How Many Bags of Concrete Do I Need? Posts, Tubes and Slabs in Bags, Yards and Cubic Metres
An 80 lb bag of QUIKRETE Concrete Mix yields about 0.60 cubic feet (17 litres) according to the manufacturer's data sheet, so a cubic yard takes 45 bags and a cubic metre takes about 59. For a fence post, a nominal 4×4 in a hole three times its width and one-third as deep as the post is long takes about 1.4 cubic feet of concrete, which is 2.3 bags. Buy three.
What one bag makes
The QUIKRETE data sheet for Concrete Mix (product numbers 1101-40, -50, -60, -80, -90, revised October 2022) lists a yield for each bag size. The yields, water and strength in this guide come from QUIKRETE's published data sheets, so they apply to QUIKRETE mixes; other brands print their own yields on the bag, and the label of the bag you buy governs. Everything else in this guide divides a volume by one of these numbers: find the volume of the space, divide by the yield of the bag you will buy, round up once, and add a waste margin that depends on how regular the hole is (a few percent for a formed slab on a flat base, about 10% for typical work, more for holes dug in loose soil, whose sides are rarely as straight as the auger diameter suggests). The concrete calculator does the slab version of this arithmetic.
| Bag size | Yield (cubic feet) | Yield (litres) | Bags per cubic yard | Bags per cubic metre |
|---|---|---|---|---|
| 40 lb (18.1 kg) | 0.30 | 8.5 | 90 | 117.6 |
| 50 lb (22.6 kg) | 0.375 | 10.6 | 72 | 94.3 |
| 60 lb (27.2 kg) | 0.45 | 12.7 | 60 | 78.7 |
| 80 lb (36.2 kg) | 0.60 | 17.0 | 45 | 58.8 |
| 90 lb (40.8 kg) | 0.675 | 19.1 | 40 | 52.4 |
The bags-per-yard and bags-per-metre columns are the yield divided into 27 cubic feet and 1,000 litres. The yield is proportional to weight: every pound of mix makes 0.0075 cubic feet. QUIKRETE's Fast-Setting Concrete Mix data sheet (revised November 2025) confirms the same ratio for the metric bags sold only in Canada: a 25 kg (55 lb) bag yields about 0.41 cubic feet (11.6 litres) and a 30 kg (66 lb) bag about 0.5 cubic feet (14 litres), alongside 0.375 and 0.45 cubic feet for its 50 and 60 lb bags. A 30 kg bag therefore takes about 71 bags to the cubic metre, and a 25 kg bag about 86.
The same sheet gives the product's character. Slump is 2 to 3 inches, the unit weight is approximately 140 lb per cubic foot wet, and compressive strength is 2,500 psi at 7 days and 4,000 psi at 28 days. It is designed for pouring concrete 2 inches thick or more.
Fence posts and deck posts
QUIKRETE's data sheet describes setting a fence post this way: dig a hole about three times the diameter of the post, make the hole depth one-third of the overall post height, and put 6 inches of dry mix at the bottom before positioning the post and filling with mixed concrete. The company's calculator adds that the hole depth should be about half the above-ground height, which for the same post gives the same depth: a 9-foot post with 6 feet above ground has 3 feet in the ground.
The concrete you need is the volume of the round hole minus the volume of the post inside it.
| Post (actual size) | Post length | Hole diameter | Hole depth | Hole volume | Post displacement | Concrete needed | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|---|---|---|
| 4×4 (3.5 in) | 6 ft | 10.5 in | 24 in | 1.20 cu ft | 0.17 | 1.03 cu ft | 1.7, buy 2 | 2.3, buy 3 |
| 4×4 (3.5 in) | 8 ft | 10.5 in | 32 in | 1.60 cu ft | 0.23 | 1.38 cu ft | 2.3, buy 3 | 3.1, buy 4 |
| 4×4 (3.5 in) | 10 ft | 10.5 in | 40 in | 2.00 cu ft | 0.28 | 1.72 cu ft | 2.9, buy 3 | 3.8, buy 4 |
| 6×6 (5.5 in) | 10 ft | 16.5 in | 40 in | 4.95 cu ft | 0.70 | 4.25 cu ft | 7.1, buy 8 | 9.4, buy 10 |
| 6×6 (5.5 in) | 12 ft | 16.5 in | 48 in | 5.94 cu ft | 0.84 | 5.10 cu ft | 8.5, buy 9 | 11.3, buy 12 |
The 3.5 in and 5.5 in dimensions are the actual sizes of nominal 4×4 and 6×6 lumber; measure your own posts, since round or specialty posts differ. Hole volume is π × (diameter ÷ 2)² × depth, and displacement is the post's cross-section times the depth.
The hole diameter matters more than the depth
Widening the hole is the biggest lever on the bag count. For an 8-foot 4×4 at 32 inches deep:
| Hole diameter | Concrete needed | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 8 in | 0.70 cu ft | 1.2, buy 2 | 1.6, buy 2 |
| 10.5 in (3× the post) | 1.38 cu ft | 2.3, buy 3 | 3.1, buy 4 |
| 12 in | 1.87 cu ft | 3.1, buy 4 | 4.2, buy 5 |
A 12-inch hole instead of a 10.5-inch one adds a bag per post, which over a long fence adds up to dozens of bags. Auger holes larger than the plan are the usual reason a project runs out.
Many posts: total first, then round
Rounding each post up and then multiplying overbuys. For ten 8-foot 4×4 posts in 10.5-inch holes, the concrete needed is 13.77 cubic feet in total. Counting bags:
| Bag | Bags for 10 posts (exact, then rounded up) | With 10% waste |
|---|---|---|
| 40 lb | 45.9, buy 46 | 51 |
| 50 lb | 36.7, buy 37 | 41 |
| 60 lb | 30.6, buy 31 | 34 |
| 80 lb | 22.95, buy 23 | 26 |
| 90 lb | 20.4, buy 21 | 23 |
If you rounded up per post you would buy 30 bags of 80 lb (3 per post) and have seven left over. Total first, then round, and add the waste at the end.
What the hole is for
Frost depth and post stability can override the one-third rule. For deck posts and other structural footings, local code controls both the depth (often below the frost line) and whether a form is required. QUIKRETE's own project sheet for tube forms says that for deck posts and load-bearing members the form should extend down below the frost line and that the exact method acceptable for load-bearing uses is subject to local building codes. Use the bag count here for the material, and the code for the depth.
Setting posts with fast-setting mix
QUIKRETE's Fast-Setting Concrete Mix (data sheet revised November 2025) is built for posts. It reaches final set in 20 to 40 minutes and a compressive strength of 1,000 psi at 24 hours, 2,500 psi at 7 days and 4,000 psi at 28 days, and the sheet allows a post to take strain after 4 hours. It also changes the method: the hole is dug the same way, at about three times the post width and one-third of the post's height deep, but the dry mix is poured straight into the hole around the standing post until it is 3 to 4 inches from the top, and then soaked with water, about 1 gallon per 50 lb bag depending on soil. For holes deeper than 30 inches, the sheet says to fill to 30 inches or less, wait until the water soaks all the way in, then add the rest of the mix and water. The top 3 to 4 inches of the hole is filled with sod or the soil that came out of it, which is why the bag count is lower than the geometry of the full hole.
The sheet gives its own sizing example: two 50 lb bags set a 3 inch square post in a 9 inch diameter hole 30 inches deep. Compare that with the geometry used earlier in this guide:
| Check | Calculation | Result |
|---|---|---|
| Hole volume, 9 in diameter × 30 in deep | π × 4.5² × 30 ÷ 1,728 | 1.10 cu ft |
| Post displacement, 3 in × 3 in × 30 in | 3 × 3 × 30 ÷ 1,728 | 0.16 cu ft |
| Concrete to fill the full depth | 1.10 − 0.16 | 0.95 cu ft |
| Bags of 50 lb at 0.375 cu ft | 0.95 ÷ 0.375 | 2.5 bags |
| Manufacturer's own count | Two 50 lb bags | 0.75 cu ft |
The manufacturer's two bags cover about 79% of the full-depth volume, consistent with the soil cap on top and some settling of the dry-poured mix. The practical reading: counts from this guide's geometry are a safe upper bound for a dry-poured post, and the sheet's example is the lower figure. For an 8-foot 4×4 in a 10.5-inch hole the geometry gives 1.38 cubic feet, which is 3.7 bags of 50 lb or 2.8 bags of 30 kg, so buy 4 and 3. Two cautions from the sheet: mix only as much material as can be placed in 20 minutes when you are mixing in a barrel or wheelbarrow rather than pouring dry, and expect set times to shift in very hot or cold weather.
Which mix, and what the project sheets say about thickness and base
QUIKRETE's project sheets for slabs, patios and footers add the specifications that bag counts do not show. The mixes themselves differ:
| Mix | What the manufacturer says | Strength figures |
|---|---|---|
| Concrete Mix | General-purpose structural mix | 2,500 psi at 7 days, 4,000 psi at 28 days |
| Fast-Setting Concrete Mix | Posts, and slabs 2 in or thicker, when rapid return to service is wanted | 1,000 psi at 24 hours, 4,000 psi at 28 days |
| 5000 | High early strength; gains strength quickly and suits cold weather | 5,000 psi at 28 days |
| Fiber-Reinforced Concrete | Eliminates the need for wire mesh; resists surface cracking and chipping | Not stated on the project sheet |
The slab sheet's specifications are the ones that change the volume. Sidewalks should be at least 4 inches thick, and simple slabs for small foundations and bases are normally 4 to 6 inches thick depending on the load. Walks slope away from buildings at about 1/4 inch per foot for drainage, and a patio slopes at 1/8 inch per foot; a 4 inch slab set 2 inches above ground level is generally suitable for a patio. For larger jobs or poor drainage the sheet says to dig deeper and add several inches of crushed stone or gravel to the subbase, and when plain Concrete Mix is used it may be desirable to add wire mesh supported at the midpoint of the slab. Control joints are cut every 6 to 8 feet in a 4 inch slab, and the sheet says the forms come off after about 48 hours. Each extra inch of base excavation is a volume of gravel, not concrete, which the gravel calculator handles.
The footer sheet applies the same logic to walls: a typical footer is twice the width of the wall and as deep as the wall is wide, resting on a 6 inch gravel base set below the frost line. Reinforcing bars sit one-third of the way up from the base, stepped footings on slopes should rise no more than 2 feet per step with at least 2 feet of overlap, and the footer should cure for at least 3 to 4 days before the forms come off. For an 8 inch wall that rule gives a 16 inch wide, 8 inch deep footer, which is the 14.8 bags per 10 feet worked in the footing example above. Local building departments set the frost depth and any footer design requirements, and they override the sheet.
Tubes and sonotubes
A tube form is a cylinder, so the volume is easy to read from a table. QUIKRETE's Quik-tube forms come in 6, 8, 10 and 12 inch nominal diameters and are 4 feet long; they can be cut to length. The company says the tube diameter should be at least double the diameter of the post it supports, and that 6 inches of gravel should go in the base of the hole.
The table gives bags of 80 lb mix for each tube diameter and depth, exact and rounded up.
| Tube diameter | 24 in | 36 in | 42 in | 48 in | 60 in |
|---|---|---|---|---|---|
| 8 in | 0.70 cu ft, 1.2, buy 2 | 1.05 cu ft, 1.7, buy 2 | 1.22 cu ft, 2.0, buy 3 | 1.40 cu ft, 2.3, buy 3 | 1.75 cu ft, 2.9, buy 3 |
| 10 in | 1.09 cu ft, 1.8, buy 2 | 1.64 cu ft, 2.7, buy 3 | 1.91 cu ft, 3.2, buy 4 | 2.18 cu ft, 3.6, buy 4 | 2.73 cu ft, 4.5, buy 5 |
| 12 in | 1.57 cu ft, 2.6, buy 3 | 2.36 cu ft, 3.9, buy 4 | 2.75 cu ft, 4.6, buy 5 | 3.14 cu ft, 5.2, buy 6 | 3.93 cu ft, 6.5, buy 7 |
| 16 in (a larger form) | 2.79 cu ft, 4.6, buy 5 | 4.19 cu ft, 7.0, buy 7 | 4.89 cu ft, 8.1, buy 9 | 5.59 cu ft, 9.3, buy 10 | 6.98 cu ft, 11.6, buy 12 |
Volume scales with the square of the diameter, so a 12-inch tube takes 2.25 times the concrete of an 8-inch one and 1.44 times a 10-inch one. Per foot of depth, an 8-inch tube takes 0.58 of an 80 lb bag, a 10-inch tube 0.91 bags and a 12-inch tube 1.31 bags.
A 4-foot tube of 12-inch diameter holds 3.14 cubic feet. Check the bag count against the tube length you actually use: tubes are often cut to the footing depth.
Slabs, pads and footings
For flat work the volume is length × width × thickness, with the thickness in feet. Bags are that volume divided by the yield.
| Size | Cubic feet | Cubic yards | 80 lb bags | With 10% waste |
|---|---|---|---|---|
| 4 ft × 4 ft × 4 in | 5.33 | 0.20 | 8.9, buy 9 | 10 |
| 10 ft × 10 ft × 4 in | 33.33 | 1.23 | 55.6, buy 56 | 62 |
| 10 ft × 12 ft × 4 in | 40.00 | 1.48 | 66.7, buy 67 | 74 |
| 12 ft × 12 ft × 6 in | 72.00 | 2.67 | 120 | 132 |
| 20 ft × 40 ft × 6 in | 400.00 | 14.81 | 666.7, buy 667 | 734 |
Every half inch of extra thickness over 100 square feet adds 4.17 cubic feet, or about 7 bags of 80 lb. A slab poured 4½ inches instead of 4 on a 10 × 12 pad needs 5 more cubic feet, which is about 8 more bags, so a slightly deeper pour into a low spot in the base is a real cost.
For a continuous strip footing 16 inches wide and 8 inches thick, 10 feet of length is 8.89 cubic feet, or 14.8 bags of 80 lb (buy 15). A 3 ft × 3 ft pad at 4 inches is 3.0 cubic feet or 5 bags.
Metric jobs
If you are working from metric dimensions, or from a ready-mix quote in cubic metres, the data sheet's 17 litres per 80 lb bag gives:
| Volume | 80 lb bags |
|---|---|
| 0.10 m³ | 5.9 |
| 0.25 m³ | 14.7 |
| 0.50 m³ | 29.4 |
| 1.00 m³ | 58.8 |
A 3 m × 3 m slab, 100 mm thick, is 0.9 m³ (31.8 cubic feet), which is 53 bags of 80 lb or 1.18 cubic yards. If you start from a ready-mix quote in cubic metres, multiply by 58.8 to see the equivalent in 80 lb bags.
Bag counts for common projects
The volume formulas above, applied to jobs people actually build. All use 80 lb bags at 0.60 cubic feet, and the right-hand column adds 10% for waste. Hole and tube sizes are assumptions to adjust to your own plan.
| Project | Assumptions | Concrete | 80 lb bags | With 10% waste |
|---|---|---|---|---|
| Mailbox post | 4×4 post, 5 ft long, 10.5 in hole, 20 in deep (one-third of the post) | 0.86 cu ft | 1.4, buy 2 | 2 |
| Landing or step pad | 4 ft × 4 ft, 4 in thick | 5.33 cu ft | 8.9, buy 9 | 10 |
| Deck footings | 6 tubes, 12 in diameter, 36 in deep | 14.14 cu ft | 23.6, buy 24 | 26 |
| Pergola | 4 posts, 6×6, 8 ft long, 16.5 in holes, 32 in deep | 13.60 cu ft | 22.7, buy 23 | 25 |
| Shed pad | 8 ft × 10 ft, 4 in thick | 26.67 cu ft | 44.4, buy 45 | 49 |
| Pole in a tube | 12 in diameter, 40 in deep | 2.62 cu ft | 4.4, buy 5 | 5 |
Two notes on these. QUIKRETE's tube-form sheet says flag, basketball and playground poles may need to be embedded to a depth of up to one-third of the pole's overall height, and to check local codes or the pole maker's recommendations, so use the manufacturer's depth for a pole rather than the fence-post rule. And a shed pad that is a 45-bag job is already a mixing marathon by hand: 45 bags is a full cubic yard, the point at which the next section suggests pricing a delivery.
When bags stop making sense
At 45 bags of 80 lb to the cubic yard, a one-yard job is 3,600 lb (about 1,630 kg) of material to lift, open and mix. The data sheet asks for at least 1 cubic foot of mixer capacity for each 80 lb bag mixed at one time and about 6 pints of water per bag to start (up to 9 pints), which means 45 bags need 270 to 405 pints of water, roughly 126 to 194 litres, and many batches in a barrel mixer. A barrel mixer of 3.5 cubic feet handles about three bags at a time, so 45 bags is fifteen batches.
There is no precise break-even (the cement calculator covers other mixes, and the homeowners hub collects the project calculators); it depends on your equipment, the number of helpers and the delivery charge for ready-mix. The data sheet's own advice for a slab is to fill to the full depth of the form and work continuously from one end to the other, avoiding partial depth fills that can leave cold joints. A pour that cannot be mixed fast enough to do that is a pour for a truck.
Mixing and curing details that change your plan
These come from the same data sheet.
| Bag size | Starting water | Maximum expected water |
|---|---|---|
| 40 lb | 3 pt (1.4 L) | 4½ pt (2.1 L) |
| 50 lb | 3½ pt (1.7 L) | 5½ pt (2.6 L) |
| 60 lb | 4 pt (1.9 L) | 7 pt (3.3 L) |
| 80 lb | 6 pt (2.8 L) | 9 pt (4.3 L) |
| 90 lb | 7 pt (3.3 L) | 10 pt (4.7 L) |
- Water. Add water until the mix is workable; if you have a slump cone, adjust to a 2 to 3 inch slump. Do not leave standing puddles.
- Curing. The data sheet calls for 5 days in warm weather at 70°F (21°C) or higher, or 7 days in colder weather at 50°F to 70°F (10°C to 21°C). Keep the surface moist with burlap, plastic sheeting or a sprinkler, or use a curing compound.
- Cold. Protect concrete from freezing for the first 48 hours.
- Posts. Wait 24 hours before a fence post is subjected to any strain.
- Safety. The product causes severe skin burns and serious eye damage; wear impervious gloves such as nitrile, a mask and eye protection.
Sources
- QUIKRETE, Concrete Mix No. 1101 data sheet (yields by bag size, unit weight, slump, compressive strength, water per bag, method for pouring a slab and setting fence posts, curing, safety; revised October 2022): https://www.quikrete.com/PDFs/DATA_SHEET-Concrete%20Mix%201101.pdf (checked October 23, 2026)
- QUIKRETE, Concrete Mix product page (bag sizes, 4000 psi average compressive strength): https://www.quikrete.com/productlines/ConcreteMix.asp (checked October 23, 2026)
- QUIKRETE, Quik-tube pillar foundations project sheet (tube diameters 6, 8, 10 and 12 inches, 4 feet long; form diameter at least double the post; frost line and local code): https://www.quikrete.com/PDFs/Projects/QuiktubePillarFoundations.pdf (checked October 23, 2026)
- QUIKRETE, concrete calculator page (post hole depth guidance, bags round up to the next whole bag): https://www.quikrete.com/calculator/main.asp (checked October 23, 2026)
- QUIKRETE, Fast-Setting Concrete Mix data sheet (yields including 25 kg and 30 kg bags, set time, strength, post-setting method and example; revised November 2025): https://www.quikrete.com/PDFs/DATA_SHEET-Fast%20Setting%20Concrete%20Mix%201004-50.pdf (checked October 23, 2026)
- QUIKRETE project sheets for sidewalks and small slabs, patios and footers for walls (thickness, slope, joints, gravel base, footer proportions): https://www.quikrete.com/PDFs/Projects/ConcreteSidewalksAndSmallSlabs.pdf , https://www.quikrete.com/PDFs/Projects/ConcretePatios.pdf and https://www.quikrete.com/PDFs/Projects/FootersForWalls.pdf (checked October 23, 2026)
- Yields for other brands are not covered; check the label of the bag you buy.