Sizing the hole
Quikrete's post-setting guide gives a simple rule: make the hole about three times the post's width, and dig it 1/3 to 1/2 of the post's above-ground height deep, plus 6 in for a gravel base. A 4 in post gets a hole about 12 in across. A 6 ft fence needs holes at least 2 ft deep. For structural posts such as decks, the hole has to reach at least 6 in below the frost line.
| Post (actual size) | Above-ground height | Hole diameter | Hole depth (incl. 6 in gravel) |
|---|---|---|---|
| 4×4 (3.5 in) | 4 ft | ~10–12 in | 22–30 in |
| 4×4 (3.5 in) | 6 ft | ~10–12 in | 30–42 in |
| 6×6 (5.5 in) | 6 ft | ~16 in | 30–42 in |
| 6×6 (5.5 in) | 8 ft | ~16 in | 38–54 in |
| Post (actual size) | Above-ground height | Hole diameter | Hole depth (incl. 15 cm gravel) |
|---|---|---|---|
| 90 × 90 mm | 1.2 m | ~25–30 cm | 55–75 cm |
| 90 × 90 mm | 1.8 m | ~25–30 cm | 75–105 cm |
| 140 × 140 mm | 1.8 m | ~40 cm | 75–105 cm |
| 140 × 140 mm | 2.4 m | ~40 cm | 95–135 cm |
Lumber is sold by nominal size: a "4×4" is really 3.5 in square and a "6×6" is 5.5 in. Enter the actual size.
How the concrete is calculated
For each hole the concrete fills the cylinder minus the post:
Concrete = π × (hole diameter ÷ 2)² × fill depth − post width² × fill depth
The fill depth is the hole depth minus the gravel layer. If you'll stop the concrete a few inches below grade and top it with soil, as Quikrete suggests, reduce the depth by that amount too. The calculator multiplies by the number of posts, adds waste, and converts to 40, 60 and 80 lb bags. If you set a gravel depth, it also reports the gravel volume.
Bags per post at common sizes
| Hole | Post | Concrete per hole | 80 lb bags per hole | 60 lb bags per hole |
|---|---|---|---|---|
| 10 in × 30 in deep | 3.5 in | 0.04 yd³ (1.2 ft³) | 2 | 3 |
| 12 in × 24 in deep | 3.5 in | 0.05 yd³ (1.4 ft³) | 3 | 4 |
| 12 in × 36 in deep | 3.5 in | 0.08 yd³ (2.1 ft³) | 4 | 5 |
| 16 in × 36 in deep | 5.5 in | 0.13 yd³ (3.6 ft³) | 6 | 8 |
| 16 in × 48 in deep | 5.5 in | 0.18 yd³ (4.7 ft³) | 8 | 11 |
| Hole | Post | Concrete per hole | 40 kg bags per hole | 25 kg bags per hole |
|---|---|---|---|---|
| 25 cm × 75 cm deep | 90 mm | 0.031 m³ (31 L) | 2 | 3 |
| 30 cm × 60 cm deep | 90 mm | 0.038 m³ (38 L) | 3 | 4 |
| 30 cm × 90 cm deep | 90 mm | 0.056 m³ (56 L) | 4 | 5 |
| 40 cm × 90 cm deep | 140 mm | 0.095 m³ (95 L) | 6 | 9 |
| 40 cm × 120 cm deep | 140 mm | 0.127 m³ (127 L) | 7 | 11 |
Fast-setting post mixes are often sold in 50 lb bags with their own yield. Sakrete, for example, says a standard 6 ft fence needs about two bags of its Fast Setting Concrete per hole. Check the yield on the bag you buy.
Fast-setting post mix
Set Concrete type to “Fast-setting (post mix)” to count bags of Quikrete Fast-Setting Concrete Mix, the product most people use for fence posts. It goes into the hole dry and is then soaked with water, and its data sheet lists about 0.375 ft³ per 50 lb bag. The calculator also shows bags per post, so you can check the rule of thumb on the bag against your hole size. With the defaults (10 posts, 10 in holes 30 in deep, 3.5 in posts, 5% waste) it's 33 bags, or 3.2 bags per post.
Set Concrete type to “Fast-setting (post mix)” to count 20 kg bags of rapid-set post mix, which goes into the hole dry and is then soaked with water. Yields vary by brand, so the calculator scales by mass from Quikrete's Fast-Setting data sheet (22.7 kg gives about 10.6 litres). That makes a 20 kg bag about 9.4 litres, so check the figure on your product. The calculator also shows bags per post. With the metric defaults (10 posts, 25 cm holes 75 cm deep, 90 mm posts, 5% waste) it's 35 bags, or 3.4 bags per post.
Worked examples
100 ft privacy fence: 15 posts, 10 in holes 30 in deep, 6 in gravel30.5 m privacy fence: 15 posts, 25.4 cm holes 76.2 cm deep, 15.2 cm gravel
Hole diameter 10 in · Hole depth 30 in · Post size 3.5 in · Number of posts 15 · Gravel at bottom of hole 6 in · Waste / overage 5 %Hole diameter 25.4 cm · Hole depth 76.2 cm · Post size 8.9 cm · Number of posts 15 · Gravel at bottom of hole 15.2 cm · Waste / overage 5 %
- Concrete volume: 0.54 yd³ (14.5 ft³)Concrete volume: 0.411 m³ (411 L)
- Concrete per hole: 0.03 yd³ (0.9 ft³)Concrete per hole: 0.026 m³ (26 L)
- 80 lb bags: 2540 kg bags: 22
- Gravel for hole bottoms: 0.15 yd³ (4.1 ft³)Gravel for hole bottoms: 0.116 m³ (116 L)
Mailbox post: one 4×4 in a 12 × 24 in holeMailbox post: one 10.2×10.2 cm a 30.5 × 61 cm hole
Hole diameter 12 in · Hole depth 24 in · Post size 3.5 in · Number of posts 1 · Gravel at bottom of hole 0 in · Waste / overage 0 %Hole diameter 30.5 cm · Hole depth 61 cm · Post size 8.9 cm · Number of posts 1 · Gravel at bottom of hole 0 cm · Waste / overage 0 %
- Concrete per hole: 0.05 yd³ (1.4 ft³)Concrete per hole: 0.04 m³ (40 L)
- 80 lb bags: 340 kg bags: 3
- 60 lb bags: 425 kg bags: 4
Pergola: four 6×6 posts in 16 × 42 in holes, 6 in gravelPergola: four 6×6 posts in 40.6 × 107 cm holes, 15.2 cm gravel
Hole diameter 16 in · Hole depth 42 in · Post size 5.5 in · Number of posts 4 · Gravel at bottom of hole 6 in · Waste / overage 5 %Hole diameter 40.6 cm · Hole depth 106.7 cm · Post size 14 cm · Number of posts 4 · Gravel at bottom of hole 15.2 cm · Waste / overage 5 %
- Concrete volume: 0.55 yd³ (14.9 ft³)Concrete volume: 0.423 m³ (423 L)
- 80 lb bags: 2540 kg bags: 23
Setting a post, briefly
- Call 811 (in the US) a few days before digging so buried utilities get marked.
- Dig the hole. Sakrete suggests a slightly wider bottom (a bell) rather than a cone narrowing toward the bottom.
- Add about 6 in of gravel and compact it so water can drain away from the post end.
- Set the post, brace it plumb on two adjacent faces, and check it with a level.
- Fill with concrete to 3–4 in below grade, then crown the top or backfill with soil so water runs away from the post (Quikrete).
- Wait for the concrete to set before hanging rails or gates. Fast-setting mixes can be ready in hours. Standard mixes need longer.
Concrete, gravel or foam?
Concrete gives the most lateral stiffness and is the usual choice for gate posts and tall privacy fences. Compacted gravel drains well and is easy to dig out later, and some fence builders prefer it for line posts in well-draining soils. Expanding foam products are sold for light fence posts. Check the manufacturer's limits, because they aren't a substitute for structural concrete. Deck and structural posts follow code or an engineer's design, which usually means concrete footings below frost depth.
Common mistakes
- Using the nominal post size (4 in) rather than the actual size (3.5 in). The difference is small, but it's free accuracy.
- Filling to the brim. Concrete flush with the soil traps water against the post. Crown it or stop below grade and backfill.
- Skipping the gravel layer, which leaves the end grain sitting in wet soil.
- Not bracing posts plumb before the concrete sets.
Checked against
Each case below is re-run against the calculator by our automated test suite.
| Case | Our result | Checked against | Result |
|---|---|---|---|
| 10 in hole, 30 in deep, 3.5 in post: concrete per hole | 0.04 yd³ (1.2 ft³) | Hand calculation | Match |
| 1 ft³ of fast-setting mix in 50 lb bags (no waste) | 3 | Quikrete Fast-Setting Concrete Mix data sheet (bag yield) | Match |
| Case | Our result | Checked against | Result |
|---|---|---|---|
| 25.4 cm hole, 76.2 cm deep, 8.9 cm post: concrete per hole | 0.033 m³ (33 L) | Hand calculation | Match |
Frequently asked questions
How many bags of concrete per fence post?
With a 10 in × 30 in hole and a 4×4 post, each hole takes about 1.2 ft³: 2 bags of 80 lb mix, or 3 bags of 60 lb.
Should I put gravel at the bottom of the post hole?
Quikrete and Sakrete both recommend about 4–6 in of compacted gravel for drainage.
How deep should a fence post be?
Quikrete says 1/3 to 1/2 of the post's above-ground height (plus 6 in for gravel), and below the frost line for structural posts.
Can I set posts without concrete?
Yes. Compacted gravel is used for some fences and drains well, but it gives less lateral strength. Gate posts are usually set in concrete.
Sources & assumptions
The results are planning estimates, and the cost figures only use prices you enter. Product yields, densities and coverage rates vary, so check the label or ask your supplier, and follow local codes. Figures marked "approximate" or "rule of thumb" are general practice, not exact standards.
- Quikrete: Setting Posts in Concrete
- Quikrete Fast-Setting Concrete Mix (No. 1004-50) data sheet: about 0.375 ft³ per 50 lb bag
- NIST Voluntary Product Standard PS 20-25 (American Softwood Lumber Standard), Table 3: dressed sizes
- Sakrete: Setting Posts & Poles
- 811: Call before you dig
- Quikrete Concrete Mix data sheet (bag yields, minimum thickness)
See how we calculate and test our formulas and the changelog. Last updated 2026-10-08.
Reviewed by the MaterialWhiz Editorial Team. This estimate uses the assumptions shown in the calculator. Review the result against your supplier's product coverage and site conditions. Last reviewed: October 10, 2026. See how we calculate · Report a correction.