Construction
Post Hole Concrete Calculator
Setting fence posts, deck posts, or mailbox posts means digging holes, dropping in posts, and pouring concrete around them. The two ways to mess this up are not enough concrete (post wobbles within a year) or way too much concrete (you spend $200 extra at the store and break your back hauling sacks you didn't need). This calculator computes the actual volume of concrete needed per hole — subtracting the post's volume — and returns bag count for both 60-lb and 80-lb fast-setting concrete mixes. Multiply by your hole count to get the total order.
60 lb bags per hole
—
- Total 60 lb bags
- —
- Cu ft per hole
- —
- 80 lb bags total
- —
How the math works
Volume of concrete per hole = hole volume – post volume.
- Hole volume = π × (diameter/2)² × depth, all in feet.
- Post volume = post width² × depth (assumes square post, e.g., 4x4 = 4" × 4").
- Bag yields: a 60-lb bag of mixed concrete = 0.45 cubic feet. An 80-lb bag = 0.6 cu ft. Fast-setting Quikrete (orange bag) = same yields, faster cure.
Worked example: a 10"-diameter × 36"-deep hole around a 4x4 post. Hole volume = π × (5/12)² × 3 = 1.64 cu ft. Post volume = (4/12)² × 3 = 0.33 cu ft. Concrete needed = 1.31 cu ft = 3 bags of 60-lb (1.35 cu ft, close enough) or 2.2 bags of 80-lb (round up to 3 for the realistic single-post order). For a 50-ft fence with 14 posts: 42 bags of 60-lb — a full pallet from Lowe's.
Hole sizing rules of thumb
- Hole diameter = 3x post width. 4x4 post → 10"-12" diameter hole (use a 12" power auger or an 8" auger plus widening). 6x6 post → 16" hole.
- Hole depth = 1/3 of total post length, plus below the frost line. A 6-ft fence uses 8-ft posts buried 30-36". A 4-ft fence uses 6-ft posts buried 24-30".
- Frost line depth varies by region: 24-30" in the Mid-Atlantic and South, 36-48" in the Midwest and Northeast, 48-60" in the upper Midwest, Maine, and most of Canada.
- Below ground line, leave 4-6" of gravel under the post before setting in concrete — lets water drain off the bottom of the post instead of pooling and rotting it.
How to use this calculator
- Hole diameter: width of the hole you'll dig in inches (10" is typical for 4x4 posts).
- Hole depth: total depth in inches, typically 30-48 depending on frost line.
- Post size: nominal width in inches (4 for 4x4, 6 for 6x6).
- Number of holes: total post count for your project.
- Output: bags per hole, total bags for the project (60-lb and 80-lb), and cubic feet of concrete per hole.
Common scenarios
100-ft wood privacy fence, 14 posts (4x4), 10" hole × 36" deep. Roughly 3 bags of 60-lb fast-setting per hole × 14 posts = 42 bags. A full pallet from the home center is 56 bags (great per-bag price) — buy the pallet and have leftover for repairs or future projects.
20-ft x 20-ft deck, 9 posts (6x6), 12" hole × 48" deep. Hole volume = 3.14 cu ft. Post volume = 1.0 cu ft. Concrete = 2.14 cu ft per hole = 5 bags of 60-lb or 4 bags of 80-lb per hole. 9 posts × 4 (80-lb) = 36 bags of 80-lb.
Mailbox post replacement, 1 post (4x4), 9" hole × 24" deep. Hole = 1.06 cu ft, post = 0.22 cu ft, concrete = 0.84 cu ft = 2 bags of 60-lb fast-set. Pour it in dry (Quikrete fast-set method), add water from a milk jug per the bag instructions, walk away. Set in 30 minutes.