Post Hole Concrete Calculator
* Prices are approximate US retail averages (May 2026). Always add 10% buffer for spillage. Fast-Setting (yellow bag) sets in 20-40 min without pre-mixing.
Frost Depth Guide by Region
Footings must extend below the frost line to prevent heaving. These are minimum depths — always verify with your local building department.
| Region | Frost Depth | Min Footing Depth | Risk Level |
|---|---|---|---|
| 🇺🇸 US South (Zone 8-10) | 0–12" | 12" min | Low |
| 🇺🇸 US Mid-Atlantic (Zone 6-7) | 24–36" | 36–42" | Medium |
| 🇺🇸 US North (Zone 4-5) | 42–48" | 48–54" | High |
| 🇺🇸 US Extreme North (Zone 3) | 60–72" | 72–84" | Very High |
| 🇬🇧 UK | 450mm (18") | 450mm | Low |
| 🇨🇦 Canada South (ON, BC) | 900mm (36") | 1000mm | Medium |
| 🇨🇦 Canada North (AB, MB) | 1500mm (60") | 1600mm | High |
| 🇦🇺 Australia | 200mm (8") | 300mm | Very Low |
Expert Guide: Deck Footings & Concrete in 2026
The concrete footing is the foundation of every deck — get this wrong and nothing above it matters. In 2026, the most common reason for deck permit failures and structural problems is under-sized or under-depth footings. This guide explains exactly what you need and why.
How Many Bags of Quikrete Do I Need?
The formula is simple: calculate the volume of your hole in cubic feet, subtract the volume of the post, then divide by the yield of your bag size. An 80lb bag yields 0.60 cubic feet of mixed concrete. A 60lb bag yields 0.45 cubic feet. A standard 12-inch diameter, 48-inch deep hole holds approximately 3.14 cubic feet — minus a 4x4 post (0.07 cu ft) = 3.07 cu ft. That's about 5 bags of 80lb Quikrete. Our concrete post hole calculator above does this automatically for every hole size and post combination.
Quikrete Fast-Setting vs Standard: Which to Use?
Quikrete Fast-Setting (yellow bag) — pour dry into hole, add water on top, sets in 20-40 minutes. No mixing required. Ideal for fence posts and deck posts where you want to continue framing the same day. Costs about $8-9 per 50lb bag. Standard Quikrete (gray bag) — must be pre-mixed in a bucket or mixer, sets in 24-48 hours. Stronger (4000 PSI vs 4000 PSI for fast-set at 28 days), but both are acceptable for deck post footings per IRC. Sakrete is a direct competitor to Quikrete with similar performance and pricing — use whichever is available at your local Lowe's or Home Depot.
Footing Size Rules (IRC 2026)
The footing diameter must be at least 3 times the post width. For a 4x4 post (3.5" actual), minimum footing diameter is 10.5" — use 12" for margin. For a 6x6 post (5.5" actual), use 16-18". The footing base must bear on undisturbed soil or bedrock. In frost-susceptible soils, use a bell-bottom or flared footing — widen the base by 50% to prevent frost heave from pushing the footing up.
Frost Depth: The Most Critical Variable
Footings that don't extend below the frost line will heave in winter, cracking the deck structure. In northern US states (Zone 4-5), frost depth is 42-48 inches — your footing must go deeper than this. The frost line map varies significantly even within states: check with your local building department or the USDA Plant Hardiness Zone Map for your exact address. Always add 6 inches below the frost line for safety margin.
Post Base Hardware vs Buried Posts
Modern best practice (and increasingly required by code) is to use adjustable post base hardware (Simpson Strong-Tie ABA66 or similar) rather than burying the post directly in concrete. Buried posts rot at the concrete/soil interface within 10-15 years even with pressure treatment. Post bases keep the post end grain above the concrete, extend post life to 30+ years, and allow post replacement without breaking up the footing.
How Many Posts Does Your Deck Need?
Post spacing is governed by the beam size above. A doubled 2x10 beam can typically span 8 feet between posts. A doubled 2x12 can span up to 10-12 feet. Most residential decks use posts every 6-8 feet along the beam lines. Calculate your total post count first, then use our concrete footing calculator to get the exact bag count for your entire project.