Concrete Calculator

To find how much concrete you need, multiply length × width × thickness in feet, then divide by 27 for cubic yards. A 10 × 10 ft slab 4 in thick is 10 × 10 × 0.3333 ≈ 33.33 cu ft, or about 1.23 cu yd. This concrete calculator does that for slabs, footings, walls, columns, post holes, steps and curbs, and counts 40, 60 and 80 lb bags.

How to use the concrete calculator

1. Choose Feet + inches or Meters + cm. Each size has its own feet and inches boxes, so a 4 in slab is simply 0 ft 4 in; no need to convert.
2. Pick the shape of the first pour and type its sizes. The diagram beside it redraws to scale, which makes a mistyped number easy to spot.
3. Use Count for identical pours (eight post holes) and + Add another pour for the rest of the job, such as a slab plus its footing.
4. Set the extra for waste. The bag yields are filled in from the manufacturers’ data; change them to match the bag you buy.
5. Add a ready-mix price or bag prices if you want a cost comparison.

The result gives cubic yards, cubic feet and cubic meters, the amount to order, and a bag table with the number of bags and the total weight of mix for each size. Show steps writes out every formula with your numbers.

Concrete volume formulas

Work in one unit. If every size is in feet, the volume is in cubic feet; then divide by 27, because a cubic yard is 3 ft × 3 ft × 3 ft = 27 cu ft.

The steps formula comes from stacking: the bottom step is one rise tall, the next is two rises tall, and so on, so the heights add up to 1 + 2 + … + n = n(n + 1) ÷ 2 rises. For metric, 1 cu ft = 0.028316846592 m³ exactly.

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

1. Thickness in feet: 4 ÷ 12 ≈ 0.3333 ft.
2. Volume: 10 × 10 × 0.3333 ≈ 33.33 cu ft.
3. Cubic yards: 33.33 ÷ 27 ≈ 1.23 cu yd.
4. Add 10% extra: 33.33 × 1.1 ≈ 36.67 cu ft, about 1.36 cu yd.
5. Bags: 36.67 ÷ 0.60 ≈ 61.1, so 62 bags of 80 lb mix. With 60 lb bags it is 36.67 ÷ 0.45 ≈ 81.5, so 82 bags; with 40 lb bags, 123 bags.

Round bags up, never down. A pour that runs short leaves a cold joint where the fresh concrete meets the old.

Worked example: fence post holes

Four holes are 10 in across and 3 ft deep, each holding a 4×4 post that measures 3.5 in on a side. The hole’s cross-section is π × (10 ÷ 12)² ÷ 4 ≈ 0.5454 sq ft, and the post fills (3.5 ÷ 12)² ≈ 0.0851 sq ft of it. So each hole takes (0.5454 − 0.0851) × 3 ≈ 1.38 cu ft, and four holes take about 5.52 cu ft. With 10% extra that is about 6.08 cu ft, or 11 bags of 80 lb mix (6.08 ÷ 0.60 ≈ 10.13, rounded up). Leaving the post out of the formula would overestimate by about 1 cu ft.

Worked example: three concrete steps

Three solid steps are 4 ft wide, with a 7 in rise and an 11 in run. Using the stacking formula: 4 × (11 ÷ 12) × (7 ÷ 12) × (3 × 4 ÷ 2) = 4 × 0.9167 × 0.5833 × 6 ≈ 12.83 cu ft, about 0.48 cu yd. With 10% extra you would need 14.12 ÷ 0.60 ≈ 23.5, so 24 bags of 80 lb mix.

Concrete bag yields

A bag’s yield is the volume of concrete it makes once mixed. These figures come from the manufacturers’ published data for their standard mixes (QUIKRETE Concrete Mix No. 1101 and SAKRETE High-Strength Concrete Mix). Other products, such as fast-setting or lightweight high-yield mixes, yield different amounts, so read the bag.

One cubic yard from 80 lb bags means 45 bags, or 3,600 lb of dry mix to carry and mix. That is why larger pours are usually ordered as ready-mix, which is sold by the cubic yard (or cubic meter). Ask your supplier about its minimum load and any short-load or delivery fees before comparing prices.

How much extra concrete to order

The calculator starts at 10% extra. Real pours use more than the drawing because the ground is never perfectly flat, forms bow a little, and some concrete is spilled or left in the mixer. Raise the allowance for hand-dug footings or uneven ground; lower it for accurate forms on a flat, compacted base. Running out in the middle of a pour costs far more than a few leftover bags.

Common concrete calculation mistakes

• Typing inches as feet. 4 in is 0.333 ft, not 0.4 ft, and certainly not 4 ft. Use the inches box.
• Dividing by 9 instead of 27. 9 converts square feet to square yards; cubic feet to cubic yards needs 27.
• Using the radius as the diameter. The round formulas here take the full width across.
• Ignoring the post. A post takes up part of the hole.
• Assuming every bag yields the same. Check the yield printed on the product you buy.
• Rounding bags down. 61.1 bags means 62.

Concrete volume practice quiz: 10 questions

Test yourself with 10 multiple-choice questions. Choose an answer and press Submit to see if you are right and why. Your score appears at the end.

Frequently asked questions

How many bags of concrete do I need for a 10 × 10 slab?

At 4 in thick, a 10 × 10 ft slab is about 33.33 cu ft. That is 56 bags of 80 lb mix with no extra, or 62 bags with 10% extra. With 60 lb bags it is 75 without extra and 82 with 10% extra.

How many 80 lb bags of concrete make a yard?

A cubic yard is 27 cu ft. At 0.60 cu ft per 80 lb bag, 27 ÷ 0.60 = 45 bags. With 60 lb bags it is 60, and with 40 lb bags it is 90.

How do I calculate cubic yards of concrete?

Convert every size to feet, multiply to get cubic feet, then divide by 27. For example, 12 ft × 12 ft × 0.5 ft (6 in) = 72 cu ft, and 72 ÷ 27 ≈ 2.67 cu yd.

How thick should a concrete slab be?

That depends on what the slab will carry, the soil under it and your local building code, so check those first. The volume grows in step with thickness: going from 4 in to 6 in adds 50% more concrete, from about 1.23 to 1.85 cu yd for a 10 × 10 ft slab.

Is it cheaper to use bags or ready-mix?

It depends on local prices and the size of the job. Type your bag price and your ready-mix quote into the calculator to compare them for your exact volume, and remember that ready-mix suppliers may add delivery or short-load fees.

Related calculators

• Square footage calculator for the area of the slab or floor
• Volume calculator for cubes, cylinders, cones and more
• Area and perimeter calculator for form lengths and areas
• Circle calculator for round slabs and columns
• Unit converter for other length and volume units

Sources

• QUIKRETE Concrete Mix (No. 1101) product data sheet (40 lb bag ≈ 0.30 cu ft, 60 lb ≈ 0.45 cu ft, 80 lb ≈ 0.60 cu ft), checked November 2026
• SAKRETE High-Strength Concrete Mix and its technical data sheet (0.30, 0.45 and 0.6 cu ft for 40, 60 and 80 lb bags), checked November 2026
• NIST Handbook 44, Appendix C: General tables of units of measurement (1 cu yd = 27 cu ft; 1 cu ft = 0.028316846592 m³), checked November 2026