A post hole concrete calculator tells you exactly how many bags of concrete mix to buy for fence posts, deck posts, mailbox poles, and similar footings. It computes the cylindrical hole volume using π × r² × depth, subtracts the wood or steel post that displaces concrete, and converts the net fill volume into a whole-bag count — so you never make an extra hardware store run mid-project.
Post Hole Estimator
Hole Dimensions
Common: 8" for 4×4, 10–12" for 6×6
Rule: 1/3 of post height, min 24"
Post Size (subtracted from hole volume)
Square cross-section assumed
Bag Size & Options
How to Calculate Concrete for Post Holes
Getting the concrete count wrong on a post hole project is surprisingly easy. A fence with 20 posts can need anywhere from 20 bags to 80 bags depending on hole size and depth — that is a significant budget swing. This post hole concrete calculator removes the guesswork by computing the exact cylindrical fill volume, subtracting the post, and rounding up to whole bags so you always know how many to put in your cart.
Step 1: Measure Your Hole Diameter and Depth
Hole diameter depends on your post size. A standard rule of thumb is to make the hole 2–3 times the width of the post. For a 4×4 post (3.5 inches actual), an 8-inch hole is typical. For a 6×6 post (5.5 inches actual), use a 10–12 inch hole. This collar of concrete grips the post firmly and resists lateral movement from wind and soil pressure.
Hole depth depends on the post height and your local frost line. The general rule is one-third of the post's total length below grade, with a minimum of 24 inches. In northern climates where the ground freezes, the bottom of the hole should extend 6 inches below the frost line — often 36–48 inches deep — to prevent frost heave from pushing the post up over winter.
Step 2: Select Your Post Size
The calculator subtracts the volume of the post itself from the hole volume because the post displaces concrete. For lumber posts, the actual (dressed) dimensions are smaller than the nominal name — a 4×4 is actually 3.5 × 3.5 inches, and a 6×6 is 5.5 × 5.5 inches. For round steel pipes, select the diameter that matches your pipe. If you are leaving the hole empty (a drainage test pit, for example), choose "No post" to calculate the full hole volume.
Step 3: Choose Your Bag Size
Concrete bags come in three common sizes. An 80 lb bag yields 0.60 cubic feet of mixed concrete and is the most economical option — you buy fewer bags and make fewer trips. A 60 lb bag yields 0.45 cubic feet and is easier to lift and carry solo, especially helpful on uneven terrain. A 50 lb bag yields 0.375 cubic feet and suits confined spaces or anyone working alone without a mixer. For most residential fence and deck projects, 80 lb bags are the go-to choice.
Step 4: Add a Waste Factor
Post holes drilled with an auger are rarely perfectly cylindrical — the walls are slightly irregular, and the bottom may be uneven. A 10% waste factor accounts for this extra concrete needed to fill voids, plus residual mix left in the tub. For hand-dug holes with rougher walls, increase the waste factor to 15%. The calculator always rounds up to the next whole bag, so you will never run short.
Step 5: Enter Cost for a Budget Estimate
Optionally enter the price per bag from your local home improvement store. The calculator multiplies total bags by bag price to give an immediate material cost estimate. Quikrete and Sakrete 80 lb bags typically cost $6–$9 each in the United States, varying by region and retailer. This estimate covers concrete only — add post lumber, hardware, and labor costs separately for a complete project budget.
The Post Hole Concrete Formula
The volume calculation behind this tool is straightforward: Hole Volume = π × (diameter ÷ 2)² × depth. Post volume for square lumber is width × width × depth. Net fill = Hole Volume − Post Volume. After applying the waste factor and dividing by the bag's yield in cubic feet, the result is rounded up to the nearest whole bag. Enter your own numbers to verify by hand or use the calculator above for instant results.
FAQ
Is this post hole concrete calculator free to use?
Yes, completely free with no signup, no paywalls, and no usage limits. Every calculation runs locally in your browser — your measurements are never sent to any server and stay entirely private on your device.
Is my data private when using this tool?
Absolutely. All calculations run client-side in your browser using JavaScript. No dimensions, quantities, or personal information are ever transmitted to a server. Your data stays on your device.
How many bags of concrete do I need for a fence post?
A typical 4×4 fence post in an 8-inch diameter hole that is 24 inches deep needs about 1 bag of 80 lb concrete per hole. For 6×6 posts in a 12-inch hole at 36 inches deep, plan for roughly 2–3 bags of 80 lb per hole. Use the calculator above with your actual hole and post dimensions for a precise count.
Should I use 50 lb, 60 lb, or 80 lb bags of concrete?
80 lb bags cover 0.6 cubic feet of mixed concrete and are the most economical choice for post holes — you need fewer bags and trips. 60 lb bags (0.45 cu ft) are easier to lift and mix, making them popular for solo projects. 50 lb bags (0.375 cu ft) suit tight spaces or when lifting heavy bags is a concern.
Why subtract the post volume from the hole?
The post itself displaces concrete inside the hole, so you only need to fill the annular space around and beneath the post. Skipping this subtraction leads to over-ordering, especially for larger posts like 6×6 lumber in narrower holes. The calculator handles this automatically when you select a post size.
How deep should a fence post hole be?
A common rule of thumb is one-third of the total post length should be in the ground, with a minimum of 24 inches below grade. In frost-prone climates, holes should extend below the frost line — typically 36–48 inches in northern US states and Canada — to prevent frost heave from pushing the post up over winter.
What diameter should a post hole be?
Post holes should generally be 2–3 times the width of the post. A 4×4 post (3.5 inches actual) works well in an 8-inch hole. A 6×6 post (5.5 inches actual) typically needs a 10–12 inch hole. The extra space allows concrete to form a footing collar around the post that resists both uplift and lateral movement.
What waste factor should I use for post holes?
A 10% waste factor is standard for post holes and accounts for slightly oversized holes from drilling, uneven hole walls, and concrete left in the mixing tub or bag. For hand-dug holes with irregular walls, increase the waste factor to 15%. Running out of concrete mid-project is more inconvenient than having a bag or two left over.