attachments Buying Guide for Small Farms & Homesteads

By the Farmer Direct Supply Team