Concrete Calculator

How much concrete do I need (bags vs. yards)?

For a slab, volume = length × width × thickness. With length and width in feet and the thickness converted from inches to feet (divide by 12), the answer comes out in cubic feet. Divide that by 27 to get cubic yards, the unit ready-mix suppliers quote, batch and deliver by. In metric, metres × metres × (centimetres ÷ 100) gives cubic metres directly.

The same volume can be bought two ways. Bagged mix from a hardware store suits small jobs: a few post holes, a shed pad, a step. Ready-mix arrives in a truck and suits anything you could not mix and place by hand before the first batch starts to set. The calculator shows both numbers, so you can see where your project falls. If you need to move between cubic feet, yards and metres for a supplier's quote, the unit converter handles volume as well as length.

How many bags of concrete do I need?

Divide the volume by what one bag yields once mixed. Quikrete's data sheet for its standard Concrete Mix lists about 0.30 cu ft for a 40 lb bag, 0.45 cu ft for 60 lb and 0.60 cu ft for 80 lb; Sakrete's standard 80 lb mix lists the same 0.6 cu ft. Those yields are what the US bag sizes here use. In round numbers, a cubic yard takes 45 bags of 80 lb, 60 bags of 60 lb or 90 bags of 40 lb.

The bag count is the volume plus your waste allowance, divided by the yield, then rounded up, since stores don't sell part-bags. The default allowance is 10%, which covers a little spillage, a subgrade that dips in places and forms that bow slightly under the load. Drop it to 5% for neat formed work such as a step, or raise it for rough ground. Not every bag yields the same: lightweight high-yield products such as Sakrete Maximizer list about 1.0 cu ft per 80 lb bag, so read the yield on the bag you plan to buy.

Worked example: a 10 × 10 ft slab, 4 inches thick

A 10 × 10 ft patio slab poured 4 inches thick, using 80 lb bags and the default 10% extra:

1. Volume: 10 ft × 10 ft × (4 ÷ 12) ft = 33.33 cu ft, which is 33.33 ÷ 27 = 1.23 cubic yards.
2. Add 10%: 33.33 × 1.10 = 36.67 cu ft (1.36 cubic yards).
3. Bags: 36.67 ÷ 0.6 cu ft per 80 lb bag = 61.1, rounded up to 62 bags.

With 60 lb bags the same slab takes 36.67 ÷ 0.45 = 81.5, so 82 bags. That is roughly 5,000 lb of dry mix either way, a strong argument for having it delivered to the site and for comparing the price against a ready-mix load (see the cost section below).

Bags of concrete per slab size

Common slab sizes at 4 and 6 inches thick. The volume and the first two bag columns include no extra; the last column adds 10%.

Going from 4 to 6 inches adds 50% to the volume, so the thickness you choose matters as much as the footprint. A 20 × 20 ft slab is already 5 to 7 cubic yards, well beyond what most people would mix from bags.

Metric units and 20 kg / 25 kg bags

Switch to Metric to enter lengths in metres and thickness, hole size and post width in centimetres. Results are given in cubic metres and litres, and the bag sizes become 20 kg and 25 kg, the common sizes outside the US. When you switch, a 40 lb choice becomes 20 kg and 60 or 80 lb becomes 25 kg; switching back maps 20 kg to 40 lb and 25 kg to 60 lb.

US manufacturers don't publish yields for metric bags, so the metric counts are worked out from density. Quikrete's data sheet gives about 17 litres of concrete from an 80 lb (36.2 kg) bag, roughly 0.47 L per kg of dry mix. That puts a 25 kg bag at about 11.7 L and a 20 kg bag at about 9.4 L, or roughly 86 × 25 kg bags per cubic metre. Products vary by brand and aggregate size, so treat these as estimates and use the litres-per-bag figure printed on your sack if it differs.

Example: a 3 × 3 m slab, 10 cm thick, is 0.9 m³ (900 L). With 10% extra that is 990 L, or 85 bags of 25 kg (77 with no extra).

Concrete for a fence post

Each post hole is a cylinder: volume = π × radius² × depth. A common rule of thumb is to dig the hole about 3 times as wide as the post and set a third to half of the post's total length into the ground, deeper where winters freeze so the base sits below the local frost line. Fence Post mode totals the concrete across however many posts you are setting.

The post itself takes up space that you don't have to fill. Lumber is sold by nominal size, so enter the actual width: a 4×4 is 3.5 in square (8.9 cm) and a 6×6 is 5.5 in (14 cm). The calculator subtracts width × width × hole depth from each hole, which assumes the post runs to the bottom. If you put a few inches of gravel under the post for drainage, enter the depth of concrete rather than the full hole depth. Enter 0 for a sonotube pier or a footing with no post in it.

For a 10 in hole, 24 in deep, the hole holds 1.09 cu ft and a 4×4 displaces 0.17 cu ft, leaving 0.92 cu ft per post. Four posts with 10% extra come to 4.05 cu ft, or 10 bags of 60 lb. A 6×6 in a 12 in × 36 in hole needs about 1.73 cu ft per post.

Bagged mix vs. ready-mix: what it costs

Ready-mix concrete runs roughly $125–$175 per cubic yard delivered, with several cost guides clustering around $130–$165. That is a rough 2026 US range; prices vary by region, so get local quotes. It is before pumping, labor, and the short-load fees many suppliers add to small orders. At that rate, the 1.36 cubic yards for the 10 × 10 ft slab above works out to about $170–$240 of concrete before any fees.

As guidance rather than a hard rule, bagged mix makes sense for post holes, pads and small repairs. Around 1 cubic yard (about 45 bags of 80 lb) and up, ready-mix is usually worth pricing: mixing that many bags by hand takes hours and is hard to finish as one consistent pour. Ask the supplier about minimum loads and short-load charges, and price bags at your local store for the comparison.

Tips for buying concrete

• Keep 5–10% extra for an uneven sub-base, spillage and measurement error. The calculator includes 10% by default; change it in "Extra for waste".
• Slabs usually sit on a compacted gravel base. Work out the stone separately with the gravel calculator for the sub-base.
• For an L-shaped or irregular slab, split it into rectangles and add up the bags, or get the total area from the square footage calculator and enter that area as the length with a width of 1.
• For anything over about 1 cubic yard, ready-mix delivery is usually less work and often cheaper than mixing dozens of bags by hand.
• Fast-setting mix is convenient for posts (no bracing needed), but standard mix cures stronger over time for structural slabs. Quikrete's standard mix is rated about 4,000 psi at 28 days and is meant for pours at least 2 in thick.
• Check your local frost line depth before setting fence posts. If they're too shallow, frost heave can push them out of the ground over a few winters.

Sources

The formulas on this page come from the published research below. Last updated October 10, 2026. This calculator gives estimates for general information. It isn't medical advice, so check anything important with a qualified professional.

1. QUIKRETE. Concrete Mix No. 1101 — product data sheet.Used for: Bag yields: 40 lb ≈ 0.30 ft³, 60 lb ≈ 0.45 ft³, 80 lb ≈ 0.60 ft³ (17 L), used for bag counts and the metric yield per kg