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Post Hole / Pier Calculator

Q: Should I use fast-set or regular concrete for post holes?

Fast-set (QUIKRETE Fast-Setting or similar) is popular for post holes because you can pour dry mix into the hole and add water — no mixing required. Standard 60 lb or 80 lb mix must be mixed in a wheelbarrow or mixer first. Both yield similar strength (3,000–4,000 psi). Fast-set costs more per bag.

Q: What diameter hole auger should I use?

General rule: hole diameter should be 3 times the post width. A 4x4 post (3.5 in actual) needs a minimum 10-inch auger; a 6x6 post (5.5 in actual) needs at least 12 inches. For fence posts, 8-inch auger is often sufficient. Check local codes — many require a minimum 8-inch diameter for structural posts.

Concrete bags and volume for post holes, deck piers, and fence posts

0 Bags (80 lb)

0 Bags (60 lb)

0.0 Total cu ft

0.00 Total cu yd

Hole Dimensions

Number of holes

Hole diameter

Hole depth

Concrete & Post

Concrete type

Post size

Waste factor

Results

0 bags (80 lb)

0.0 Total cu ft

0.00 Total cu yd

0 80-lb bags

0 60-lb bags

0.0 Cu ft / hole

0 Bags / hole

Ready-mix order: — (minimum order typically 1 CY)

Hole Cross-Section

Concrete Volume per Hole (cu ft)

Material Checklist

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Concrete bags (80 lb) 0 bags
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Recommended auger —
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Gravel base (3" layer) 0.0 cu ft
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Approximate water 0 qts

Frost Depth by US Region

Region	Climate Zone	Frost Depth	Example Cities
Deep South / Hawaii	Zone 1–2	0–12"	Miami, New Orleans, Houston
South / Southwest	Zone 3	12–18"	Atlanta, Dallas, Phoenix
Mid-Atlantic / Pacific NW	Zone 4	30–42"	DC, Baltimore, Seattle, Portland
Midwest / Ohio Valley	Zone 5	42–54"	Indianapolis, Columbus, Kansas City
Great Lakes / Northeast	Zone 5–6	42–54"	Chicago, Detroit, Cleveland, Boston
Northern Plains	Zone 6–7	54–72"+	Minneapolis, Fargo, Duluth
Mountain West	Zone 6–7	48–60"	Denver, Salt Lake City, Billings
Alaska / Extreme North	Zone 7–8	60–100"+	Anchorage, Fairbanks

Rule of thumb: Dig the hole at least 6 inches below the local frost line. For a 42" frost line, use a minimum 48" deep hole. Always verify with your local building department — code minimums vary by jurisdiction.

Minimum Embedment Depth by Use

Application	Min Depth	Notes
Deck post (attached)	Frost + 6"	Must be below frost line per IRC R507
Deck post (freestanding)	Frost + 6"	Same standard — frost heave lifts both
Fence post (6 ft panel)	2 ft minimum	1/3 of total post length is a common rule
Pergola/arbor column	Frost + 6"	Wind uplift requires adequate depth
Mailbox post	18–24"	No structural load; frost depth still preferred
Sign post (large)	Frost + 6"	Wind load controls; engineer for large signs

Frost heave: Water in soil expands ~9% when frozen, generating forces of hundreds of pounds per square foot. Posts not anchored below the frost line will be pushed up and out of plumb each winter.

Deck Beam Span & Post Spacing (IRC R507)

Post Size	Max Post Spacing	Typical Beam	Notes
4×4	6–8 ft	Double 2×8 – 2×10	Limited to single-level decks ≤ 8 ft above grade (IRC)
4×6	8–10 ft	Double 2×10 – 2×12	Increased beam depth needed for longer spans
6×6	10–12 ft	Triple 2×10 or LVL	Required above 8 ft deck height in most codes
Round tube (6")	8 ft	Steel or LVL beam	Common for concrete piers / Sonotube forms
Round tube (8")	10–12 ft	Steel beam or LVL	Engineered connections required

Important: These are general guidelines. Always verify against IRC Table R507.5 and your local amendments. Actual span depends on lumber species, grade, tributary width, and live/dead loads.

Fence Post Spacing

Fence Type	Post Spacing	Post Embedment
Wood privacy (6 ft panel)	8 ft o.c.	2 ft minimum (1/3 total length)
Wood privacy (8 ft panel)	8 ft o.c.	2.5–3 ft
Chain-link (4 ft)	10 ft o.c.	30 in (end/corner posts), 24 in (line posts)
Chain-link (6 ft)	10 ft o.c.	36 in (end/corner posts), 30 in (line posts)
Split rail (2-rail)	8–10 ft o.c.	24–30 in
Vinyl / composite	8 ft o.c.	Per manufacturer — typically 2–3 ft

Pergola & Structure Spacing

Structure	Typical Column Spacing	Recommended Post
Garden pergola	8–12 ft	4×4 or 6×6
Attached patio cover	8–10 ft	4×4 (single level)
Freestanding shade structure	10–14 ft	6×6 minimum
Carport / porte-cochere	10–16 ft	Engineered steel or 6×6+

How to Calculate Post Hole Concrete

Determine hole dimensions — note the number of holes, diameter (typically 3× the post width), and required depth (local frost line + 6 inches).

Calculate volume per hole — a cylindrical hole: V = π × (diameter ÷ 2)² × depth. Convert diameter from inches to feet by dividing by 12.

Subtract the post volume — for a 6×6 post the actual section is 5.5 × 5.5 in (0.21 sq ft). This calculator uses the full hole volume as a conservative estimate, which accounts for waste.

Add waste factor — add 10–15% for spillage, mixing losses, and uneven hole walls (not perfectly cylindrical after digging).

Choose bag size — 80 lb bags yield 0.60 cu ft; 60 lb bags yield 0.45 cu ft. For more than ~1 cubic yard total, consider ordering ready-mix concrete.

Key Terms

Frost Line — The maximum depth to which soil freezes in winter. Post holes must extend below this depth to prevent frost heave. Ranges from 0 inches in South Florida to 100+ inches in Alaska. Check your local building code or USDA frost depth map.

Frost Heave — The upward movement of soil (and anything embedded in it) caused by ice crystal formation during freeze-thaw cycles. Posts anchored above the frost line can be pushed out of plumb each winter, eventually failing.

Sonotube / Form Tube — A round cardboard tube form used to create cylindrical concrete piers and footings. Available in 6–24 inch diameters. Peeled away after concrete cures. Essential for piers that extend above grade.

Fast-Set Concrete — A concrete mix formulation that cures in 20–40 minutes after wetting. Can be poured dry into the hole and water added — no mixing required. Reaches structural strength in 4 hours vs. 24+ hours for standard mix.

Cubic Yard — The standard unit for ordering concrete. 1 cubic yard = 27 cubic feet. Most ready-mix suppliers have a minimum order of 1 cubic yard (~$150–$200) plus short-load fees. Bagged concrete is more economical for small jobs under ½ yard.

Auger — A drill-type tool used to bore cylindrical holes in soil. Hand augers handle 6–8 inch holes in loose soil; gas-powered one-person augers handle 6–12 inch holes; two-person or tractor-mounted augers handle 12–24 inch holes in hard soil.

Frequently Asked Questions

How deep should a deck post hole be?

Post holes must extend at least 6 inches below the local frost line to prevent frost heave. In most of the continental US this means 36–54 inches deep. Always check your local building code — many jurisdictions specify minimum depths in their residential code tables.

How many bags of concrete do I need per post hole?

A 12-inch diameter hole 3.5 feet deep holds about 3.3 cubic feet of concrete, requiring roughly 6 bags of 80 lb mix or 8 bags of 60 lb mix. Add a 10% waste factor for spillage and mixing losses. Use the calculator above for exact numbers based on your dimensions.

Should I use fast-set or regular concrete for post holes?

Fast-set concrete (QUIKRETE Fast-Setting or similar) is popular because you pour dry mix directly into the hole and add water — no mixing required. Standard 60 lb or 80 lb mix must be mixed in a wheelbarrow or mixer first. Both achieve similar strength (3,000–4,000 psi). Fast-set costs more per bag but saves significant labor time.

What diameter auger do I need for my post?

General rule: hole diameter should be 3× the post width. A 4×4 post (3.5 in actual) needs at least a 10-inch auger; a 6×6 post (5.5 in actual) needs at least 12 inches. For fence posts, 8 inches is often sufficient. Check local codes — many require a minimum 8-inch diameter for structural posts.

Do I need gravel at the bottom of a post hole?

A 3–6 inch layer of crushed stone at the bottom of each hole improves drainage and reduces wood rot for posts set in concrete. This is especially important in clay soils that hold moisture. The gravel base also provides a flat, compacted surface for consistent post depth.