Fence Post Concrete Calculator
Find out exactly how many bags of concrete you need per post — and for the whole fence line — plus water, weight and cost.
Your post holes
Rule of thumb: 3× the post width.
≈ ⅓ of post height above ground.
= 14 posts (13 spans + 1)
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Per-post breakdown
2 × 80 lb bags per post
Mixing time
48 m
One person, 8 bags
Post length planner
The short answer
A typical 4×4 fence post in a 10 inch wide, 24 inch deep hole needs about 2 × 80 lb bags of concrete. Work it out precisely by finding the hole’s volume, subtracting the post, adding 5–10% waste, and dividing by the bag’s yield — 0.60 cu ft for an 80 lb bag. Multiply by however many posts you’re setting.
Bags per post, by hole size
Figures below are per post, with a 4×4 post fitted, no waste added.
| Hole size | Concrete needed | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|
| 8″ wide × 24″ deep | 0.63 cu ft | 2 | 2 | 3 |
| 10″ wide × 24″ deep | 1.09 cu ft | 2 | 3 | 4 |
| 10″ wide × 30″ deep | 1.36 cu ft | 3 | 4 | 5 |
| 12″ wide × 30″ deep | 2.09 cu ft | 4 | 5 | 7 |
| 12″ wide × 36″ deep | 2.51 cu ft | 5 | 6 | 9 |
Most people buy 2 bags per standard fence post and accept a little surplus. Running out halfway through setting a post is the worse outcome by far.
How deep and how wide should the hole be?
| Post height above ground | Recommended depth | Recommended hole width |
|---|---|---|
| 3 ft (fence rail) | 12″ | 9–10″ |
| 4 ft (standard fence) | 16″ | 10″ |
| 6 ft (privacy fence) | 24″ | 10–12″ |
| 8 ft (gate / corner post) | 30–36″ | 12″ |
The rule of thumb is a depth of one third of the post’s above-ground height, with a practical minimum around 24 inches for most residential fences — deeper again if your area has a frost line. Hole width runs about three times the post’s own width.
How to calculate concrete for a fence post
The post takes up space inside the hole, so the concrete you actually need is the hole’s volume minus the post’s volume.
- Find the hole’s volume. π × (diameter ÷ 2)² × depth, all in feet.
- Subtract the post. For a square post, side² × depth. For round, π × (diameter ÷ 2)² × depth.
- Multiply by the number of posts. This gives the total concrete for the whole fence.
- Add waste. 5–10% covers spillage and over-dug holes.
- Divide by bag yield and round up. 0.60 cu ft for 80 lb, 0.45 for 60 lb, 0.30 for 40 lb — round up per post, not just the total, so you don’t come up short on any single hole.
Worked example
A 4×4 post in a 10″ wide, 24″ deep hole, using 80 lb bags:
- Hole volumeπ × 0.417² × 2 = 1.09 cu ft
- Post volume (3.5″ square)0.292² × 2 = 0.17 cu ft
- Concrete per hole1.09 − 0.17 = 0.92 cu ft
- Plus 10% waste1.01 cu ft
- Divide by bag yield1.01 ÷ 0.60 = 1.7
- Round up2 × 80 lb bags per post
For four posts that’s 8 bags total — about 640 lb to carry and roughly 45 minutes of mixing for one person.
Standard vs fast-setting mix
| Standard concrete mix | Fast-setting post mix | |
|---|---|---|
| Preparation | Mix with water before placing | Pour dry, add water in the hole |
| Working time | 30–60 minutes | Sets in 20–40 minutes |
| Bracing needed | Yes, until it firms up | Usually not — sets fast enough |
| Best for | Multiple posts, larger jobs | Single posts, quick repairs |
| Full strength | ~28 days | ~28 days |
Fast-setting mix is convenient for a post or two, but on a full fence line, mixing standard concrete in a tub gives you more control and is usually cheaper per bag.
Setting the post: a quick checklist
- Dig the holeRoughly 3× the post width, to a depth of about ⅓ the post’s above-ground height.
- Add a gravel base2–3 inches of gravel improves drainage and stops the post base sitting in standing water.
- Set and plumb the postCheck level on two adjacent faces before pouring, not after.
- Pour and pack the concreteFill around the post, tamping to remove air pockets, and crown the top so water sheds away.
- Brace itTwo stakes and scrap wood, angled to the post, hold it plumb while it cures.
- Wait before loading it24–48 hours before attaching rails or panels; full strength at about 28 days.
Common mistakes
- Forgetting the post displaces concreteA 4×4 in a 10″ hole takes up real volume. Ignore it and you over-buy on every single post.
- Digging the hole too narrowConcrete needs room around the post to actually hold it. Too tight and you’re basically setting the post in dirt with a thin concrete shell.
- Rounding up the total instead of per postRound up bags per post, then add them up — rounding the grand total first can leave individual holes short.
- Not bracing the postStandard concrete stays workable for up to an hour. An unbraced post can lean before it sets.
- Skipping the gravel baseWithout drainage, water pools under the post and accelerates wood rot from the bottom up.
- Loading the post too soonHanging a gate or heavy panel within a day of pouring risks knocking the post out of plumb before it’s cured.
What this calculator assumes
Adjust your result if your project differs from these.
- Bag yields of 0.60, 0.45, 0.375 and 0.30 cu ft for 80, 60, 50 and 40 lb bags
- Post holes treated as true cylinders (real holes are usually slightly wider at the top)
- Actual lumber dimensions for post displacement (a 4×4 is 3.5″, a 6×6 is 5.5″)
- Standard concrete mix, not fast-setting or high-strength products
- Water of roughly 3 quarts per 80 lb bag
- Mixing at about 6 minutes per 80 lb bag, per person
- Post count from fence length rounds up to a whole number of posts, spaced evenly
- Prices are whatever you enter; they are not live market rates
Common questions
How many bags of concrete per fence post?
Most 4×4 posts in a 10″ wide, 24″ deep hole need about 2 × 80 lb bags per post. Larger posts or deeper holes can need 3 to 5 bags.
How deep should a fence post hole be?
A common rule is one third of the post’s above-ground height, with a practical minimum of 24 inches for most residential fences. Cold climates may need to go below the frost line.
How wide should a fence post hole be?
About three times the width of the post. A 4×4 post typically gets a 10 to 12 inch wide hole.
Do you have to remove the post to add concrete?
No. The post sits in the hole while concrete is poured and packed around it. The post’s volume simply displaces some of the concrete, which is why it’s subtracted from the total.
How much water does a bag of fence post concrete need?
About 3 quarts (2.8 L) per 80 lb bag, 2.25 quarts for a 60 lb bag, and 1.5 quarts for a 40 lb bag. Add gradually to a firm, workable consistency.
Can I mix the concrete dry and add water in the hole?
Fast-setting post mix is designed for exactly this — pour the dry mix around the post and add water on top. Standard concrete mix is meant to be mixed first, though many people dry-set posts successfully in damp ground.
How long does concrete take to set around a fence post?
Fast-setting mix is typically firm enough to hold a post upright in 20 to 40 minutes. Standard mix needs 24 to 48 hours before you can attach rails or panels, and about 28 days for full strength.
Should I use gravel instead of concrete for fence posts?
A gravel base under the post improves drainage either way. Gravel alone is faster and cheaper but holds posts less rigidly, which suits lightweight or temporary fencing better than a heavy gate post.
How many 50 lb bags of Quikrete for one fence post?
For a typical 4×4 post in a 10″ wide, 24″ deep hole, about 2 × 50 lb bags of fast-setting mix is standard, matching what most manufacturers list on the bag.
Do I need to brace the post while the concrete sets?
Yes, for standard concrete mix — it stays workable for 20 to 40 minutes and needs 24 to 48 hours to firm up. Two stakes braced against the post keep it plumb while it cures.
How many bags for a whole fence line?
Multiply the bags needed per post by the total number of posts. A 100 ft fence with posts every 8 ft needs around 13 to 14 posts, which at 2 bags each is roughly 26 to 28 × 80 lb bags.
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Fence Post Concrete Estimate
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materialcalculate.com/fence-post-concrete-calculator/
Project
| Hole size | — |
| Post type | — |
| Number of posts | — |
| Waste allowance | — |
Per post
| Concrete per hole | — |
| 80 lb bags per post | — |
Total shopping list
| 80 lb bags | — |
| 60 lb bags | — |
| 40 lb bags | — |
| Water needed | — |
| Dry weight to move | — |
| Estimated cost | — |
Notes
Estimate generated by materialcalculate.com. Assumes cylindrical post holes and standard concrete mix with yields of 0.60, 0.45, 0.375 and 0.30 cu ft for 80, 60, 50 and 40 lb bags. Confirm quantities with your supplier before buying.