Primary material
62 bags
80 lb concrete mix
Use the calculated bag count as your buying starting point, then verify yield and pallet limits with the store.
Post hole concrete calculator
Post hole concrete calculator for fence and deck posts.
Enter hole diameter, depth, number of posts, and waste allowance to estimate concrete volume, bag count, and rough material cost.
4 holes, 8 in x 2.5 ft
7 bags
0.14 yd³ with default 10% waste
Formula
pi r² d
Cylinder volume per hole
Best for
Posts
Fence, deck, mailbox, sign posts
Quick post-hole answers
Common post-hole layouts
4 fence posts
8 in diameter x 2.5 ft deep
7 bags
0.14 yd³ with default 10% waste
8 fence posts
8 in diameter x 2.5 ft deep
13 bags
0.28 yd³ with default 10% waste
6 deck posts
12 in diameter x 3 ft deep
26 bags
0.58 yd³ with default 10% waste
Depth and diameter matter
Increasing diameter adds volume fast because the radius is squared. A deeper frost line or oversized auger can change bag count more than expected.
Shopping list
Ready to price and order
Use the calculated quantities as a starting cart, then compare store availability, delivery fees, pallet limits, and local ready-mix minimums before buying.
Large pours
Ready-mix quote check
For high bag counts, compare the bagged route against local ready-mix minimums, delivery, access, and unload time.
Optional base
Gravel base
Base depth is project-specific. Estimate it separately before adding bags or bulk gravel to the order.
Optional forms
Forms and stakes
Plan lumber, stakes, screws, release, and bracing around the perimeter and any step-downs or curves.
Store buttons open retailer search results for now. Confirm exact product yield, pickup, delivery, and pallet rules before ordering.
Formula shown
How the post-hole calculation works
1. Convert diameter to radius in feet
radius ft = (diameter in / 2) / 12
2. Calculate one hole
one hole ft³ = pi x radius ft² x depth ft
3. Multiply, add waste, and estimate bags
total ft³ = one hole ft³ x count; bags = ceil(total ft³ with waste / bag yield)
Post-hole edge cases
The calculator assumes a clean cylindrical hole. Bell-shaped bottoms, loose soil, water in the hole, oversized auger cuts, and gravel at the base can all change the concrete amount.
Sources
- Quikrete product yields — bagged concrete mix yield references
- Sakrete bagged concrete references — common bag sizes and fast-setting concrete guidance
- ConcreteNetwork concrete price guide — ready-mix and installed concrete cost context
Reviewed for estimating accuracy
Last updated: June 20, 2026
Estimate only.
This calculator is not a quote. Verify product yield, price, delivery minimums, and local conditions with your supplier or contractor before buying materials.
Related calculators
Other concrete tools
Concrete slab Use for patios, shed pads, sidewalks, and driveway panels. Concrete calculators Browse all concrete shapes and cost tools. Concrete footing Use for continuous footings or pad footings with rectangular volume. Fence post calculator Post holes are mostly for fence posts: count, depth, and concrete per hole.
FAQ
Post-hole concrete questions
Answers for fence post bag counts, hole diameter, fast-setting concrete, waste allowance, and quote limitations.
How much concrete do I need for fence post holes? + -
Enter the hole diameter, depth, number of posts, and your waste allowance. The calculator treats each hole as a cylinder, multiplies by the post count, then adds the entered waste allowance.
How many 80 lb bags do I need for 4 fence posts? + -
For four 8 inch diameter holes at 2.5 feet deep, the calculator estimates about 7 bags of 80 lb concrete mix with the default 10% waste allowance. You can edit waste for tighter or more conservative planning.
What hole diameter should I use? + -
A common starting point is a hole about three times the post width, but soil, frost depth, wind load, post type, and local code can change the requirement.
Should post holes use fast-setting concrete? + -
Fast-setting concrete is convenient for fence and mailbox posts, but always follow the product instructions for water, placement, and bracing time.
Is this post-hole estimate a quote? + -
No. It estimates material quantity and rough material cost only. Confirm hole size, depth, frost requirements, and product yield with your supplier or contractor.