A standard 4-inch / 100 mm diameter post in a 10-inch / 254 mm wide hole requires roughly 0.8 cubic feet / 0.023 m³ of concrete per foot of depth — but the actual volume depends on your hole diameter, hole depth, and post size. Get those three numbers right, and waste drops to near zero.
How to calculate post hole concrete volume
The formula treats the hole as a cylinder and subtracts the volume of the post itself:
Volume = π × r² × depth − post cross-section × depth
Where r is the radius of the hole. In practice, the post volume is small enough that most contractors skip the subtraction for wood posts — it matters more for large-diameter steel or concrete posts.
Worked example: a 10-inch / 254 mm diameter hole, 36 inches / 914 mm deep, for a 4×4 (3.5-inch / 89 mm actual) wood post.
Hole radius = 5 inches / 127 mm = 0.417 ft / 0.127 m
Hole volume = π × (0.417)² × 3 ft = 1.636 ft³ / 0.0463 m³
Post cross-section = 3.5 × 3.5 = 12.25 in² = 0.0851 ft²
Post volume = 0.0851 × 3 = 0.255 ft³ / 0.0072 m³
Net concrete volume = 1.636 − 0.255 = 1.38 ft³ / 0.039 m³
That is just under one 60 lb bag. For a fence project with 20 posts, that is 20 bags minimum before adding a 10% waste factor.
Skip the manual calculation — the post hole concrete calculator handles this for any hole size, depth, and post dimension, and outputs total bags across multiple holes at once.
Post hole concrete volume by common hole and depth combinations
The table below shows net concrete volume (after subtracting a standard 4×4 post) for the most common post hole configurations.
| Hole diameter | Depth | Net volume (ft³) | 60 lb bags needed |
| 8 in / 203 mm | 24 in / 610 mm | 0.62 | 1 |
| 8 in / 203 mm | 36 in / 914 mm | 0.93 | 2 |
| 10 in / 254 mm | 36 in / 914 mm | 1.38 | 2 |
| 10 in / 254 mm | 48 in / 1219 mm | 1.84 | 3 |
| 12 in / 305 mm | 42 in / 1067 mm | 2.46 | 4 |
| 12 in / 305 mm | 48 in / 1219 mm | 2.82 | 4 |
| 16 in / 406 mm | 48 in / 1219 mm | 5.10 | 7 |
A 60 lb bag yields approximately 0.45 ft³ / 0.013 m³. An 80 lb bag yields roughly 0.60 ft³ / 0.017 m³. The bag counts above assume 60 lb bags with a 10% overage built in.
How deep do post holes need to be?
Depth is determined by two factors: frost depth and the structural load the post carries.
In cold climates, the bottom of the footing must sit below the local frost depth or frost heave will push the post up over successive freeze-thaw cycles. Frost depth ranges from 0 in parts of Florida and southern California to over 60 inches / 1524 mm in Minnesota and northern Canada. The frost depth and footing depth calculator gives region-specific depths by US zip code and Canadian province.
The rule of thumb used by most fence and deck contractors is to set posts at least one-third of their above-ground height into the ground. A 6-foot / 1.83 m fence post needs a minimum 2-foot / 610 mm embedment, but in freeze-prone zones that often extends to 3–4 feet / 914–1219 mm.
For structural applications — deck posts carrying live loads, carport columns, pergola supports — engineered footings sized by a structural engineer replace rule-of-thumb depths. In those cases, the concrete column and pier calculator is the more appropriate tool.
Common mistakes when filling post holes with concrete
Adding too much water to the mix. Contractors and DIYers frequently add extra water to make bagged mix easier to pour into the hole. Every additional pint of water above the recommended ratio raises the water-cement ratio and drops final compressive strength. A mix that pours like soup may hit only 2,000 psi / 14 MPa instead of the 4,000 psi / 28 MPa printed on the bag. Add water conservatively — the mix should be stiff enough to hold a mound shape before it settles into the hole.
Skipping the gravel drainage layer. Placing concrete directly on undisturbed soil in clay-heavy ground traps moisture against the post base. The correct approach is 4–6 inches / 100–152 mm of compacted gravel at the bottom of the hole before pouring. This allows water to drain away from the post and reduces post rot in wood applications.
Undersizing hole diameter for the post. A hole only slightly wider than the post leaves no room to plumb the post accurately before the concrete sets, and the thin concrete shell around the post lacks the bearing area to resist lateral loads. The minimum hole diameter is 3× the post width for wood posts — so a 4×4 / 89 mm post needs at minimum a 12-inch / 305 mm hole for structural applications (though 10 inches / 254 mm is accepted for light fence work).
Pouring in cold weather without precautions. Fresh concrete that freezes before reaching initial set will not gain strength and will be permanently weakened. Below 40°F / 4°C, use warm mix water and cover the exposed hole opening with insulating blankets for at least 24–48 hours. Below 20°F / -7°C, postpone the pour or use a heated enclosure.
Related calculators you might need
Once you know how many cubic feet of concrete each hole requires, the concrete bags calculator converts total volume into exact bag counts across 40 lb, 60 lb, and 80 lb sizes — useful when buying in bulk from a home centre. For larger post and column footings poured from a ready-mix truck, the concrete truck load calculator helps you determine whether you need a full load or will be paying a short-load surcharge. If your project also includes a deck or slab over the posts, the concrete footing calculator handles pad-style and continuous footings. For total project budgeting, the full concrete project estimator rolls materials, labour, and delivery into a single cost figure.
Frequently asked questions
How many bags of concrete do I need for a fence post?
For a standard 8-inch / 203 mm diameter hole at 30 inches / 762 mm deep, one 60 lb bag is usually sufficient. For a 10-inch / 254 mm hole at 36 inches / 914 mm deep, plan on two 60 lb bags. These figures include a small overage allowance. Multiply by your total post count and add 10% for the full project quantity. The post hole concrete calculator gives exact figures for any combination of hole size and depth.
What is the right concrete mix for post holes?
A minimum 3,000 psi / 21 MPa mix is standard for fence posts; 4,000 psi / 28 MPa is preferred for deck and structural posts. Most bagged fast-setting mixes (Quikrete Fast Setting, SAKRETE, and equivalents) reach 4,000 psi at 28 days. Do not use general-purpose sand mix — it lacks the coarse aggregate needed for load-bearing footings.
Can I pour dry concrete into a post hole without mixing first?
Yes, for fast-setting mixes specifically labelled for dry-pour use. You pour dry mix into the hole, then add water on top; the mix absorbs moisture from the surrounding soil and the added water. This technique works in moist soil conditions. In dry, sandy soil the mix may not hydrate fully — in that case, mix in a bucket first. Standard bagged concrete should always be pre-mixed before pouring.
How long before I can hang a gate on a concrete-set post?
Fast-setting concrete reaches sufficient strength to accept light loads (fence rails, gate hardware) in 4 hours at 70°F / 21°C. Standard concrete needs 24–48 hours before loading. Full design strength takes 28 days regardless of mix type. The concrete curing time estimator adjusts cure timelines by temperature, so you can plan accurately in cold or hot conditions.
How deep should a post hole be for a 6-foot fence?
The structural minimum is one-third of above-ground height — 2 feet / 610 mm for a 6-foot / 1.83 m fence. In frost-prone climates, the code minimum is often 3–4 feet / 914–1219 mm to clear the frost line. Check your local building code; most jurisdictions require permits and inspections for fence installations on boundary lines.
Does the shape of the hole bottom affect how much concrete I need?
Yes. A flat-bottomed hole requires less concrete than a rounded or irregular bottom. Hand-dug holes in rocky ground often have irregular profiles that can add 15–20% more volume than the calculated cylinder. Flare the bottom of structural footings — a bell shape 1.5× the hole diameter — to increase bearing area in soft soil, but account for that added volume when ordering bags.
Concrete is the most widely used building material on Earth, yet some of the most important decisions behind it are still made with rough guesses, outdated spreadsheets, and conflicting advice from random websites.
We thought that was a problem worth fixing.
Every calculator and guide on this platform exists for one reason: to replace uncertainty with reliable numbers. Whether you’re estimating a slab, planning footings, calculating reinforcement, comparing mix designs, or budgeting a project, the goal is the same—help you make decisions with confidence before concrete is poured.
We don’t publish calculators for the sake of having calculators. Every formula is researched, tested, and built around real construction requirements. Every guide is written to answer practical questions that contractors, builders, engineers, and homeowners actually face on job sites—not questions invented for search engines.
Because concrete is unforgiving.
A small mistake in volume estimation can waste thousands of dollars. An undersized footing can create structural problems. Incorrect reinforcement calculations can compromise performance. Once concrete is placed, many mistakes become expensive—or impossible—to undo.
That’s why accuracy matters.
Our approach is simple: start with the engineering, verify the calculations, explain the reasoning, and present the results in a way that anyone can use. No inflated claims. No copied content. No generic formulas stripped of context.
Just practical tools, clear guidance, and calculations designed to solve real-world construction problems.
The internet has plenty of articles about concrete.
We’re building a resource that helps people work with it.