Post Hole Concrete Calculator

When to use this calculator

Use this post hole concrete calculator when setting fence posts, deck supports, mailbox posts, gate posts, signs, or small pole structures. The calculator estimates the concrete that fills the space around the post, not the entire drilled hole.

• Fence lines where many similar post holes are being poured.
• Mailbox, gate, sign, or light posts with round drilled holes.
• Deck or porch posts where the concrete quantity is needed before buying bags.
• Comparing post hole diameter and depth choices before digging.

Input guide

• Number of posts is the count of similar holes being filled.
• Hole diameter is the auger or form diameter in inches.
• Post diameter or width is the approximate post size that displaces concrete.
• Hole depth is the concrete-filled depth in inches.
• Bag yield should match the concrete mix label.

Formula and estimation method

Step-by-step worked example

Twelve 12 inch holes, 30 inches deep, around 4 inch posts need about 20.9 cubic feet before waste after subtracting the post displacement. With 10% waste and 80 lb bags yielding 0.80 cubic feet, plan on about 29 bags.

Before you order materials

Use the Post Hole Concrete Calculator result as a starting quantity, then convert it into the way your supplier actually sells the material. Some stores sell by bag, each, pallet, strap, ton, cubic yard, or delivered load. Ask whether the quoted price includes delivery, fuel surcharge, pallet deposit, short-load fee, tax, unloading limits, or return rules. For outdoor projects, also check access for trucks, where material can be staged, whether the ground is firm enough for delivery, and how long the work area can stay open if weather changes.

For a safer order, write down the measurements used in the calculator, the waste percentage, the product size or density, and the final rounded quantity. Bring those notes to the supplier or contractor so they can catch unit mistakes before money is spent. If the project affects drainage, support, vehicle traffic, steps, walls, posts, or anything inspected by code, treat the calculator as a planning aid and verify the final material list with a qualified local source.

Common mistakes to avoid

• Using post height above ground instead of hole depth.
• Forgetting that gate posts, corner posts, and terminal posts may need larger holes.
• Assuming soil, frost depth, wind load, and fence height do not matter.
• Setting posts without checking alignment, bracing, and drainage.
• Buying too little concrete for deeper end posts or sloped ground.

Planning notes and limitations

• Check local frost depth, fence height, soil type, and post manufacturer guidance.
• Brace posts before pouring so they stay plumb while concrete sets.
• Consider gravel drainage at the bottom where appropriate for the post type and soil.
• Large structural posts may require engineered footings or code inspection.

This calculator estimates material quantity only. Follow local code, fence or deck plans, utility marking requirements, frost-depth guidance, and professional advice for structural or safety-critical posts.