Concrete Post Hole Calculator

How it's calculated

The calculator treats each post hole as a cylinder — π × (diameter ÷ 2)² × depth — with the post's own rectangular cross-section subtracted out, since concrete only fills the space around the post, not the space the post occupies. That per-hole volume is multiplied by the quantity you enter, so a whole run of identical posts calculates at once instead of one hole at a time. A waste percentage is added on top to cover spillage and irregular hole walls, and the adjusted cubic yardage is multiplied by concrete's density to get the weight in tons. Bag counts divide the adjusted cubic feet by each bag's yield; ready-mix truckloads divide the adjusted cubic yards by the truck's capacity.

Worked example

Six fence posts, each set in a 10-inch-diameter hole 30 inches deep, with a nominal 4×4 post (actual 3.5" × 3.5") centered in each: per hole, the hole's cross-section is π × (5 ÷ 12)² = 0.545 ft², minus the post's footprint of (3.5 ÷ 12) × (3.5 ÷ 12) = 0.085 ft², leaving 0.460 ft² of concrete. At 30 inches deep, that's 0.460 × (30 ÷ 12) = 1.15 cubic feet per hole — × 6 holes = 6.90 cubic feet total. In cubic yards, that's 6.90 ÷ 27 = 0.26 yd³. Adding a 10% waste factor: 0.26 × 1.10 = 0.28 yd³. At 2.02 tons per cubic yard, that's 0.28 × 2.02 = 0.57 tons — or 0.55–0.57 tons across the 1.95–2.03 density range. In 60 lb bags, the adjusted 7.59 cu ft (6.90 × 1.10) needs 17 bags for all six holes combined.

How deep should it be?

Both the depth and diameter of a post hole are decided by factors this calculator doesn't know about — how deep your local frost line runs, how tall and heavily loaded the post above ground is, and what your local building code or fence/deck permit requires for embedment and clearance around the post. None of that is something a generic calculator should guess at; check with your local building department, a permit's engineering notes, or the International Residential Code's deck and fence provisions for the actual numbers your project needs. What this calculator does know is the geometry once you have those numbers: it treats a post hole as a cylinder with the post's own footprint subtracted out, since concrete only fills the gap around the post, not the space the post itself occupies. Enter the post's actual milled dimensions, not its nominal size — a nominal "4x4" post, for example, is actually 3.5" × 3.5" after milling, and using the nominal number overstates how much space the post takes up. Use the quantity field to calculate a whole run of identical posts at once instead of re-running the calculator per hole.

Common mistakes

• –Using a post's nominal lumber size (e.g. "4x4") instead of its actual milled dimensions (3.5" × 3.5") — the smaller actual size is what displaces concrete, so nominal sizes overstate the post's footprint and understate how much concrete you need.
• –Guessing at hole depth or diameter instead of checking local frost depth and your code's embedment requirements — both are set locally and by the post's load, not by a generic rule of thumb.
• –Forgetting to subtract the post's own volume. Only the gap between the post and the hole wall needs concrete; skipping the subtraction overstates the order, sometimes significantly for a large post in a narrow hole.
• –Running the calculator once and multiplying the result by hand instead of using the quantity field — small rounding differences per hole add up across a full run of posts.
• –Pouring directly onto undisturbed dirt at the hole bottom without a gravel drainage layer — standing water at the base of a post hole is a common cause of post rot and frost heave; check your local code for whether a drainage layer is required.

Where the density figure comes from

Unlike the landscape materials on this site, concrete is a manufactured product with a tightly controlled mix, so its density varies far less than gravel, sand, or mulch — and the same figure applies whether it's poured as a slab, a footing, or a post hole. CEMEX, a major ready-mix producer, puts a solid unbroken pour at about two tons (4,000 lbs) per cubic yard. ACI 318 and ASTM C138 define normal-weight concrete at 145–150 lb/ft³ — ACI 318 uses 150 lb/ft³ as its default structural value — which works out to 3,915–4,050 lbs (1.96–2.03 tons) per cubic yard. The 1.95–2.03 ton/yd³ range here reflects that normal-weight span, not moisture variation. Lightweight and heavyweight concrete mixes fall outside this range and aren't covered by this calculator. We use 2.02 tons/yd³ as the display figure, drawn from:

• CEMEX — Ready-Mix Concrete Calculator
• ACI 318 & ASTM C138 — normal-weight concrete density, 145–150 lb/ft³

Read more in our density methodology.

FAQ

How deep should a post hole be?

It depends on your local frost depth and the post's height and load — your local building department or the International Residential Code's deck and fence provisions size that for your specific situation. There's no universal number here; check with your building department before you dig.

What diameter should a post hole be?

It depends on your post's size and your local code's clearance requirements around it — typically enough room for concrete to fully surround the post plus whatever margin your code specifies. Check your local building department rather than assuming a number; this calculator only converts whatever diameter you enter into a volume.

Why does the calculator subtract the post's volume from the hole?

Because concrete only fills the space around the post, not the space the post itself occupies — pricing or ordering based on the hole's full cylinder volume overstates what you need. Enter the post's actual milled dimensions for this to be accurate: a nominal "4x4" is actually 3.5" × 3.5", not a full 4 inches square.

Can I calculate concrete for multiple posts at once?

Yes — enter the quantity of identical holes in the Quantity field and the calculator multiplies the per-hole volume automatically. This only works for holes that are all the same diameter, depth, and post size; calculate different post sizes separately and add the results together.

Should I use bags or ready-mix for post holes?

It depends on scope. A handful of post holes are usually small enough that bags make sense on their own. A long fence run with many posts adds up fast — past roughly half a cubic yard total, ready-mix delivery is generally cheaper and far less labor than mixing bag after bag by hand for every hole.

Why does concrete's weight vary so little compared to gravel or sand?

Concrete is a manufactured product with a tightly controlled mix ratio, so its density is far more consistent than a quarried or organic material's. The range on this site comes from ACI 318 and ASTM C138's normal-weight concrete specification (145–150 lb/ft³), the same figures used for the slab and footing calculators, not from moisture swings the way gravel, sand, or mulch vary.

How many ready-mix truckloads will I need?

Divide your adjusted cubic yardage by the truck's capacity — standard ready-mix trucks carry about 10 cubic yards per full load. The calculator above uses 10 cubic yards as a default and rounds up, since a partial load still requires dispatching a truck, and most plants charge extra for small or split loads.